Method for treating brain tumors
REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of commonly assigned U.S. patent application Ser. No. 08/184,021, U.S. Pat. No. 05,442,678 entitled X-RAY SOURCE WITH IMPROVED BEAM STEERING, filed Jan. 21, 1994, and of commonly assigned U.S. application Ser. No. 08/104,338, U.S. Pat. 5,452,720 entitled METHOD FOR TREATING BRAIN TUMORS, filed Aug. 9, 1993, both of which are continuations-in-part of U.S. patent application Ser. No. 07/955,494, U.S. Pat. No. 05,369,679 entitled LOW POWER X-RAY SOURCE WITH IMPLANTABLE PROBE FOR TREATMENT OF BRAIN TUMORS, filed Oct. 2, 1992, assigned to the assignee of the present application, which is a continuation-in-part of U.S. patent application Ser. No. 07/577,883, now U.S. Pat. No. 5,153,900, entitled &#34;Miniaturized Low Power X-Ray Source,&#34; filed Sep. 5, 1990, assigned to the assignee of the present application. 
    
    
     BACKGROUND OF DISCLOSURE 
     The present invention relates to methods of treating brain tumors, particularly using a low power, programmable x-ray source for use in delivering low-levels of substantially constant or intermittent x-rays to a specified region. 
     Conventional medical x-ray sources are large, fixed position machines. Generally, the head of the x-ray tube is placed in one room and the control console in an adjoining area, with a protective wall, equipped with a viewing window, separating the two. The x-ray tube typically is approximately 20 to 35 centimeters (cm) long, and approximately 15 cm in diameter. A high voltage power supply is housed within a container located in a corner of the room containing the x-ray tube. Patients are brought to the machine for diagnostic, therapeutic, or palliative treatment. 
     Diagnostic x-ray machines are typically operated at voltages below 150 kilovolts (kV), and at currents from approximately 25 to 1200 milliamps (mA). By contrast, the currents in therapeutic units typically do not exceed 20 mA at voltages which may range above 150 kV. When an x-ray machine is operated at nominal voltages of 10 to 140 kV, the emitted x-rays provide limited penetration of tissue, and are thus useful in treating skin lesions. At higher voltages (approximately 250 kV), deep x-ray penetration is achieved, which is useful in the treatment of major body tumors. Super voltage machines, operable in the 4 to 8 megavolt (MV) region, are used to ablate or destroy all types of tumors, except superficial skin lesions. 
     A conventional x-ray tube includes an anode, grid, and cathode assembly. The cathode assembly generates an electron beam which is directed to a target, by an electric field established by the anode and grid. The target in turn emits x-ray radiation in response to the incident electron beam. The radiation absorbed by a patient generally is that which is transmitted from the target in the x-ray tube through a window in the tube, taking into account transmission losses. This window typically is a thin section of beryllium, or other suitable material. In a typical x-ray machine, the cathode assembly consists of a thoriated tungsten coil approximately 2 mm in diameter and 1 to 2 cm in length which, when resistively heated with a current of 4 amps (A) or higher, thermionically emits electrons. This coil is surrounded by a metal focusing cup which concentrates the beam of electrons to a small spot on an opposing anode which also functions as the target. In models having a grid, it is the grid which both controls the path of the electron beam and focuses the beam. 
     The transmission of an electron beam from cathode to anode is influenced by electron space charge forces which tend to become significant in conventional x-ray machines at currents exceeding 1 A. In such conventional machines, the beam is focused on the anode to a spot diameter ranging anywhere from 0.3 to 2.5 millimeters (mm). In many applications, most of the energy from the electron beam is converted into heat at the anode. To accommodate such heating, high power medical x-ray sources often utilize liquid cooling and a rapidly rotating anode, thereby establishing an increased effective target area, permitting a small focal spot while minimizing the effects of localized heating. To achieve good thermal conductivity and effective heat dissipation, the anode typically is fabricated from copper. In addition, the area of the anode onto which an electron beam is incident requires a material of high atomic number for efficient x-ray generation. To meet the requirements of thermal conductivity, effective heat dissipation, and efficient x-ray generation, a tungsten alloy typically is embedded in the copper. 
     In use, the total exposure from an x-ray source is directly proportional to the time integral of the electron beam. During relatively long exposures (e.g. lasting 1 to 3 seconds), the anode temperature may rise sufficiently to cause it to glow brightly, accompanied by localized surface melting and pitting which degrades the radiation output. However, thermal vaporization of the tube&#39;s coiled cathode filament is most frequently responsible for conventional tube failure. 
     While the efficiency of x-ray generation is independent of the electron beam current, it is highly dependent on the acceleration voltage. Below 60 kV, only a few tenths of one percent of the kinetic energy from an electron is converted to x-rays, whereas at 20 MV that conversion factor rises to 70 percent. An emitted x-ray spectrum is composed in part of discrete energies characteristic of transitions between bound electron energy levels of the target element. The spectrum also includes an x-ray energy continuum, known as bremsstrahlung, which is caused by acceleration of the beam electrons as they pass near target nuclei. The maximum energy of an x-ray cannot exceed the peak energy of an electron in the beam. Further, the peak of the bremsstrahlung emission curve occurs at approximately one-third the electron energy. 
     Increasing the electron current results in a directly proportional increase in x-ray emission at all energies. However, a change in beam voltage results in a total x-ray output variation approximately equal to the square of the voltage, with a corresponding shift in peak x-ray photon energy. The efficiency of bremsstrahlung radiation production increases with the atomic number of the target element. The peak output in the bremsstrahlung curve and the characteristic spectral lines shift to higher energies as the atomic number of the target increases. Although tungsten (Z=74) is the most common target material used in modern tubes, gold (Z=79) and molybdenum (Z=42) are used in some specialty tubes. 
     X-rays interact in several ways with matter. For biological samples, the following two types of interactions are most important: Compton scattering of moderate-energy x-rays with outer shell electrons; and, photoionizing interactions of inner shell electrons. In these processes, the probability of atom ionization decreases with increasing photon energy in both soft tissue and bone. For the photoelectric effect, this relationship follows an inverse third-power law. 
     One disadvantage of present x-ray devices used for therapy is the high voltage required when directed to soft tissue within or beneath bone. One example is in directing x-rays to areas of the human brain, which is surrounded by bone. High energy x-rays are required to penetrate the bone, but often damage the skin and brain tissue. Another example in radiation therapy is in directing the x-rays to soft tissue located within the body cavity, couched among other soft tissue, or within an internal calciferous structure. Present high-voltage machines are limited in their ability to selectively provide desired x-ray radiation to such areas. 
     Another disadvantage of the high voltage output of present x-ray sources is the damage caused to skin external to the affected organ or tissue. Therefore, high voltage devices of present systems often cause significant damage not only to the target region or tissue, but also to all surrounding tissue and surface skin, particularly when used for human tumor therapy. However, since present devices apply x-ray radiation to target regions internal to a patient from a source external to the target region, such incidental tissue damage is practically unavoidable. 
     Specifically as to brain tissue, which lacks any substantial regenerative ability, the treatment of brain tumors requires precise techniques to bring about specific tissue destruction. The use of conventional x-ray devices in brain tumor therapy often lacks the precision needed in volumetric irradiation, resulting in the damage of non-cancerous tissue of the brain and associated glandular structures. 
     An alternative form of tumor therapy, called brachytherapy, involves implanting encapsulated radioisotopes in or near the tumor to be treated. While such use of radioisotopes may be effective in treating certain types of tumors, introduction of the isotopes requires invasive procedures which have potential side-effects, such as the possibility of infection. Moreover, brain swelling may occur in some applications because the emission from the isotope cannot be controlled. Further, there is no ability to provide selective control of time dosage or radiation intensity. Handling and disposal of such radioisotopes involves hazards to both the individual handler and the environment. 
     Invasive techniques of the brain require precise control of irradiation through the choice and concentration of isotopes used. Intracranial penetration poses a significant risk as is well known in the art. 
     In view of the above requirements and limitations to the use of x-rays from present machines in therapeutic, diagnostic, palliative, or evaluative environments, there remains a need for a relatively small, easily manipulated, low-energy, x-ray device. Such a device operating at low energy and power will be suitable for many of the applications described herein. 
     Accordingly, it is an object of the present invention to provide an improved method for treating brain tumors. 
     It is another object of the present invention to provide a treatment method for brain tumors using an implantable, low-power x-ray device. 
     SUMMARY OF THE INVENTION 
     Briefly, the invention is a treatment planning and operation room procedure for a Photonic Radiosurgery Source (PRS), used to deliver single fraction doses of x-rays to small metastatic brain lesions. The procedure begins with a CT scan of the patient, with a stereotactic head frame attached, to determine tumor location, size, and shape, as well as suitable point of entry for a stereotactic biopsy. A biopsy is performed using the coordinates derived from the stereotactic frame. Preferably, while the biopsy is being performed, a series of checks are performed on the PRS to ensure that the output is optimized and in calibration. Following a short warm-up period, beam alignment and output of the PRS are checked for symmetry by a photodiode array, and for absolute calibration via ion chamber. A transfer calibration factor to convert the exposure measured in air to dose in water at 1 cm from the source is applied. Subsequent to a positive pathology report, the PRS, is coupled to the stereotactic frame and inserted into the patient, through the path of the biopsy needle. Scintillation detectors are located on either side of the head in the stereotactic frame, or near the head, to monitor the output of the device during the treatment. Signals from these detectors are recorded and displayed on a PC video monitor. After the treatment, similar checks of the output are made. The invention is particularly well suited for use with an easily manipulated PRS having a low-level, electron beam (e-beam) activated x-ray source of preselected, or adjustable, duration, effective energy and intensity, for example as disclosed in U.S. Pat. No. 5,153,900 and U.S. patent application Ser. No. 955,494. The PRS (or &#34;probe&#34;) may be fully or partially implanted into, or surface-mounted onto a desired area of a patient to irradiate a region with x-rays. As noted above the PRS is &#34;preferably assembled in combination with a reference frame, for example, a stereotactic frame, and an associated coupler for use in the treatment of brain tumors. 
     With the method of the invention, the apparatus operates at a relatively low voltage, for example, in the range of approximately 10 kV to 90 kV, with small electron currents, for example, in the range of from approximately 1 nA to 100 mA. To achieve a desired radiation pattern over a desired region, while minimally irradiating other regions, x-rays may be emitted from a nominal, or effective &#34;point&#34; source located within or adjacent to the desired region-to-be-irradiated. In some applications, a low dose rate of x-rays irradiates any part of the desired region, either continually or periodically, over extended periods of time. For use with a reference frame for treatment of brain tumors, a high dose rate for single dose irradiation is generally preferred. 
     The PRS apparatus preferably includes a controllable, or programmable, power supply located outside the desired region-to-be-irradiated to enable variations in voltage, current, and timing of an electron beam. The electron beam is controlled to pass along a desired beam axis and to be incident on a target which is preferably located in the patient&#39;s body. The axis may be straight, or curved. The composition and/or geometry of the target, or x-ray emitting, material is selected to provide a customized pattern of x-rays. Shielding at the emission site, or around the target, further enables control of the energy and spatial profile of the x-ray emission to match the preselected distribution of radiation throughout the desired region. The use of a small target, relative to the cross-sectional area of the x-ray producing electron beam, can provide a stable and reproducible source of x-rays. 
     Thus, the present invention provides a method of treating malignant cells, such as found in tumors, in vivo, utilizing the apparatus described above. Generally, the method involves identifying and locating malignant cells with a device generally available in the art, such as by computed tomography (CT) scanning or magnetic resonance imaging (MRI). A needle-type biopsy of the tumor may be performed to confirm the diagnosis. Then the region of treatment is selected and the radiation dosage determined. Such radiation treatment planning involves defining the size and shape of the tumor determining precisely its location in the body, identifying radiation-sensitive critical biological structures surrounding the tumor, deciding on the proper radiation dose distribution in the tumor and surrounding tissue and the entry path in to the tumor of the implanted portions of the apparatus. For spherical tumors, treatment planning can be performed manually using CT or MRI data. However, for more complex geometries, close-by critical structures, or higher precision procedures, computer-based &#34;3-D&#34; imagery is performed. In that case, tumors and critical structures are, for example, manually or semiautomatically segmented on a series of digitized CT scans, and a 3-D composite is rendered, which allows viewing the tumor from any direction. Various software systems have been-developed for radiosurgical procedures, such as for the linac and gamma knife, and some are commercially available. For example, Radionics Software Applications of Brookline, Mass. offers for sale software which images a CRW and BRW stereotactic frame affixed to a graphically transparent skull. Isodose profiles are overlaid on the tumor and other brain tissue. Similar software may be used with the invention disclosed in U.S. patent application Ser. No. 955,494 which effects imaging with respect to a stereotactic frame, for use with the x-ray-radiating electron beam target imbedded in the tumor. Isodose contours around the target are superimposed on the tumor and adjacent tissue. The absolute radiation dosage delivered along each contour is determined by experimental dosimetry performed to calibrate the probe. In these tests, the dose is measured at multiple locations around the target immersed in water or a biological tissue-simulating phantom. Such plastic, &#34;solid water,&#34; phantoms are commercially available (RMI, Middleton, Wis.) and simulate various body tissues, e.g., soft tissue of the brain. Either thermoluminescent detectors (TLD) or calibrated x-ray sensitive film (e.g., gafchromic film from Far West Technologies, Goleta, Calif.) can be positioned in the solid water to measure the dose directly. Using the imaging and dosimetry results from the radiation treatment planning, a low-power electron beam source and a selectively shaped x-ray radiation pattern generating target and shield assembly are positioned within or proximal to a region containing the cells to-be-irradiated, generally tumor cells, for example, in conjunction with a stereotactic frame assembly, such as disclosed in U.S. patent application Ser. No. 955,494. Other positioning assemblies, or methods, may be used. 
     Pursuant to the present invention, the target and shield assembly geometry and materials are shaped and selected in accordance with the characteristics of the desired region-to-be-irradiated. A programmable power supply is provided, which may be used to vary the voltage, current, and duration of the electron beam source to establish, in accordance with dosimetry information, a desired electron beam which is directed to the desired region-to-be-irradiated. Finally, x-rays emitted from the target and shield assembly are transmitted to the desired region-to-be-irradiated for selective destruction of the cells in that region. 
     In particular, the treatment of a tumor can be carried out utilizing an apparatus of the present invention comprising the combination of a low-power x-ray source for generation of a controllable irradiation pattern, with a device for accurately positioning the x-ray source in the body. The x-ray source can thus be precisely located near or in the tumor. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which: 
     FIG. 1 is a perspective view of a low power x-ray source embodying the present invention; 
     FIG. 2 is a schematic representation of a sheath adapted for use with the apparatus of FIG. 1; 
     FIGS. 3A and 3B are a perspective view and sectional view, respectively, of a surface-mountable apparatus embodying the present invention; 
     FIG. 4 is a schematic block diagram of the embodiment of FIG. 1; 
     FIGS. 5A and 5B are graphical representations of the x-ray emission spectrum of tungsten- and molybdenum-targets, respectively; 
     FIG. 6 is a detailed block diagram of a representative power supply of the embodiment of FIG. 1; 
     FIG. 7 is a detailed schematic diagram of power supply of FIG. 6; 
     FIG. 8 is a perspective view of a beam steering assembly embodying the present invention; 
     FIG. 8A is a cross-section view of the assembly of FIG. 8, taken along lines 8a; 
     FIG. 9 is a perspective view of a brain tumor x-ray treatment system incorporating a stereotactic frame for positioning the x-ray source; 
     FIG. 10 is an exploded perspective view of an x-ray apparatus and the coupling assembly of the system of FIG. 9; 
     FIG. 11 is a cross-section view of the x-ray device of the system of FIG. 9; and 
     FIG. 12 is a schematic representation of the high voltage power supply of the x-ray device of the system of FIG. 9. 
     FIG. 13 shows in block diagram form, at brain tumor treatment system; 
     FIG. 14 shows in exemplary equipment and personnel layout for a brain tumor room; and 
     FIG. 15A-B shows an exemplary sweep chart of a scintillation detector in use in the system of FIG. 13. 
     FIG. 16 shows a cross-sectional view of a low power x-ray source having an internal beam steering assembly which includes a feedback loop for electron beam positioning, and an internal scintillation detector/counter. 
     FIG. 17 shows a cross-sectional view of a low power x-ray source having an external beam steering assembly which includes a feedback loop for electron beam positioning and an external scintillation detector/counter. 
     FIG. 18 shows a cross-sectional view of the tubular probe. 
     Like numbered elements in each FIGURE represent the same or similar elements. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is a method for treating brain tumors, particularly adapted for use with a relatively small, electron-beam activated, low power x-ray apparatus. The method is advantageously used where the x-ray apparatus is coupled to a stereotactic frame affixed to a patient&#39;s head. The method may be used for therapeutic or palliative radiation treatment of tumors, or for other purposes. 
     Generally, the preferred apparatus for use with the present invention includes an electron-beam (e beam) activated x-ray source which operates at relatively low voltages, i.e. in the range of approximately 10  kV to 90 kV, and relatively small electron beam currents, i.e. in the range of approximately 1 nA to 100 mA. At those operating voltages and currents, the x-ray output is relatively low, and the apparatus may be made quite small and be adapted for implantation in medical therapeutic applications. In view of the low level x-ray output, adequate tissue penetration and cumulative dosage may be attained by locating the x-ray source adjacent to or within the region to be irradiated. Thus, the x-rays are emitted from a well-defined, small source located within or adjacent to the region to be irradiated. In the preferred embodiment, a low dose rate of x-rays may be applied to any part of a tumor, either continually or periodically, over extended periods of time, e.g., up to one month. In use with a stereotactic frame for the treatment of brain tumors, a high dose rate may be applied to a tumor for shorter periods of time (i.e., on the order of 5 minutes to 3 hours). 
     The present invention provides interstitial radiotherapy similar to that achieved with implanted capsules, needles, tubes, and threads containing natural or artificial radioactive isotopes, known as brachytherapy. However, a programmable power supply may be included in the x-ray source of the present apparatus to vary energy, intensity, and duration of the radiation. This differs from brachytherapy in that the intensity and penetration depth of the x-rays may be changed without surgically or invasively replacing the isotopes. Furthermore, the present invention is not limited by the half-life of a particular isotope, and does not pose a radiation hazard when turned off. 
     FIG. 1 shows an x-ray apparatus 10 for use with the present invention. Apparatus 10 includes a housing 12 and an elongated cylindrical probe 14 extending from housing 12 along a reference axis 16. The housing 12 encloses a high voltage power supply 12A (illustrated in electrical schematic form in FIGS. 6 and 7). The probe 14 is a hollow tube having an electron beam generator (cathode) 22 adjacent to the high voltage power supply 12A. Cathode 22 is located in close proximity to an annular focusing electrode 23 typically at the same potential as the cathode 22. An annular anode 24 is positioned approximately 0.5 cm or more from the annular focusing electrode 23. A hollow, tubular probe 14 extends along the same axis as the cathode, grid, and the hole in the anode. Probe 14 is integral with the housing 12 and extends toward a target assembly 26. In various embodiments, pans of the probe 14 may be selectively shielded to control the spatial distribution of x-rays. In addition, the probe 14 may be magnetically shielded to prevent external magnetic fields from deflecting the beam away from the target. 
     The electron beam generator 22 may include a thermionic emitter (driven by a floating low voltage power supply) or a photocathode (irradiated by an LED or laser source). The high voltage power supply establishes an acceleration potential difference between the cathode of generator 22 and the grounded anode 24 so that an electron beam is established along the reference axis 16, through the center hole of the anode and to the target assembly 26, with the region between anode 24 and the target assembly 26 being substantially field free. The beam generation and acceleration components are adapted to establish a thin (e.g. 1 mm or less in diameter) electron beam within the probe 14 along a nominally straight axis 16. 
     In a preferred embodiment, the probe 14 is a hollow, evacuated cylinder made of a beryllium (Be) cap and a molybdenumrhenium (Mo--Re), molybdenum (Mo), or mu metal body and a stainless-steel base extension. The cylinder is 16 cm long, with an interior diameter of 2 mm, and an exterior diameter of 3 mm. The target assembly 26 includes an emission element consisting of a small beryllium (Be) target element 26a coated on the side exposed to the incident electron beam with a thin film or layer 26b of a high-Z element, such as tungsten (W), uranium (U) or gold (Au). By way of example, with electrons accelerated to 30 keV-, a 2.2 micron thick tungsten film absorbs substantially all the incident electrons, while transmitting approximately 95% of any 30 keV-, 88% of any 20 keV-, and 83% of any 10 keV- x-rays generated in that layer. In a preferred form of the x-ray source, the beryllium target element 26a is 0.5 mm thick with the result that 95 % of these x-rays generated in directions normal and toward the substrate, and having passed through the tungsten target, are then transmitted through the beryllium substrate and outward at the distal end of probe 14. While the target element 26a shown in FIG. 3b is in the form of a disc, other shaped elements may be used, such as those having hemispherical or conical outer surfaces. 
     For use with some forms of the invention, the target element 26a assembly may include a multiple layer film 26b, where the differing layers may have different emission characteristics. By way of example, the first layer may have an emission (vs. energy) peak at a relatively low energy, and the second (underlying) layer may have an emission (vs. energy) peak at a relatively high energy. With this form, a low energy electron beam may be used to generate x-rays in the first layer (to achieve a first radiation characteristic) and high energy electrons may be used to penetrate through to the underlying layer (to achieve a second radiation characteristic). As an example, a 0.5 mm wide electron beam is emitted at the cathode and accelerated to 30 keV- through the anode, with 0.1 eV transverse electron energies, and arrives at the target assembly 26 sixteen centimeters downstream from the anode, with a beam diameter of less than 1 mm at the target. X-rays are generated in the target assembly 26 in accordance with preselected beam voltage, current, and target element 26a composition. The x-rays thus generated pass through the beryllium target element 26a in the probe with minimized loss in energy. As an alternative to beryllium, the target element 26a may be made of carbon, or other suitable material which permits x-rays to pass with a minimum loss of energy. An optimal material for target element 26a is carbon in its diamond form, since that material is an excellent heat conductor. In some embodiments, a discrete x-ray transmissive window separate from the target assembly 26 may be used. Using these parameters, the resultant x-rays have sufficient energy to penetrate into soft tissues to a depth of a centimeter or more, the exact depth dependent upon the x-ray energy distribution. 
     The apparatus of FIG. 1 is particularly adapted for full implantation into a patient, where the housing 12 has a biocompatible outer surface and encloses both a high voltage power supply circuit 12A for establishing a drive voltage for the beam generator 22, and an associated battery 12B for driving that circuit 12A. In this case, an associated controller 12C establishes control of the output voltage of the high power supply circuit 12A, in the manner described below. 
     The apparatus of FIG. 1 may also be used in a manner where only the probe 14 is inserted into a patient while the housing remains outside the patient, i.e., a transcutaneous form. In the latter form, the various elements shown within housing 12 may alternatively be remotely located. 
     In the transcutaneous form, the apparatus 10 may be used with an elongated closed end (or cup-shaped) sheath 34, as shown in FIG. 2, having a biocompatible outer surface, for example, fabricated of medical grade aliphatic polyurethane, as manufactured under the trademark Tecoflex® by Thermedics, Inc., Woburn, Mass. With this configuration, the probe 14 is first inserted into the sheath 34. The sheath 34 and probe 14 are then inserted into the patient through the skin. Alternatively, a port may be inserted through the skin and attached to it, as for example a Dermaport® port manufactured by Thermedics Inc., Woburn, Mass. The probe 14 is then inserted into the port. 
     The lining of the sheath or port can be configured as an x-ray shield by introducing barium sulfate or bismuth trioxide, or other x-ray shielding materials, into the sheath. If necessary, the probe 14 and housing 12 can be secured to the patient&#39;s body to prevent any relative motion during the extended time of treatment. An exemplary sheath 34 is shown in FIG. 2. 
     In one embodiment of the apparatus as shown in FIG. 1, the main body of the probe 14 can be made of a magnetically shielding material such as a mu-metal. Alternatively, the probe 14 can be made of a non-magnetic metal, preferably having relatively high values for Young&#39;s modulus and elastic limit. Examples of such material include molybdenum, rhenium or alloys of these materials. The inner or outer surface of probe 14 can then be coated with a high permeability magnetic alloy such as permalloy (approximately 80% nickel and 20% iron), to provide magnetic shielding. Alternatively, a thin sleeve of mu-metal can be fitted over, or inside of, the probe 14. The x-ray apparatus 10 can then be used in environments in which there are dc and ac magnetic fields due to electrical power, the field of the earth, or other magnetized bodies nominally capable of deflecting the electron beam from the probe axis. 
     In implantable configurations, the power supply 12A and target assembly 26 are preferably enclosed in a metal capsule to prevent current flow from the x-ray source to the patient. The closed housing 12 and probe 14 are, thus, encapsulated in a continuous outer shell of appropriate shielding material such as those mentioned previously. 
     The high voltage power supply 12A in each of the illustrated embodiments preferably satisfies three criteria: 1) small in size; 2) high efficiency to enable the use of battery power; and 3) independently variable x-ray tube voltage and current to enable the unit to be programmed for specific applications. A high-frequency, switch-mode power converter is used to meet these requirements. The most appropriate topology for generating low power and high voltage is a flyback voltage converter working in conjunction with a high voltage, Cockroft-Walton-type multiplier. Low-power dissipation, switch-mode power-supply controller-integrated circuits (IC) are currently available for controlling such topologies with few ancillary components. 
     In order to provide active control of the x-rays, a preferred embodiment of the present invention establishes independent control of cathode voltage and current without the use of a grid electrode. In that form of the invention, a radio frequency ohmic heating current is provided to a thermionic cathode 22, preferably using a transformer-coupled 0.6 volt, 0-300 mA filament power supply floating at the cathode potential of 40 kV. 
     FIGS. 3A and 3B show an alternative embodiment 10&#39; of the invention adapted for superficial usage, that is for direct placement on the skin of a patient. This form of the invention is particularly useful for x-ray treatment of skin lesions or tumors, or other dermatological applications. In FIGS. 3A and 3B, elements that correspond to elements in the embodiment of FIG. 1 are denoted with the same reference designations. Apparatus 10&#39; generates an electron beam in a channel 40 enclosed within housing 12, where that channel 40 corresponds to probe 14. In the present embodiment, of FIGS. 3A and 3B, the target assembly 26 (elements 26a and 26b) functions as the anode as well as an x-ray emitter. Otherwise, the apparatus 10&#39; is similar to apparatus 10. As with the configuration of FIGS. 3A and 3B, low power x-rays may be directed to a desired skin region of a patient. 
     In all of the above-described embodiments, the x-ray emission element of the target assembly is adapted to be adjacent to or within the region to be irradiated. The proximity of the emission element to the targeted region, e.g. the tumor, eliminates the need for the high voltages of presently used machines, to achieve satisfactory x-ray penetration through the body wall to the tumor site. The low voltage also concentrates the radiation in the targeted tumor, and limits the damage to surrounding tissue and surface skin at the point of penetration. For example, the delivery of 4000 rads, as is required after a mastectomy, with a 40 kV, 20 μA electron beam, may require approximately 1 to 3 hours of radiation. However, since the x-ray source is, in this preferred embodiment, insertable proximate to, or into, the region-to-be-irradiated risk of incidental radiation exposure to other parts of the patient&#39;s body is significantly reduced. 
     Further, specificity in treating tumors may be achieved by tailoring the target and shield geometry and material at the emission site. This tailoring facilitates the control of energy and the spatial profile of the x-ray emission to ensure homogenous distribution of the radiation throughout the targeted tumor. 
     FIG. 4 is a schematic representation of the x-ray source apparatus 10 shown in FIG. 1. In that configuration, the housing 12 is divided into a first portion 12&#39; and a second portion 12&#34;. Enclosed within the first housing portion 12&#39; is a rechargeable battery 12B, a recharge network 12D for the battery 12B, which is adapted for use with an external charger 50, and a telemetry network 12E, adapted to be responsive to an external telemetry device 52 to function in the manner described below. That portion 12&#39; is coupled by cables to the second housing portion 12&#34;. The second housing portion 12&#34; includes the high voltage power supply 12A, controller 12C and the probe 14, as well as the electron beam generating element 22. In the illustrated apparatus 10, the electron beam generator includes a thermionic emitter 22 driven by the power supply 12A. In operation, power supply 12A heats the thermionic emitter 22, which in turn generates electrons which are then accelerated toward the anode 24. The anode 24 attracts the electrons, but passes them through its central aperture toward the target assembly 26. The controller 12C controls the power supply 12A to dynamically adjust the cathode voltage, the electron beam current, and temporal parameters, or to provide pre-selected voltage, beam current, and temporal parameters. 
     Also illustrated, is an alternative electron beam generator which includes a photoemitter 22 irradiated by a light source 56, such as a diode laser or LED, powered by a driver 55. The light is focussed on the photoemitter 22 by focusing lens 58. 
     In the illustrated embodiment, device 52 and network 12E cooperate to permit external control (dynamic or predetermined) control over the power supply 12A and temporal parameters. In embodiments when the housing 12&#34; is not implanted, but where only probe 14 extends into a patient&#39;s body, the controller 12C may directly be used to control operation; in that case there is no need for network 12E. 
     In an important aspect, the target assembly 26 may be shaped to emit x-rays in a radiation pattern having a predetermined spatial distribution, and in a predetermined spectral range. This spectral target shaping may be achieved in part by selecting target materials of known characteristics. For example, as shown in FIGS. 5A and 5B, the emission spectra for tungsten targets (FIG. 5A) and molybdenum targets (FIG. 5B) are distinct. FIG. 5A shows the x-ray emission spectrum from a tungsten target tube operating at 30 and 50 kV. Note that the bremsstrahlung spectrum predominates, and that x-rays are supplied in a wide energy range. FIG. 5B shows the emission spectrum from a molybdenum target tube, also operating at 30 and 50 kV. Note the near absence of bremsstrahlung x-rays. Note also that the change in tube potential from 30 to 50 kV results in a minor change in the shape of the emission spectrum from a molybdenum target x-ray tube. Thus, the x-ray emission from target assembly 26 may effectively be shaped by selecting the target material to provide the desired radiative penetration of tissue, e.g., the tumor. 
     The x-ray spatial distribution may be also shaped by altering the geometric configuration of target element 26A. By way of example, the target element 26A of the target assembly 26 may be shaped such that the electrons directed from the anode will be incident at a predetermined angle or may be selectively directed to different areas of the region from which emission is to occur. As shown in the above-described embodiments, the apparatus 10 of FIG. 1 includes a power supply 12A. FIG. 6 is a block diagram of a representative supply 12A. FIG. 7 shows a more detailed schematic of the supply of FIG. 7. As shown in FIGS. 6 and 7, that embodiment includes a flyback switching converter and regulator 280, a 30:1 voltage transformer 282 coupled to a control voltage (or high voltage multiplier input) terminal 282a, and a 10 stage voltage multiplier 284 coupled to a high voltage terminal 22a, and adapted to drive the filament of a thermionic emitter 22. A filament radio frequency power driver and voltage-to-frequency (V/F) converter 290 and an associated radio frequency filament driver 292 are coupled through current control terminal 292a and capacitor C o  by way of a filament drive circuit 286 to the filament of emitter 22. 
     A difference amplifier 294 establishes a current feedback loop by driving the radio frequency power driver and V/F converter 290 in response to the detected difference between a current feedback signal on line 295 and an applied emission control signal on line 296. The latter signal may be selectively controlled to establish a desired temporal variation in the x-ray tube cathode current in filament of emitter (thermionic cathode) 22. 
     A high voltage amplitude feedback loop is established by the switching converter and regulator 280 in response to the detected difference between a voltage feedback signal on line 297 and an applied high voltage control signal on line 298. The latter signal may be selectively controlled to establish a desired amplitude variation of the potential at the filament of emitter (thermionic cathode) 22. 
     A more detailed description of the power supply shown in FIGS. 6 and 7 is provided in U.S. Pat. No. 5,153,900 and also in parent application U.S. Ser. No. 955,494, which are incorporated herein by reference. 
     As a further feature of the invention, steering may be used to direct the emitted electron beam to selected surfaces on the emission element, for example, where the target has different emission characteristics in different spatial regions. Control of the electron beam may be achieved under the control of telemetry, or by pre-programming the power source prior to implantation of all or part of the apparatus 10. 
     FIG. 8 shows an exemplary electrostatic beam steering assembly. In the illustrated embodiment, the cathode 22 generates electrons in a manner consistent with the above-described embodiments. The electrons are accelerated through a focusing electrode 23 toward the anode 24, and pass through an aperture 24A toward the target assembly 26. Enroute to target assembly 26, the electrons pass through an electrostatic deflection assembly 30, shown in cross-section at FIG. 8A. The assembly includes four deflectors 32. By varying the voltages applied to the opposing pairs of the deflectors 32, the electrons of the beam entering the assembly along axis 16A are deflected, or &#34;steered&#34; as they travel toward the target assembly 26 along axis 16B. Thus, the beam axis may be controlled to be straight or curved, as desired. As described below, electromagnetic techniques may alternatively be used to establish beam steering. In the latter case, the electrostatic deflective plates 32 may be replaced with magnetic deflector coils which are driven by currents to establish magnetic fields necessary to achieve a featured beam deflection. 
     In another form of the beam-steering embodiment, rather than pass through an electrostatic deflection assembly 30, the electron beam passes through a set of magnetic field-generating coils. The coils can be arranged in a configuration similar to the electrostatic deflection plates of the assembly 30. By varying the current through the coils, the resultant magnetic field is produced in a predetermined manner so as to influence the path of the electron beam. 
     In such a fashion, the electron beam may be steered to hit certain physical locations on a cone-shaped target assembly (FIG. 8), or a target of any other specific geometric configuration. By way of example, in the illustrated embodiment, a beam hitting the angled side of target assembly 26 will result in x-rays emitted off to that side, with little or no incidental radiation transmitted through to the opposite side of the target assembly. 
     In another form of the beam-steering embodiment, the x-ray emission characteristics may be controlled by spatially varying the emission parameters (such as radiation peak vs. energy) of the target assembly. By changing the emission peak (as a function of energy) at various points in the target assembly 26, for example, with a &#34;bullseye&#34; spatial pattern, the beam may be steered to regions of relatively high energy x-ray emission, or to regions of relatively low energy x-ray emission. Thus, the beam may be selectively directed to regions of the target assembly to achieve the required x-ray emission characteristic and direction. 
     As one skilled in the art will appreciate, the beam steering assembly of FIG. 8, can also be used in combination with the target assembly 126 of FIGS. 16 and 17. 
     FIG. 9 shows an exemplary system 300 adapted for x-ray treatment of brain tumors. System 300 includes a stereotactic frame 302 in combination with a low-power x-ray device 10A coupled thereto. In that configuration, x-ray device 10A is generally similar to the x-ray device 10 shown in FIG. 1, but has a cylindrical geometry. Corresponding elements of the two x-ray devices 10 and 10A are identified with the same reference designations. In general, stereotactic frames provide a fixed reference structure relative to the cranium of a patient. While the preferred embodiment described above is particularly adapted for use with this stereotactic frame, other embodiments of the invention might be similarly adapted for use with other frames or with general reference frames, for example, one establishing and operating fixture fixedly referenced to a part of the body other than the head. In the illustrated embodiment of FIG. 9, the stereotactic frame 302 is substantially similar to the Cosman-Roberts-Wells system manufactured by Radionics Inc., Burlington, Mass. 
     In the illustrated embodiment, the frame 302 establishes a reference XYZ coordinate system disposed about a desired origin point O. The frame 302 includes a generally U-shaped support element 304 defining a reference plane. Four arms 306A, 306B 306C and 306D (not shown) extend out from support frame 304. Each ann has a positioning pin 308. The pins 308 extend generally towards each other from the respective distal tips of arms 306A, 306B, 306C and 306D. In use, the four pins 308 are positioned against a patient&#39;s skull to establish a fixed positional relationship between the frame 302 and the patient&#39;s cranium. Thus, the frame 302 defines the reference XYZ coordinate system with respect to the patient&#39;s cranium. 
     An x-ray device support member 310 is coupled to the support element 304 by way of a pair of rotational coupling assemblies 312 and a pair of linear coupling assemblies 314. The x-ray device support member 310 includes an arcuate support track 310A. An x-ray device 10 is coupled to support track 310A by a coupling assembly 316. Coupling assembly 316 provides controlled movement of the x-ray device 10 on a circular path along track 310A and between an inner limit point and an outer limit point along axes (exemplified by axis 316&#39;) extending radially inward from the circular path of arcuate track 310A toward the origin point O. 
     In addition, rotation about the hubs of rotational coupling assemblies 312 allows the x-ray device support member 310 to be rotatably moved about the X axis. The x-ray device support member 310 is translocatable in a direction normal to the plane defined by the X and Y axes (the X-Y plane) by movement along tracks 314A, of the linear coupling assemblies 314. In the illustrative embodiment, a T-groove in tracks 314A mates with a tenon of block 314B which is affixed to member 304, permitting linear motion in the direction perpendicular to the X-Y plane. Set screws 332 in block 3 14B may be adjusted to lock the x-ray device support member 310 at a set height relative to the support frame 304. 
     X-ray support member 310 may be moved in the direction of the Z axis by movement of the tenons extending from member 310 in tracks 304A of support element 304. A controlled position of the member 310 along the tracks 304A can be established using locking screws 334. 
     In addition, support element 304 can be adjustably positioned in the direction of the X axis by sliding member 304 relative to its support member 305, and may be adjustably positioned with three degrees of freedom to establish a desired location of origin point O within the skull of a patient. 
     The coupling assembly 316 is shown together with an x-ray device 10A, in exploded form, in FIG. 10. As shown, the coupling assembly 316 includes a receiver block 316A, a bushing element 316B, together with complementary shaped portions of the x-ray device 10. As shown, the central axis of probe 14 of x-ray device 10A is coaxial with axis 316&#39;. The electron beam axis 16 of probe 14 is nominally coaxial with axis 316&#39;, but may be adjustably varied as described above in conjunction with FIGS. 8, and 8A, and below in conjunction with FIG. 10. 
     The cylindrical bushing element 316B is positioned partially within and coaxially with the receiver block 316A. The bushing element 316B is slidable (in the direction of radial axis 316&#39;) and may be selectively locked in place relative to block 316A using a set screw 318A. The bushing element 316B includes a central bore (with diameter D) extending along its central axis. 
     As noted above, the x-ray device 10A is similar to the x-ray device 10 shown in FIG. 1, but has a generally cylindrically shaped housing 12; the probe 14 includes a cylindrical shoulder portion 14A (having a diameter slightly less than D) immediately adjacent to housing 12, with a main portion with a small diameter (3.0 mm in the preferred embodiment). With this configuration, the x-ray device 10A may be positioned with its axis 16 coaxial with axis 316&#39; and the shoulder portion 14A slidingly positioned within the bore of bushing element 316B. The relative position of x-ray device 10A may be fixed along axis 316&#39; using set screws 320 of element 316B. 
     The x-ray device 10A is shown in sectional form in FIG. 11. As shown in FIG. 11, X-ray device 10A may include a magnetic deflection subsystem for its electron beam. The deflection subsystem includes magnetic deflection coils 32&#39; positioned about axis 16 within shoulder portion 14A. These coils are driven to adjustably control the position of the beam axis so that the beam is incident on the target of assembly 26 in a desired manner. In the preferred form, radiation generated by device 10A is monitored (for example, by an x-ray detector positioned outside the patient) and the deflector coils are driven accordingly by steering control currents on deflection X1, X2, Y1 and Y2 lines (generated in supply 12&#39;) applied to the deflection coils. 
     The microprocessor-based controller may not be disposed within the housing 12, but located external to the housing 12 in a control unit 342. Control unit 342 is coupled to x-ray device 10A by way of cable 342&#39;. The elongated probe 14 of x-ray device 10 is configured so as to allow the probe 14 to pass through the track left by a biopsy needle, thereby permitting easy insertion of the probe 14 into the brain of a patient. For tumors composed of hard tissue, and where a biopsy needle smaller in width than the probe 14 is used, proper penetration into the tumor may require first widening the track left by the biopsy needle with intermediate sized needles. 
     With this configuration, the tip of probe 14 contains the x-ray emitting target and can be moved in and out relative to the cranial insertion site by movement along the axis 316&#39;. The x-ray device 10A can be secured at a given position along by set screws 318A and 320. The length of probe 14 of x-ray device 10A is chosen such that the tip of probe 14, when fully inserted down to the lower limit point along the axis 316&#39; of 316A, exactly contacts the origin point O; when the x-ray apparatus 10 is fully withdrawn to the upper limit point along axis 316&#39;, the distal tip of the probe 14 is intended to be outside the patient&#39;s skull. The coordinates of the arcuate support track 310A can be set such that the origin point O is located at the desired epicenter of irradiation. Thus, by the rotation of x-ray device 10A support member 310 and the positioning of the x-ray device 10&#39; along the circumferential track of the arcuate support track 310A and along axis 316&#39;, a user can choose the appropriate path (preferably of least destruction) for insertion of probe 14 into a patient&#39;s skull, the tip of probe 14 always contacting the origin point O upon full insertion of the probe 14 to the lower limit point. 
     FIG. 12 shows a schematic diagram of a preferred high voltage power supply 12A for use with the x-ray device 10A of FIGS. 10 and 11. In that power supply, the HV drive signal is a 0 to 9 Volt pulse density modulated drive signal. This signal drives the Flyback Switching FET Q1, which in turn drives the HV Flyback transformer. The HV Flyback transformer steps up the +12 Volts to several thousand volts. The HV multiplier, D1 to D28, in turn steps up the voltage to the desired output voltage of 15 to 40 kV. The voltage FB line provides feedback information to controller 12C, so that the output voltage of the HV multiplier can be held at a constant value. 
     The Filament + and - lines provide complementary 9 Volt 250 kHz square wave drive signals to FET&#39;s Q2 and Q3. These FET&#39;s chop the variable Filament DC voltage into an AC voltage, and drive the Filament/HV Isolation Transformer T2. Using a high frequency signal to drive this transformer permits a single turn secondary to drive the x-ray tube filament. This in turn permits miniaturizing the transformer while maintaining the necessary high voltage isolation. The current FB line allows controller 12C to sense the beam current, and the controller then adjusts the Filament DC Voltage for the desired beam current, by providing the appropriate heating current to the thermionic emitter 22. The Deflection X1, X2, Y1, Y2 lines provide current drive signals to the magnetic beam deflection coils. 
     As discussed above with respect to FIG. 1, the apparatus 10 includes beam generation and acceleration components to generate and accelerate electrons, prior to those electrons entering the probe 14. The generated electron beam then flows through probe 14, impacts the target 26b, and thereby produces x-rays. In the absence of magnetic fields, the electrons flowing through the probe 14 follow a straight-line trajectory. Consequently, the probe 14 is typically rigid without any bends. 
     A miniature x-ray device, such as disclosed in generally in U.S. Pat. No. 5,153,900 and U.S. patent application Ser. No. 955,494, has recently been developed for the purpose of treating small intracranial lesions, less than 3 cm in diameter. Unlike linear accelerators which have dual, independent, calibrated dose monitors to verify that the correct treatment dose has been given, the miniature x-ray device may not have a built-in dose monitor and may rely upon several external monitoring devices and strategies. 
     The above described x-ray source and stereotactic frame assembly, or similar assemblies, may be used with the present invention. FIG. 13 shows, in schematic form, such a configuration (system 400) in integrated form for treating brain tumors. FIG. 14 shows a treatment room 402, showing equipment and personnel layout. System 400 includes a CRW stereotactic frame 410 affixed to a patient. The frame 410 is shown with the PRS 414 and its associated controller 420. The controller 420 is coupled to a computer based treatment planning system 426 (which evaluates CT date representative of the patient and calibration data for the PRS 414 and determines appropriate control signals for the PRS controller 420 to apply to PRS 414. As described below, the controller 420 receives calibration/monitoring data by way of a computer 430 which is coupled to an electrometer 432 which monitors an ionization chamber 434. Also, a treatment radiation monitor 440 is coupled to controller 420. A PDA 442 and straightener 444 are coupled to controller 420. The ionization chamber is supported by the ionization chamber holder (IC Holder 446). 
     The treatment planning system 426 is coupled to a dosimetry computer 446 which monitors a dosimetry tank 448 via associated control electronics 450. The computer 446 may monitor the output of a radiochromic film reader 454, which measures the exposure of radiochromic films, located in tissue-equivalent phantoms 456, to x-ray dose. 
     With this system 400, after a short warm-up of the device and check of the PRS 414 for straightness and then optimizing the output by steering adjustments to the electron beam, the dose rate of the PRS 414 in air is checked immediately prior to the procedure using a thin window, parallel plate ion chamber 434 in a fixed geometry. This dose rate is related to the dose rate in water by a transfer factor derived on previous occasions by sequential in-air and in-water measurements. This converted, in-water, dose rate is used as the basis for determining the treatment time with the PRS 414. If, the output of the PRS 414 drops, in which case, the delivered dose will be less than the prescribed dose, changes may be made. To correct for this type of problem, should it occur, two scintillation detectors 440 are placed in the stereotactic frame close to the patient&#39;s head on either side of the x-ray device. These detectors 440 monitor the radiation during the treatment in the following way. At the beginning of the treatment the count rate from these devices is determined over a minute. Based on the prescribed treatment time, it is then possible to predict the final count at the end of the treatment. If the device should fail completely, it is possible to determine how much dose has been given, and if the dose rate should drop during the treatment, the necessary time to complete the prescribed treatment is determined. 
     FIGS. 16 and 17 show an alternate beam steering assembly which includes a feedback loop system to accurately position the electron beam on the x-ray target 126B. In the illustrative embodiment, the deflection assembly 30 is substantially similar to that shown in FIG. 8, (except that it is a magnetic deflection system located outside of the probe) and an x-ray detector 142 is arranged to monitor x-rays emitted from the x-ray target 126B. The x-ray detector 142 can be positioned off axis with the electron beam, as shown, or placed on axis behind the cathode 22. A photon counter 142A is also provided, to allow monitoring of the radiation by scintillation counting. 
     Changes in the trajectory of the electron beam can be measured when there are concomitant changes in the x-ray emission from the target 126B. A deflection controller 144, which is preferably driven by a microprocessor, can utilize the data from the x-ray detector 142 and, by controlling the voltages applied to the deflectors 32 of the deflection assembly 30, can appropriately position the electron beam. 
     For instance, the feedback loop system can be used to center the electron beam on a small x-ray target 126B. However, while a change in the monitor signal does indicate that the center of the beam has moved from the center of the target, there is no immediate information as to which direction the movement has taken place. Hence it may be necessary to periodically deflect the beam in a known direction and observe the behavior of the monitor signal in order to recenter the beam. 
     The monitor signal required to keep the beam positioned on the x-ray target 126B can be obtained-by placing an x-ray detector 142 and in some cases a photon counter 142A behind the electron optics 138 to monitor x-ray photons which are emitted back along or slightly offset from the axis 16 of the probe 14. In FIGS. 16 and 17, the monitored x-rays 140 are shown to pass to one side of the electron optics 138. However, if the cathode is thin enough to be transparent to the x-ray photons, it is possible to design the system such that the x-ray photons 140 pass through the electron optics 138 and the cathode 22. The detector 142 and photon counter 142A can be place either within or outside of the principal housing 12 as shown in FIGS. 16 and 17, respectively. In the embodiment of FIG. 17, the principal housing 12 may be positioned within a secondary housing (not shown) so that the photon counter 142A is in the region between the primary housing 12 and the secondary housing. 
     As illustrated in FIG. 17, if the detector 142 and photon counter 142A arc located outside of the housing 12, an x-ray transmissive window 148 is preferably located in the wall of the housing to provide optical coupling of the detector 142, photon counter 142A, and x-ray target 126B. Thus with the configuration of FIGS. 16 and 17, detector 142 and counter 142A may provide a measure of radiation generated at the target element by detecting generated x-ray photons propagating along or slightly off-set from axis 16. In the preferred form, detector 142A is a scintillation detector which generates an x-ray photon signal in response to each x-ray photon incident thereon. Other x-ray photon detectors may be used in other embodiments. 
     With the scintillation detector embodiment, an x-ray photon generates a burst of optical photon. When the number of optical photons in a burst exceeds a preset level (chosen to minimize noise), on x-ray photon signal. Counter 142A provides a signal representative of the cumulative number of x-ray photon signals thereto. When that signal corresponds to a predetermined value, treatment can be terminated, where that value represents a predetermined x-ray photon count corresponding to a desired dose. 
     After the beam has been accurately centered on the x-ray target 126B, the feedback system of FIGS. 16 and 17 can be used to optimize the electron-beam focus for maximum output of x-rays. For instance, this can be accomplished by maximizing the signal monitored by the feedback system by using the deflection controller 144 to adjust the voltages on the focus elements (such as focusing electrode 23) of the electron optics 138. 
     The beam positioning feedback system illustrated in FIGS. 16 and 17 can also be used with the target assembly 26 shown in FIGS. 1 or 8. By way of example, the beam positioning feedback systems can be used to position The electron beam so as to be incident upon a particular point of an emission element having regions of different emission characteristics (such as the bullseye spatial pattern described above). Additionally, the feedback system can be employed to control the acceleration voltage of the electron optics. 
     Additional details relating to X-ray target 126B are set forth ha commonly assigned co-pending application U.S. Ser. No. 184,021, which is incorporated herein by reference. 
     Throughout the treatment, the count rate from both scintillation detectors may be monitored graphically on a laptop personal computer (PC) 430. This device 430 provides the first warning of any change in the operating conditions of-the probe. FIG. 15 shows a trace of the scintillation monitor 440 count rate as displayed on the PC 430. These curves show a nearly constant output rate. The slight decay has been well characterized and is substantially constant over repeated treatments and is accounted for in the treatment time calculation. The first dose pulse shown at `200` is the period when the count rate is first sampled. This time is included in the overall treatment time. 
     In the system of FIG. 14, variations in the x-ray absorbance alone the x-ray propagation path between the target assembly and the scintillation detector, i.e., through the normal soft tissue, the tumor, and the skull, are a source of uncertainty in the calculation of the radiation dose delivered to the tumor. In the systems of FIGS. 16 and 17, these sources are avoided since the propagation path of the x-rays does not pass through tissue, but rather passes through the probe along a path in which the x-ray absorbance may be readily determined and does not vary from patient to patient or from probe location to probe location within a patient. The configurations of FIGS. 16 and 17 thus allow a more accurate calculation of the radiation dose delivered to the tumor than is possible using the system of FIG. 14. Moreover, the detector is preferably rigidly affixed to the housing, thereby eliminating the need for recalibration of the probe after insertion into a patient. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.