Radiotherapy apparatus
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to apparatus for applying intracavitary radiotherapy. 
     2. Description of Related Art 
     Apparatus for this purpose is disclosed in U.S. Pat. No 4,233,517 issued Nov. 11, 1980 to Van&#39;t Hooft, which in turn refers to a French Pat. No. 1,593,557. In the type of apparatus described in these patents, radioactive material is conveyed to a desired location within the patient by transporting a chain of balls pneumatically to and from an appropriately inserted applicator, the ball being made up of radioactive balls appropriately spaced by non-radioactive metallic balls. The ball chain is assembled and stored prior to use and sorted after use in a control unit within which the balls are transported pneumatically and sorting is carried out magnetically. This entails that the spacer balls are of magnetic material, in practice steel. The radioactive balls consist of a radioactive filling within a comparatively thin stainless steel shell, and the spacer balls usually substantially outnumber the radioactive balls. 
     The balls are subjected to a great deal of manipulation both in the control unit and in transit to and from the patient, for example, the ball chain must be returned to the control unit every time a nurse or visitor wishes to enter the room in which the patient is being treated, and every time it is desired to alter the configuration of the ball chain. The massive nature of the spacer balls results in considerable battering of the radioactive balls during such manipulation, with the attendant risk of rupture or leakage, and this severely limits the amount of manipulation which can be tolerated since a ruptured or leaky ball is likely to distribute radioactive material throughout the system entailing a time-consuming, hazardous and very expensive clean-up and rebuilding operation. This problem restricts the possibilities for changing the ball chain configuration during the course of a treatment, and also severely restricts access to the patient by other personnel. Even with appropriate precautions, the risk of ball leakage or rupture still exists. A further disadvantage of the steel spacer balls is that they provide partial shielding of the radioactive balls, requiring compensation when calculating radiation dosage, and that they are radiographically opaque which limits the information which can be obtained from check-film radiography. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a system in which the wear and tear on the radioactive balls can be greatly reduced, and which can allow greater flexibility in operation. 
     According to the invention, a ball chain for use in radiotherapy treatment comprises a plurality of radioactive balls separated by non-radioactive spacer balls of radiographically transparent material of lower density and surface hardness than the radioactive balls. 
     The invention also extends to radiotherapy treatment apparatus comprising a storage, sorting and assembly system for radioactive balls and non-radioactive spacer balls of radiographically transparent material of lower density and surface hardness than the radioactive balls, the system including a sorter comprising a radiation detector, means to transport the balls past the radiation detector, and means receiving the radiation detector output and operative to divert the balls into different ball storage compartments depending upon whether or not the radioactive detector output exceeds a predetermined threshold, an assembler comprising means for selectively and successively releasing balls from said ball storage compartments into a ball chain store to assemble a ball chain, means for pneumatically conveying a ball chain between the ball chain store and a remote applicator tube for location within the body of a patient, and means for pneumatically conveying a ball chain from the ball chain store to the sorter. 
     Preferably the ball chain includes an end ball of larger diameter and having a physical property detectably different from that of the remaining balls. Typically this ball may be of ferromagnetic material, and it is restricted to movement along a path between the ball row store and the applicator. By use of techniques for detecting such material, correct location of the ball chain can readily be verified. Damage to radioactive balls by the ferromagnetic ball can readily be avoided since spacer balls will be interposed between the end ball and any of the radioactive balls. 
     A further feature of the invention provides improved means for transporting the ball chain between the ball chain store and the applicator tube and verifying location of the chain. In the prior art, the pneumatic pressure utilized to transport the ball chain to the applicator tube is maintained during treatment, and the pressure in a tube through which the chain is transported is sensed to determine whether it is sufficient to maintain the chain in the applicator tube. This arrangement does not necessarily provide positive verification of proper positioning of the chain, since it is possible for balls to lodge short of their proper positions without always causing the pressure sensed to drop to a level that will indicate a malfunction. In order to overcome this problem, the applicator tube may include means to sense the presence of the end ball into a predetermined location, for example by sensing the inductance of a coil wound around the tube at that location, and the pneumatic transportation means includes means for supplying air to the tube through two alternative ducts, one of which includes a choke, and a valve seat engageable by said end ball at the predetermined location within the applicator tube, such that the air supplied through the choked duct is insufficient to maintain pressure within the system unless the end ball is seated on the valve seat. The end ball may be retained in the chain store when returning the remaining balls to the sorter by arranging that the choked duct enters the ball store upstream of the normal position therein of the end ball, and that the other duct enters the chain store downstream of the end ball but upstream of the remaining balls. Said other duct is used only during transportation of the ball chain, whilst the choked duct is used whenever the balls are to be moved to or maintained in the applicator duct. 
     Further features of the invention will become apparent from the following description of a preferred embodiment with reference to the accompanying diagrammatic drawings. 
    
    
     SHORT DESCRIPTIONS OF THE DRAWINGS 
     FIG. 1 is a diagrammatic sectional view of radiotherapy treatment apparatus embodying the invention; 
     FIG. 2 is a diagrammatic sectional detail showing an applicator tube incorporated in the apparatus, showing a ball chain therein; and 
     FIG. 3 is a diagrammatic sectional detail showing a ball chain storage tube incorporated in the apparatus. 
     Referring to FIG. 1, the apparatus broadly comprises a sorter 2, a ball store 4, a ball selector 6, a channel selector 8, a function selector 10, ball chain stores 12 and applicator tubes 14. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An applicator tube 14 is shown in more detail in FIG. 2. This tube has a blind ended outer tube 15 intended for insertion in a body cavity of a patient and is connected by a suitable quick-connect coupling 17 to a carrier tube 16 which comprises two concentric flexible nylon tubes 18 and 20 which connect the applicator tube to the remainder of the apparatus, which can be located in a different room from the appropriately shielded treatment room containing the patient. The inner tube 18 is connected within the applicator tube to a source chain tube 22 of somewhat smaller diameter, the far end of which is bridged by a nylon stopping pin 24 which prevents the passage of a chain of radioactive balls 26 and spacer balls 28, but permits the passage of air. Typically the balls 26 and 28 are 25 mm in diameter, whilst the tube 22 is 2.65 mm in internal diameter, which allows movement of the balls in a chain into the tube except for an end ball 30 of 3 mm diameter. The internal diameter of the tube 18 is typically 3.2 mm, permitting passage of all of the balls. 
     To initiate treatment, pneumatic pressure is applied to the tube 18 to move a chain of balls 26, 28 and 30 into the applicator tube 14 so that the balls 26 and 28 fill the tube 22 and the ball 30 seats against a shoulder 19 between the tubes 18 and 22, this both holding the balls 26 and 28 in position and cutting off air flow through the tubes 18 and 22. The ball 30 is of ferromagnetic material, typically steel, so that its location may be verified by measuring the inductance of a coil 33 wound around the end of the tube 18. Connecting leads to the coil may pass along the space between the tubes 18 and 20. 
     Typically, there will be 48 balls 26 and 28 in the chain, the arrangement of the two types of ball depending upon the required pattern and magnitude of the radiation dosage to be administered to a patient. Usually each radioactive ball 26 will be flanked on each side by one or more inert spacer balls. The radioactive balls are of known construction, comprising a relatively thin stainless steel shell with a filling containing an appropriate radiation source. Typical balls are 2.5 mm diameter spherical Cs-137 sources such as type CDC from Amersham Corporation. 
     The balls 28 should be non-radioactive, should be of material that remains stable under the influence of radioactivity, should be of low density, should be softer than the balls 26, should retain their shape and be wear resistant and should be substantially radiographically transparent. Whilst a number of synthetic plastics materials have the potential to meet these requirements, suitable balls made from nylon are commercially available, and this material meets all of the above criteria as well as having an advantageous self-lubricating property. The low density of the balls 28 minimizes impact on the balls 26 during transit or when the ball chain is arrested, for example by the pin 24, whilst their relative softness minimizes wear on the balls 26. Their transparency to radiation avoids any masking of the radiation from the balls 26 and also means that they do not show up in radiographic images of the treatment area, thus simplifying interpretation. The lightness of the balls 28 substantially reduces the total weight of the chain, thus providing greater assurance of effective transport of the chain at a given air pressure. 
     In order to return the chain to the remainder of the apparatus, air pressure is applied to the tube 20, which is normally connected to the atmosphere via a suitable filtering system (not shown) to trap and detect any radioactive particles, and the tube 18 is vented via the filtering system. This expels the balls from the tube 22 and back down the inher tube 18 of the carrier tube 16. 
     A typical apparatus will have provision for loading several applicator tubes 14. Their carrier tubes 16 end at a coupling 32 at which air is exhausted from or admitted to the tubes 20, whilst the tubes 18 continue to a group of orifices in a valve plate 34 of the function selector 10. Groups of orifices also connect the valve plate to the channel selector 8, and to tubes 36 extending to the sorter 2. The function selector 10 also includes a valve member 38 having a group of spigots 39 movable into alignment with any of the groups of orifices in the plate 34, and means 40 to lift the spigots into coupling engagement with the orifices. The spigots are connected by tubes 42 to the ball chain stores 12. 
     Referring now also to FIG. 3, in which details of a chain store 12 are shown on a larger scale, each chain store comprises a storage tube 41, typically of 3.2 mm diameter, terminating at its lower end in a top connection of a T-connector 44, the side and bottom connections 46, 48 of which are selectively connected by valves (not shown) to a source of compressed air, typically at 1 atmosphere, or to exhaust via the filter already mentioned. The bottom connection 48 has a valve seat 50 in which the ball 30 is receivable, when the ball chain is in the store, so that air from the side connection 46 is applied between the ball 30 and the remaining balls in the chain. In order to transfer the chain from the store 12 to the associated applicator tube 14, the valve member 38 is moved and lifted to couple the tubes 42 to the tubes 18 and air pressure is applied first to the connection 48 and then the connection 46. Pressure is applied to the connection 48 through a flow restrictor 52. The air applied through the connection 48 will lift the ball 30 out of the valve seat 50, whereafter the air applied through the connection 46 will convey the entire chain through the tubes 42 and 18 to the applicator tube 14. Once arrival of the ball 30 has been detected by the coil 33, the valve controlling the connection 46 is closed. If there is any leakage of air through the system resulting from improper positioning of the chain, then the resulting pressure drop will be detectable because of the choke provided by the restrictor 52. 
     During return of the chain to the chain store 12, the connections 46 and 48 are vented through the filter. The choking of the connection 48 may provide some cushioning of the chain as the ball 30 enters the seat 50. An infrared source 54 and detector 56 disposed on opposite sides of the tube detect the passage of successive balls, generating pulses which enable the balls to be counted so as to verify the return of the entire chain. The source and detector are so disposed that when the entire chain is present the topmost ball will block passage of light from the emitter to the detector, thus providing further and continuing verification that a complete chain is present in the chain store. 
     In order to return ball chains from a chain store 12 to the sorter 2, the valve member 38 of the function selector 10 is aligned and raised so that the tubes 42 are connected to the tubes 36, and air pressure is applied to the connections 46 of the appropriate chain store 50 so as to convey the balls 26 and 28, but not the ball 30, to the sorter. The applied pressure will hold the ball 30 on the seat 50 so that it remains in the store 12. An apertured slide 58 in the sorter 2 allows balls to drop one by one onto a shutter 60 which supports them in front of a radiation detector 62. According to whether the output of the detector is above or below a predetermined threshold, a diverter member 64 is moved so that when the shutter 60 is then opened, the supported ball falls into a radioactive ball store formed by a tube 66 or a spacer ball store formed by a tube 68. Movement of the ball into the appropriate tube is verified by photoelectric detectors 70. By comparing the output of detector 62 with additional thresholds and providing additional tubes 66, it is possible to sort radioactive balls of different activity, thus improving the flexibility of the system in providing desired radiation dosages. 
     In order to assemble a ball chain in a ball chain store 12, the valve member 38 of the function selector is aligned and raised so that the tubes 42 are connected to the orifices in the plate 34 communicating with the channel selector 8. A diverter member 72 in the channel selector 8 is aligned so that balls entering the channel selector are directed into the appropriate tube 42 and hence the selected chain store 12. Balls are admitted to the channel selector 8 from the ball selector 6, which comprises apertured slides 74 which can be selectively actuated to release balls one at a time from any one of the ball stores 66, 68 onto a shutter 76 which holds the balls temporarily whilst a radiation detector verifies whether or not they are radioactive, thus providing a double check on the sorting procedure. Assuming satisfactory verification, the balls are then released by the shutter in the channel selector. 
     Preferably the mechanical movements, various slides and shutters and the function selector are operated pneumatically by electrically operated valves and the various valves controlling transport of the balls are also operated electrically. These valves are sequenced in a preferred arrangement by electrical control signals generated by a computer under program control, the computer also monitoring the operation of the system and being programmed to initiate appropriate shut-down and alarm signals in the event of malfunction. In FIG. 1, the mechanical parts are shown in a rest condition, and points at which mechanical motion may be selectively applied are shown by half arrows. Points at which pneumatic pressure may be selectively applied are shown by full arrows. 
     It is found that the use of low density spacer balls of relatively soft material enormously reduces wear and tear on the radioactive balls, which means that restrictions on manipulation of the balls are largely eliminated. This in turn means that restrictions on access to the patient are largely eliminated since the ball chain may be withdrawn without risk of excess wear and tear whenever someone wishes to enter the treatment room, and this withdrawal may be achieved automatically under control of a treatment room door interlock. Moreover, the ball chain may be withdrawn and re-assembled at intervals during treatment so as to provide more uniform or more precisely tailored therapy without significantly increasing the risk of damage to the balls.