before the subject invention is described further , it is to be understood that the invention is not limited to the particular embodiments of the invention described below , as variations of the particular embodiments may be made and still fall within the scope of the invention . it is also to be understood that the terminology employed is for the purpose of describing particular embodiments , and is not intended to be limiting . in this specification , the singular forms “ a ,” “ an ” and “ the ” include plural reference unless the context clearly dictates otherwise . unless defined otherwise , all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs . the details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below . other features , objects , and advantages of the invention will be apparent from the description and drawings , and from the claims . the circular radiation beam asymmetry is defined based on the area ratios of integrated half - beam profiles from the central axis ( 100 %) to the 50 % fall - off point . this is illustrated in fig1 , where asymmetry is defined by equation 1 and where a 1 and a 2 are 100 %- 50 % half profile integrations . the principle of asymmetry measurement as expressed is based on the assumption that if the ion chamber &# 39 ; s 20 collecting length or volume lays across the half - beam , its readings , r 1 and r 2 , as shown in fig2 a , 2b , would represent a good approximation of the calculated area , a 1 or a 2 , as shown in fig1 , from a profile . thus the asymmetry can be rewritten based on these readings as , a typical 0 . 6 cc farmer - type ion chamber 20 has an effective collecting length of about 2 . 5 cm . for a circular radiation field with a diameter of 5 . 0 cm , by placing the tip of the ion chamber at the field central axis , the ion chamber reading would reflect the integrated exposure , or dose of the half - beam profile , and as a consequence of a 1 and a 2 in equation 1 being substituted with r 1 and r 2 , asymmetry is expressed by equation 2 , fig1 a , 2 b . the method was tested on a cyberknife ™ unit by shifting the radiation field of a 5 . 0 cm collimator , in relation to measurement setup , to create field asymmetry . fig3 shows a comparison of the field asymmetry obtained by the new method , as computed by equation 2 , and the conventional method as computed by equation 1 , with respect to different amounts of profile shift . the result shows that the new method , using the preferred embodiment , is sensitive to about a 0 . 2 mm of the profile shift . the advantage of this method is that there is no need to obtain a full profile in order to calculate the integrated half - beam profile . the results also show that up to 2 % beam asymmetry can be accurately detected by the method of the invention . when the value of beam asymmetry is over 2 %, the method of the invention overestimates the asymmetry , fig3 , which is a conservative safe guard . the method can be used to assess the whole field symmetry of a circular radiation field by rotating the ion chamber tip around the field central axis with the ion chamber tip fixed at the field central axis as shown in fig4 a . the result of this measurement can be presented by the polar profile , as shown in fig4 b , in which the magnitude of measurement from the ion chamber at corresponding angular positions is represented by the radius 2 . if the beam is isotropically symmetrical , one would obtain a perfect circular or unity polar profile 3 , as shown in fig4 b . any deviation from the unity polar profile is an indication of field asymmetry 4 , as shown in fig4 b . the polar profile as in fig4 b , can be used to efficiently adjust the beam because it represents the beam symmetry of the entire field . the polar profile can be obtained not only in an integrated mode as described above but also in a singular mode . the singular mode of a polar profile can be achieved by placing a micro detector at a certain distance from the beam central axis and rotating the detector around the beam central axis . although the field symmetry derived from the singular mode of measurement no longer falls into the same definition as the integrated method , it is obvious to one skilled in the art that both the integrated mode and the singular mode of polar profile are effective for the field symmetry assessment . the present invention is directed at monitoring the field symmetry of circular radiation beams , such as in x - ray based or charged particle based srs systems . the invention is implemented by using an integrated beam qa platform (“ qa platform ). with reference to fig5 and 6 , the qa platform is comprised of the following : a table - mount base 5 ; a rotating platform 10 with grooves 11 for assembly with a sliding chamber slab 15 , and an ion chamber 20 . these components are assembled for use with any existing equipment for srs systems . in the preferred embodiment , fig7 a and 7b , when the invention is being used with the cyberknife ™, the qa platform is mounted to a bird cage 45 , fig7 a , 7b . the bird cage 45 is a simple device designed by the manufacturer of the cyberknife ™, which is used to mount a radiation detector in a fixed position of approximately 80 cm from the radiation source , for radiation output measurement . the bird cage 45 consists of a rigid metal frame with a distal end 40 that attaches to the qa platform , as shown in fig7 a , 7b , and a proximal end that attaches to the linear accelerator . one of ordinary skill in the art to which this invention belongs is familiar with the components and function of the bird cage and / or variations thereof . the bird cage 45 requires a slight modification in order to work with the qa platform and function accurately to bring about the desired results of the invention . with the slight modification of the bird cage 45 , the rotating platform 10 is attached to the bird cage 45 at the distal end 40 using three supporting adaptors 50 , fig7 a and 7b . the design allows the centering of the rotating platform 10 to the beam central axis . once the rotating platform 10 is aligned with the beam , the sliding chamber slab 15 can then glide into the grooves 11 of the rotating platform 10 , fig8 b . there are two alignment markers 55 , 60 on the sliding chamber slab 15 , as shown in fig9 b , allowing alignment of the ion chamber 20 center and the ion chamber 20 tip to the beam central axis . in another embodiment , the qa platform is mounted on the treatment table 30 for a srs system , hereinafter referred to as the table - mount embodiment , fig8 a and 8b . for the table - mount embodiment , first , the table - mount base 5 is rigidly attached to the treatment table 30 , then the rotating platform 10 is fitted into to the table - mount base 5 , fig8 a . the assembly is completed by inserting the sliding chamber slab 15 into the grooves 11 on the rotating platform 10 , ensuring proper alignment of the components , fig8 b . the table - mount embodiment requires robot manipulation to align the central axis of the radiation beam to the qa platform . upon complete assembly of the qa platform in the preferred , bird - cage mounted embodiment , the table - mounted embodiment , or any other embodiment that is obvious to one of ordinary skill in the art to which this invention belongs , the assessment of the field asymmetry of circular beams can be undertaken . the circular radiation beam asymmetry is monitored after the assembly of the qa platform by rotating the tip of the ion chamber 20 at varying angular positions , and thereby obtaining the integrated half - beam profile , fig9 a , 9b , 10 a , 10 b , 10 c , 11 a , 11 b , and 11 c . the integrated half - beam profiles are collected in pairs , with each member of a pair being 180 degrees from the other . the asymmetry is calculated from the paired readings , r 1 and r 2 , using equation 2 , as shown in fig2 a and 2b . both the preferred , bird cage - mount embodiment and the table - mount embodiment allow free rotation of the ion chamber orientation , fig1 a , 10a , 10 b , 10 c , 11 a , 11 b , and 11 c . to perform beam field asymmetry measurements , use alignment markers 55 , 60 to position the tip of the ion chamber 20 at the center of the beam , fig9 a and 9b ; and utilizing a 5 . 0 cm collimator 25 with the srs system , acquire readings at desired angular positions as shown in fig1 a , 10b , 10 c , 11 a , 11 b , and 11 c ; finally use the equation , asymmetry = 2 ( r 1 − r 2 )/( r 1 + r 2 ) to compute asymmetry for each 180 ° position pair obtained from the readings . the contents of all patents , patent applications , published articles , books , reference manuals and abstracts cited herein are hereby incorporated by reference in their entirety to more fully describe the state of art to which the invention pertains . as various changes can be made in the above - described subject matter without departing from the scope and the spirit of the invention , it is intended that all subject matter contained in the above description , shown in the accompanying drawings , or defined in the appended claims will be interpreted as descriptive and illustrative , and not in a limiting sense . many modifications and variations of the present invention are possible in light of the above teachings . it is therefore to be understood that within the scope of the appended claims , the invention may be practiced otherwise than as specifically described . those skilled in the art will recognize , or be able to ascertain using no more than routine experimentation , many equivalents to the specific embodiments of the invention described herein . such equivalents are intended to be encompassed by the claims . reference is made to the following journal articles and other references ( e . g . textbooks ) that contain definitions and methods and means for carrying out basic techniques , encompassed by the present invention . 1 . comprehensive qa for radiation oncology , aapm tg no . 40 , medical physics , 21 ( 4 ), 1994 2 . stereotactic radiosurgery , aapm tg no . 42 , american association of physicists in medicine , june 1995
a method for routine monitoring and quality assurance of field asymmetry of high energy circular radiation beam producing equipment . the quality assurance process of field symmetry for devices such as stereotactic radiosurgery systems is simplified by directly measuring the integration of the half - beam profile . the method of the invention provides that the field symmetry is obtained by positioning the tip of an ion chamber , with a collecting length approximately half the diameter of the beam , at the central axis of the beam , and rotating the ion chamber at varying angular positions , acquiring and comparing readings at desired angular positions . each pair of readings from positions 180 degrees opposed from each other , are plugged into the equation , asymmetry = 2 / to compute asymmetry .