COLLIMATOR EXCHANGE EFFECT OF MEGAVOLTAGE PHOTON BEAMS AND ITS IMPACT ON CLINICAL DOSIMETRY

Abstract

AIM To determine the Collimator Exchange Effect (CEE) for telecobalt unit (Bhabhatron -II TAW) and Linear accelerator unit (Varian Trilogy). MATERIALS AND METHOD The study was carried out in Bhabhatron-II TAW Telecobalt machine and Varian Trilogy Linear Accelerator. The study was done to find the collimator scatter factor (S¬c) for rectangular fields at 5 and 10gm/cm2 depths using indigenously designed mini phantom. Three sets of electrometer reading were noted for the irradiation of each field size and the average was taken for calculation. Sc values for different rectangular fields were then calculated from these data sets and the CEE at two depths (5 and 10 gm/cm2) for the 6MV and 15MV photon and Co-60 gamma beam were calculated. RESULTS The values of Sc obtained for the rectangular fields as alternatively defined by X & Y jaws are different for high energy photon beams indicating CEE. The maximum percentage difference between the Sc of the corresponding collimator settings for the Bhabhatron-II TAW unit for depths 5 and 10gm/cm2 were found to be 0.42% and 0.5% respectively. Sc values for 6MV and 15MV photon beams were found to be higher when Y-jaw (upper jaw) acts as the longer side of the rectangular field. The maximum percentage difference between the Sc values of the corresponding collimator settings for 6MV at depths 5 and 10gm/cm2 were 2.74% and 2.87% respectively whereas for 15 MV the differences were 3% and 2.99%. CONCLUSION The CEE of Cobalt Teletherapy units can be ignored in clinical dosimetry. However, the CEE of Linear Accelerators having energies 6MV & 15MV should be taken into consideration. A two dimensional table of Sc should be generated for rectangular fields during MU calculations in Linear Accelerators. For TPS based calculations, the generated data should be incorporated during beam modeling for accurate dose delivery in clinical dosimetry.