SU-E-T-248: Determining Output Factor of Irregular Electron Cutouts at Extended SSD
Purpose: To calculate the output factor (OPF) of any irregularly shaped electron beam at extended SSD Methods: Using Cerrobend block, circular cutouts with diameter ranging from 2.0cm to the maximum possible size for 15×15 applicator of Varian 2100C and 14×14 applicator of ELEKTA Synergy LINACs were prepared. In addition, two irregular cutouts were made. For each cutout, percentage depth dose (PDD) at standard SSD and point doses at different SSD were measured in water using Scanditronix diodes and PTW 0.125 cm3 ion-chambers, respectively. In addition, the distance at which electron fluence along the central axis becomes independent of cutout size was determined using EDR2 films. On each LINAC, electron beam energies of 6, 9, 12, and 15/20 MeV were used. Results: Using the open applicator as the reference field, the lateral buildup ratio (LBR) for each circular cutout as a function of depth was calculated. From the LBR value and radius of the cutout, the corresponding lateral spread parameter (σR(z)) was determined. Taking the cutout side dependence of σR(z)) into account, PDD of the irregular cutouts at the standard SSD were calculated. From the point dose measurement at different SSDs, the effective SSD of each cutout was calculated. Taking the calculated PDD of the irregular cutout and effective SSD (SSDeff) of the circular cutout into account, OPFs of the irregular cutout at extended SSD values were calculated. Finally, the calculated output factors were compared with the measured values. Conclusion: In this research, the improved LBR method has been generalized to calculate output factor of irregular cutouts at extended SSD. In a clinically useful region, the percentage difference between the measured and the calculated output factors of the irregular cutouts was within 2%. Similar results were obtained for all available electron energies of both Varian 2100C and ELEKTA Synergy machines.