SU-F-T-610: Comparison of Output Factors for Small Radiation Fields Used in SBRT Treatment




In order to fundamentally understand our previous dose verification results between measurements and calculations from treatment planning system (TPS) for SBRT plans for different sized targets, the goal of the present work was to compare output factors for small fields measured using EDR2 films with TPS and Monet Carlo (MC) simulations.


6MV beam was delivered to EDR2 films for each of the following field sizes; 1×1 cm2, 1.5×1.5 cm2, 2×2 cm2, 3×3 cm2, 4×4 cm2, 5×5 cm2 and 10×10 cm2. The films were developed in a film processer, then scanned with a Vidar VXR-16 scanner and analyzed using RIT113 version 6.1. A standard calibration curve was obtained with the 6MV beam and was used to get absolute dose for measured field sizes. Similar plans for all fields sizes mentioned above were generated using Eclipse with the Analytical Anisotropic Algorithm. Similarly, MC simulations were carried out using the MCSIM, an in-house MC code for different field sizes. Output factors normalized to 10×10 cm2 reference field were calculated for different field sizes in all the three cases and compared.


For field sizes ranging from 1×1 cm2 to 2×2 cm2, the differences in output factors between measurements (films), TPS and MC simulations were within 0.22%. For field sizes ranging from 3×3cm2 to 5×5cm2, differences in output factors were within 0.10%.


No clinically significant difference was obtained in output factors for different field sizes acquired from films, TPS and MC simulations. Our results showed that the output factors are predicted accurately from TPS when compared to the actual measurements and superior dose calculation Monte Carlo method. This study would help us in understanding our previously obtained dose verification results for small fields used in the SBRT treatment.