Poster ‐ 23: Dosimetric Characterization and Transferability of an Accessory Mounted Mini‐Beam Collimator

Abstract

Objective:The dosimetric characterization of an accessory‐mounted mini‐beam collimator across three beam matched linear accelerators.Materials and Methods:Percent depth dose and profiles were measured for the open and mini‐beam collimated fields. The average beam quality and peak‐to‐valley dose ratio (PVDR), the ratio of average peak dose to average valley dose, were obtained from these measurements. The open field relative output and the mini‐beam collimator factor, the ratio of the mini‐beam dose to open field dose at the beam center, were measured for square fields of side 2, 3, 4, and 5 cm. Mini‐beam output as a function of collimator inclination angle relative to the central axis was also investigated.Results and Discussion:Beam quality for both the open and mini‐beam collimated fields agreed across all linacs to within ±1.0%. The PVDR was found to vary by up to ±6.6% from the mean. For the 2, 3, and 4 cm fields the average open field relative output with respect to the 5 cm field was 0.874±0.4%, 0.921±0.3%, and 0.962±0.1%. The average collimator factors were 0.450±3.9%, 0.443±3.9%, 0.438±3.9%, and 0.434±3.9%. A decrease in collimator factor greater than 7% was found for an inclination angle change of 0.09°.Conclusion:The mini‐beam collimator has revealed a difference between the three linacs not apparent in the open field data, yet transferability can still be attained through thorough dosimetric characterization.