SU-GG-T-100: Comparison of Measured D-Max for Irregular Electron Cutout Fields to Treatment Planning System Prediction and Nominal Open Field D-Max

Abstract

Purpose: For irregular electron cutouts, not only output factors but also depths of maximum dose (D‐max) are often measured experimentally, increasing workload of clinical physicists and slowing clinical workflow. The purpose of this study is to investigate whether the D‐max of the corresponding open field or the D‐max of the cutout derived from the treatment planning system (TPS) can be used. Method and Materials: Seven cutouts for electron beams with energies ranging from 9 to 21 MeV were planned for patients using Pinnacle 8.0m without heterogeneity correction. To determine D‐max, the depth doses (PDDs) of these cutouts were generated in the TPS with flat‐surfaced water phantom geometry. PDDs were then compared with PDDs measured in a PTW scanning water phantom and corresponding open field PDDs. Because of the broad characteristic of PDDs near D‐max of these energy electron beams, a range of depths with PDD values > 99.5% on either side of D‐max were compared. Results: Within 0.5% dose variation, the D‐max ranges obtained from the TPS overlapped with those from measurements and nominal D‐max of open fields. For five of the seven cutouts, output factors measured at depths obtained from TPS, measurement, or corresponding open field resulted in less than 1% difference. For the other two cutouts, which showed the least overlap, less than 2% difference in output factors measured at the nominal depth of open‐field or measured Dmax was observed. Conclusion: For irregular electron cutouts, it is not necessary to measure the unique D‐max for the cutout field. Cutout factors within 2% measured at either nominal D‐max of the corresponding open field or the calculated D‐max from the treatment planning system can be achieved.