Feasibility study for dose calculation with a radiation treatment planning system using a fixed-size electron cone applicator for small electron fields

Objective: This study aims to evaluate the feasibility of radiation treatment planning using a commercial treatment planning system (TPS) for small fixed-size electron cone electron applicators not natively supported by the TPS. Methods: Dosimetric characteristics, including beam profiles and output factors (OFs), were compared between a 6 MeV electron beam collimated by a small fixed-size electron cone applicator and a cerrobend cutout-based general applicator. Measurements were performed using a micro-diamond detector in a water phantom for field sizes of 2, 3, 4, and 5 cm. The monitor units (MUs) from the TPS were compared with direct measurements. To estimate the MU for the fixed-size electron cone applicator using the TPS, the relative OFs were defined as the ratio of the OFs for the fixed-size electron cone and general applicators. Dose distributions obtained from the TPS were validated against measurements using Gafchromic films, ensuring accuracy. Results: Gamma analysis showed a passing rate >95% with 1%/1 mm criteria for depth dose comparisons and >99% with 2%/2 mm criteria for beam profiles. The general applicator’s OFs were consistently higher across all measured field sizes. The MU difference between the TPS and measurements was within 2.0%, while the difference between indirect TPS calculations and direct measurements for the fixed-size electron cone applicator remained within 1.0%. Dose distribution analysis showed >99% agreement (3%/3 mm) between the 2D dose distribution obtained using film in the fixed-size electron cone applicator and that calculated by the TPS of cerrobend cutout-based applicator. Conclusion: The results demonstrate the feasibility of calculating monitor units and dose distributions for small fixed-size electron cone applicators using a commercial TPS combined with relative output factors. This approach offers a reliable method for dose calculation in specialized electron therapy applications.