Breast intraoperative electron radiotherapy: Image-based setup verification and in-vivo dosimetry

Introduction: Single fraction nature of intraoperative radiotherapy highly demands a quality assurance procedure to qualify both beam setup and treatment delivery. The aim of this study is to evaluate the treatment setup during breast intraoperative electron radiotherapy (IOERT) and in-vivo dose delivery verification. Materials and methods: Twenty-five breast cancer patients were enrolled and setup verification for each case was performed using C-arm imaging. The received dose by surface and distal end of target was measured by EBT2 film. The significance level of difference between obtained dosimetry results and predicted ones was evaluated by the T statistical test. Results: Acquired C-arm images in two different oblique views revealed any misalignment between the applicator and shielding disk. The mean difference between the measured surface dose and expected one was 1.8% +/- 1.2 (p = 0.983) while a great disagreement, 11.1% +/- 1.5 (p < 0.001), was observed between the measured distal end dose and expected one. This discrepancy is mainly correlated to the backscattering effect from the shielding disk. Target depth nonuniformities can also contribute to this remarkable difference. Conclusion: Employing the intraoperative imaging for IOERT setup verification can considerably improve the treatment quality. Therefore, it is suggested to implement this imaging procedure as a part of treatment quality assurance. Favorable agreement between the predicted and measured surface doses demonstrates the applicability of EBT2 film for dose delivery verification. The results of in-vivo dosimetry showed that the electron backscattering from employed shielding disk can affect the received dose by the distal end of tumor bed.