Characterization of Plastic Scintillator Detector for In Vivo Dosimetry in Gynecologic Brachytherapy.

(1) Background: High dose gradients and manual steps in brachytherapy treatment procedures can lead to dose errors which make the use of in vivo dosimetry (IVD) highly recommended for verifying brachytherapy treatments. A new procedure was presented to obtain a calibration factor which allows fast and robust calibration of plastic scintillation detector (PSD) probes for the geometry of a compact phantom using Monte Carlo simulations. Additionally, characterization of PSD energy, angular, and temperature dependences was performed. (2) Methods: PENELOPE/PenEasy code was used to obtain the calibration factor. To characterize the energy dependence of the PSD, the signal was measured at different radial and transversal distances. The sensitivity to the angular position was characterized in axial and azimuthal planes. (3) Results: The calibration factor obtained allows for an absorbed dose to water determination in full scatter conditions from ionization measured in a mini polymethylmethacrylate (PMMA) phantom. The energy dependence of the PSD along the radial distances obtained was (2.3 ± 2.1)% (k = 1). The azimuthal angular dependence measured was (2.6 ± 3.4)% (k = 1). The PSD response decreased by (0.19 ± 0.02)%/°C with increasing detector probe temperature. (4) Conclusions: The energy, angular, and temperature dependence of a PSD is compatible with IVD.

On the need of in vivo verifications as quality control for intraoperative electron radiotherapy in breast cancer.

INTRODUCTION: Intraoperative electron radiotherapy (IOERT) is a technique aiming to deliver radiotherapy during oncological surgery. In breast IOERT, the applicator and shielding disc placement are correlated with organs at risk (OAR) irradiation, in vivo verification of these parameters is scarcely reported. The aim of our study is to report and analyze possible causes of the misalignment using radiochromic films and compare our results to others reported in the bibliography. METHODS: From November 2019 to April 2023, in vivo verifications were performed for 33 patients. IOERT was performed using a LIAC 10 MeV (Sordina, Italy) electron accelerator. We attached a radiochromic film to the upper side of the polytetrafluoroethylene cover of the shielding disc. The percentage of the irradiation area outside the disc was recorded and various parameters (applicator angulations, prescription depth, tumor location and breast size) were analyzed to find possible correlations. RESULTS: For 29 patients, 20 Gy were prescribed while 10 Gy were prescribed to 4 patients. The average irradiated area outside the disc was 19% (0-56%) corresponding to a surface of 4.5 cm2 (0-17.4 cm2). The applicator of 5 cm was used for most of the patients. The mean prescription depth was 1.4 cm (0.5-2.5 cm). We found no correlation between the analyzed parameters and misalignment. CONCLUSION: This study confirms the presence and magnitude of the misalignments. We strongly recommend in vivo verifications as a quality check during IOERT procedures. The misalignment has no correlation with tumor localization parameters, so the solution could be based on technical improvements of the applicator.