ESTRO 2021 Abstract Book

S1354

ESTRO 2021

The objective of this work was to describe a breast HDR-BT template designed to perform IVD during these treatments without altering the usual procedure. MOSFET detectors were calibrated reproducing the geometry of this template to minimize angular dependence. Temperature and distance correction models studied previously 1 were applied to improve the results. Materials and Methods A template was designed reproducing the one used in clinical practice but adding extra holes (equidistant from the original) for extra needles placement to perform IVD inserting detectors in them. The template has been manufactured with 3D printing technology using biocompatible photopolymerizable and sterilizable resin. Scan was performed to verify there were no artifacts preventing a correct reconstruction of the needles. MOSFETs TN-502RDM were used. Their tiny dimensions allow their insertion into common needles used. MOSFET calibration was made in a water tank, fixing the template inside so the needles inserted could be positioned vertically. Calibration factors (CF), defined as the ratio between calculated dose and detector response (cGy/mV), were obtained for three detectors. Each detector was irradiated with 3 Gy reproducing the geometry which will take place during clinical measurements but using a larger equilateral triangle for minimizing the dose uncertainty due to the position. CF for each detector was calculated averaging 15 total measurements reproducing the experimental setup three times. Its repeatability and reproducibility variation are also evaluated.

CFs obtained were corrected for their use in patients’ measurements using correction models resulting from a previous study.

Results Figure 1 shows the manufactured template. Holes connected with a line are for treatment needles. Scan images of the template showed no artifacts preventing the correct reconstruction of the needles. Correction factor calculated from correction models, considering temperature and distance dependence, was 1,04±0,03 (k=1). Results for CF obtained are shown in table 1.

Conclusion The designed template permits to perform IVD and does not alter significantly the usual procedure in breast cancer HDR-BT treatments. MOSFET detectors are calibrated to implement this template, which minimizes angular dependence so only temperature and distance corrections may be applied, in clinical practice. 1. Ruiz-Arrebola, R. et al. Characterization of microMOSFET detectors for in vivo dosimetry in high-dose rate brachytherapy with 192Ir. Med. Phys. 2020 .

PO-1633 EPID-based 3D in vivo dosimetry in head and neck radiotherapy