Session Item

The Zeiss INTRABEAM system is an electronic brachytherapy (eBT) device designed for intraoperative radiotherapy (IORT) applications. The source can be combined with spherical applicators to treat the resection cavity in different anatomical regions such as breast and brain. To date, the INTRABEAM treatment planning relies on calibration depth dose data in water provided by the manufacturer. This approach fails to provide 3D dose distributions required for adjuvant treatment plans with IORT. As an alternative, the AAPM TG-43 parameters were recently provided for the INTRABEAM bare probe by Ayala Alvarez et al (2020 Phys. Med. Biol. 65 245041). In the present work, the TG-43 parameters of the source with the spherical applicators are determined. With these data, it is now possible to calculate the 3D dose distribution from INTRABEAM and provide a more accurate treatment plan for the patients.

The dose distribution in water around the INTRABEAM source was determined with Monte Carlo (MC) calculations using egs_brachy, a user code of EGSnrc. Following a modified version of the TG-43 formalism (DeWerd 2015 Brachyther. 14 405-8), the dose to water at a reference point in water (1 cm from the applicator surface) is determined from the air-kerma rate at 50 cm from the source tip by means of a dose-rate conversion coefficient. The air-kerma rate was calculated with MC for each applicator and the dose-rate conversion coefficient, radial dose function and 2D anisotropy function were determined. The MC model of the source with applicators was validated by comparing calculated depth dose profiles with the calibration data provided by the manufacturer.

The total uncertainties on the calculated depth dose curves in water along the beam central axis were all below 2.4%. The MC calculations agreed with calibration depth dose curves for all applicators within uncertainties. The calculated air-kerma rates were within the range (0.952-1.608)x10-5 Gy/s, leading to dose-rate conversion coefficients in the range 182.6-683.5 for all applicators. The simulated radial dose functions (Figure 1) show the beam hardening effect caused by an internal aluminum filter present in the smallest applicators. The 2D polar anisotropy at 1 cm from each applicator’s surface is presented in Figure 2. A larger anisotropy is observed near the source axis indicating a strong contribution from primary and back-scattered components of the beam compared to the transverse dose.

This work presents the MC calculated TG-43 parameters for the INTRABEAM with spherical applicators, which constitute the necessary data required by conventional brachytherapy TPS to accurately and rapidly generate 3D dose to water distributions in clinical IORT applications. This paves the way towards standardization of the dosimetry of eBT sources and is the first step towards the inclusion of IORT dose distribution in adjuvant treatment plans with complementary external beam radiotherapy techniques.