ESTRO 2023 - Abstract Book

S1495

Digital Posters

ESTRO 2023

Several studies have reported the misalignment between the collimator and shielding disc in intraoperative electron radiotherapy (IOERT) as the main uncertainty during this procedure. This misalignment can lead to CTV underdosage and/or organs at risk irradiation. The goals of the present study are: to present an aligner that addresses and solves misalignment issues in IOERT and to evaluate its dosimetric impact in the dose distribution within the CTV. Materials and Methods The aligner was designed to prevent any movement between the collimator and the disc while assuring a correct irradiation of the CTV. It consists of three parts. Firstly, the cover of the metallic disc is a substitute of the polytetrafluoroethylene cover for the shielding disc provided by the manufacturer (Sordina, Italy), but it includes a hole to attach the screw. This cover avoids an overdose to the CTV due to electron backscatter in the metallic surface. Secondly, we designed screws of different lengths, for the different energies (thus prescription depths) available. The function of the screw is to rigidly secure the cover with the wheel. Finally, the wheel is fixed inside the walls of the collimator. It avoids any undesired shifts and rotations between the collimator and the disc while it also flattens the tumour bed. The design was produced using a 3D printer with a polyamide sterilizable material (printing courtesy of Avinent). We evaluated dose profiles in the CTV region with the aforementioned aligner using Monte Carlo simulation with PENELOPE. We simulated 10E7 histories without variance reduction techniques. The source was located 2 cm above the wheel with 0.01 cm height and diameter equal to the collimator. The dose profiles were obtained for the most commonly used energy and collimator (8 MeV and 5 cm respectively) at two representative depths (0.7 cm and 1.4 cm) (Figure 2). Results MonteCarlo simulations show good homogeneity in presence of the aligner and no underdosage occurs within the CTV area in comparison with the situation where no aligner is used (Figure 2). Although in Figure 2 we can see areas of underdosage in the profile, those areas are always located within the screw. Due to electron scatter, we can see some areas with higher doses in the first mm surrounding the screw. The minimum normalized dose in each depths are 97% and 87% for 0.7 cm and 1.4 cm, respectively.

Conclusion Although some underdosage is observed in the analysed dose profiles, this effect is constrained to the region inside the screw, so no impact should be expected in the CTV area. The aligner design could be a simple solution for the reported misalignment between the applicator and the shielding disc. However, further investigation is necessary before clinical implementation.