A novel approach for Venezia ovoid commissioning with a comprehensive analysis of source positions in high-dose-rate brachytherapy

Purpose: The lunar design of a Venezia ovoid makes commissioning of the applicator very challenging with traditional autoradiography. In this study, we propose a novel solution to ovoid commissioning and a quality assurance (QA) workflow to effectively assess the entire source path. Methods and materials: A two-step commissioning process, using electron radiation and radiochromic films, was developed to verify the most distal source position. The ovoid was first attached to a film and was irradiated with a 12 MeV linac beam. This process was repeated on a separate, unexposed film, followed by irradiating it with a HDR source at the most distal position. Two lengths, including the ovoid thickness and the distance between the irradiated spot and the ovoid's outer surface, were obtained from the films’ intensity maps. The offset value was calculated from the subtraction of the two measured lengths. Besides acquiring the offset, a source positional simulator (SPS) and a series of planar x-rays from two orthogonal orientations were used to characterize source movement within the ovoid. Results: Compared to x-ray-based autoradiography, the electron exposure significantly improved the ovoid's visibility on film. Our approach did not use surrogate, which further improved measurement outcomes by decreasing inherent uncertainties. The SPS results suggested the source movement was complex within the cervicovaginal area, but it was predictable with the proposed QA workflow. Conclusion: We introduced a novel, surrogate-free method to commission the Venezia ovoid, which facilitated a manual applicator reconstruction. Additionally, we recommended QA multiple source positions to safely use the ovoid in clinical settings.