A novel technique for direct verification of Ir‐192 source position in a multi‐channel vaginal cylinder using a mobile CBCT

Background For gynecological brachytherapy, multichannel vaginal cylinders (MCVC) offer improved control of the dose distribution as compared to single channel cylinders due to their peripheral channels. Evaluation of the source position in each channel is a crucial step when commissioning these applicators that can help to reduce systematic uncertainties, but is challenging with standard 2D techniques. Purpose In this study, we developed a novel technique that utilizes cone‐beam computed tomography (CBCT) to directly verify the position of the Ir‐192 source inside MCVC peripheral channels. Methods A mobile CBCT device was used to acquire 3D images of the source dwelling in the most distal position of the central and each peripheral channel of a MCVC using the same scan protocol and field of view. The source remained in the distal position inside each catheter while acquiring CBCT. All 3D images were combined to create a maximum intensity projection (MIP) image, showing the source in each catheter in a single image. The MIP image was imported into the treatment planning system and aligned with the applicator model. The position of the Ir‐192 source center was measured and directly compared with the digitized source position. Two plans were developed to determine the ideal catheter indexer length for the peripheral catheters. Results The Ir‐192 source position can be directly measured on the CBCT images. The ideal indexer length was determined, which showed an average deviation between the manually identified and digitized source of 0.7 ± 0.2 mm in the peripheral catheters. In addition, a 0.9 mm shift in the anterior‐posterior direction was observed in the central tandem due to gravity. Conclusion The proposed novel technique using CBCT allows for direct measurement of the source position in the MCVC applicator, which improves the source digitization process and reduces uncertainties in treatment planning and delivery.