Feasibility of 4D HDR brachytherapy source tracking using C-arm tomography: Monte Carlo investigation

High dose rate (HDR) brachytherapy is a standard radiation therapy for skin, vaginal, cervical, uterine and prostate cancers. Currently, there are few practical approaches to tracking HDR source position in vivo due to dose, shielding, accessibility and workflow challenges, hence internal dosimetry typically cannot be directly verified. Accurate source tracking capabilities are desired to reduce uncertainty in the delivered versus planned treatment dose, which may limit underdose and overdose of the prescribed treatment volume and healthy tissues and improve clinical outcomes. This work investigates the feasibility of using an isocentric C-arm x-ray imaging system to achieve 4D HDR brachytherapy source tracking in vivo using cone-beam tomography and highly constrained reconstruction methods. Monte Carlo simulations using the MC-GPU platform and XCAT anthropomorphic phantom were performed to simulate the imaging workflow for such a system, and investigate the impact of detector, image acquisition and reconstruction parameters on its achievable spatiotemporal resolution. Tradeoffs in the system's spatial, temporal, and dose characteristics are determined. The results indicate that 4D resolutions on the order of 1 mm and 1-2 second may be achieved, which is acceptable for all current HDR brachytherapy applications.