Author: Yibin Ling, Qihui Lyu, Ke Sheng 👨🔬
Affiliation: University of California Berkeley, Department of Radiation Oncology, University of California, San Francisco 🌍
Purpose:
High Dose Rate (HDR) brachytherapy enables radiation to concentrate on the tumor by sending the high-dose-rate radioactive source directly to the treatment site. On the other hand, the sharp dose falloff also makes the treatment vulnerable to errors and uncertainties. To address this, we developed a real-time source localization and anatomical registration method for HDR brachytherapy.
Methods:
We simulated a flat-panel detector to capture the exit gamma-ray photons emitted by the HDR 192Ir source for real-time source localization. A 50cm x 50cm flat-panel detector was positioned beneath the couch, which had multiple embedded tungsten ball-bearings (BB). The gamma-ray projection was computed using a ray-tracing forward projection method assuming 380 keV photon energy. The source location was computed by triangulating the BB locations on the projections with their known physical locations. We also simulated a dummy wire with 6 unevenly spaced tungsten BBs inserted through one of the implanted catheters, and then introduced a 10-degree rotation to mimic patient motion. The wire-BB locations were reconstructed by optimizing the rigid transformation to match wire-BB shadows on the projection images. We evaluated the source localization accuracy as a function of BB positions, sizes, BB-to-source-distance (BSD), and BB-to-detector-distance (BDD).
Results:
Without introduced motion , the proposed system achieves sub-millimeter accuracy. BSD and BDD are the most critical factors impacting the accuracy. The source deviation increases from 0.3mm to 1mm when BSD increase from 10cm to 24cm, and increases from 0.5mm to 1.2mm when the BDD decreases from 30 cm to 10cm. With introduced rigid motion, the proposed recovered the transformation using the wire BBs registration with 1.2mm accuracy.
Conclusion:
The study demonstrates the feasibility of using an external flat-panel detector, couch-embedded markers and a dummy wire for precise localization of the HDR brachytherapy sources with anatomical context.