Author: Zachary R. Grelewicz, Kevin C. Jones, Yixiang Liao, Andrew Ogilvy, Julius V. Turian 👨🔬
Affiliation: Rush University Medical Center 🌍
Purpose: To demonstrate the accuracy and QA application of a time-resolved 3D gamma camera, we measured the path traversed by the Ir-192 High-Dose-Rate (HDR) brachytherapy source passing through a ring applicator.
Methods: A brachytherapy ring applicator was mounted to a multi-modal registration jig. A 10-dwell HDR plan was delivered to the applicator, and a gamma camera (22-pinhole collimator mounted to an amorphous-Si x-ray detector) triangulated the 3D, time-resolved (30 Hz) position of the HDR source. Through the registration jig, the gamma camera data was registered to (i) simultaneously exposed radiochromic film, (ii) a kV image of the applicator, and (iii) a CT image of the applicator. By comparing the different modalities, the accuracy of the gamma camera was demonstrated, and the ring applicator was characterized.
Results: Visual inspection of the gamma source positions overlaid on the ring applicator CT and kV images show agreement, with gamma-determined source positions lying within <1 mm of the ring channel of the applicator. The ring diameter measured with a fiducial-embedded dummy wire (kV image), the gamma camera, and the radiochromic film is 32.0, 30.2, and 29.4 mm, respectively. The maximum difference between radiochromic- and gamma-measured dwell position is 1.4 mm. Although the afterloader was programmed to deliver dwells separated by 10 mm, the dwell separation measured by film and gamma camera was as low as 7.5 and 5.5 mm, respectively. From 4 repeated HDR deliveries, the largest difference in gamma-measured dwell position was <1 mm.
Conclusion: As demonstrated through measurements of a brachytherapy ring applicator, the time-resolved 3D gamma camera can track the HDR source with <1.5 mm accuracy. Given this accuracy and the difficulty inherent to analyzing exposed film, the gamma camera may be a preferred tool for the commissioning of brachytherapy applicators and a potential tool for in vivo source measurements.