Purpose: Quality assurance (QA) validation of HDR brachytherapy dwell positions and times often relies on analysis of exposed film and subjective stopwatch measurements. A gamma camera was constructed and characterized to provide automatic, precise, and objective time-resolved image tracking of the source.
Methods: A pinhole-based gamma camera was constructed to provide time-resolved images of an Ir-192 afterloader-controlled source. The 1 mm diameter pinhole was mounted to a 700 Î¼m CsI x-ray flat panel detector. The Ir-192 gamma emissions were collected to create images during delivery with a straight and ring applicator. The source position was automatically identified through fitting to the sum of Gaussian and Lorentzian profiles.
Results: By analyzing positions identified in consecutive images, the source position can be identified with a precision (standard deviation) of < 0.07 mm (48 cm source-to-pinhole, 6 Hz) and < 0.08 mm (37 cm source-to-pinhole, 10 Hz). To assess possible in vivo application, adding 30 cm of attenuating solid water reduces source position identification precision from 0.05 mm to 0.28 mm (44 cm source-to-pinhole, 2 Hz). For the straight applicator, expected 10 mm steps were measured with a range of 9.9-10.1 mm. For the 30 mm diameter ring applicator, a radius of 14.8-15.0 mm was measured. Steps along the ring were observed to be 9.2-10.1 mm (10 mm expected). For three back-to-back deliveries with the ring applicator, the dwell position reproducibility at 9 positions was within 0.21 mm.
Conclusion: A simple pinhole-collimator based gamma camera is capable of identifying the HDR source position with a precision of < 0.08 mm. With a calibrated system, the camera will allow for accurate and auto-analyzable QA and applicator commissioning procedures.