Purpose: During radiation therapy, photons are scattered out of the megavoltage beam. By collecting these scattered photons, the irradiated anatomy may be imaged in real-time without additional imaging-specific dose. The goal of this IRB-approved study is to report on a prototype scatter-imaging system for tracking lung tumor motion during stereotactic body radiation therapy (SBRT). The first scatter images collected of patients during radiation treatment are presented.
Methods: A mobile, crane-mounted scatter imaging camera was constructed with a flat panel x-ray detector and pinhole collimator. Optimal collimator thickness was characterized with phantom studies. Scatter images of 5 patients undergoing lung SBRT treatment were collected at 2 Hz during the delivery of 15 fractions. To avoid collision with the treatment machine, the scatter camera was positioned outside of the gantry rotation plane at a distance > 50 cm from the isocenter. For comparison, the expected patient-specific scatter images were simulated at each camera position and control point.
Results: Thirteen different camera positions were used to collect > 6,000 scatter images during lung SBRT treatment. The tumor and chestwall are identifiable in some scatter images, but the large object-to-imager distance (50-110 cm) results in poor spatial resolution (> 2 cm) relative to previous phantom studies, where sub-millimeter lung tumor monitoring was observed. Simulated and experimental images are in good agreement. Increasing the collimator thickness from 8 to 35 mm lead equivalent resulted in an improved phantom scatter image contrast-to-noise ratio (x3.6).
Conclusion: We present the first scatter images collected of patients during radiation therapy. The prototype scatter imaging camera allows for flexible placement, but improvements in spatial resolution are necessary to achieve scatter-image guidance during lung SBRT. We are investigating improvements to camera sensitivity (thicker scintillator), spatial resolution at large distances (parallel hole collimator), and camera placement (intrafraction movement).
Funding Support, Disclosures, and Conflict of Interest: We acknowledge funding from Swim Across America.