Room: Track 2
Purpose: We present the first prompt gamma-ray spectroscopy scans obtained during patient treatment to verify the range of proton therapy beams.
Methods: The prompt gamma-ray spectroscopy system is an in-house development that consists of a collimated array of LaBr3 scintillators mounted in a 7-axis robotic system for alignment with the patient. An energy- and time-resolved prompt gamma-ray spectrum is acquired during the delivery of each proton pencil-beam. These spectra are compared with a nuclear reaction model to determine the range deviation of the pencil-beam as compared to a Monte Carlo calculation of the treatment plan. In this first clinical study, we scan patients receiving proton therapy for brain tumors. One field is measured once weekly.
Results: Scans during the treatment course of one patient have been completed. We obtained 5 weekly measurements of a superior-anterior oblique field treating a skull base meningioma. The field passed through relatively complex tissue heterogeneities. We found that the range deviations are largely consistent between all fractions. The mean range deviation was 1 mm water equivalent. At the 90% confidence level, the range uncertainty was 4 mm, which is 3% of the incident range.
Conclusion: Prompt gamma-ray spectra from proton-nuclear interactions have been successfully measured for the first time during a proton therapy treatment and used to verify the range of the delivered pencil-beams. Initial results show systematic range deviations, which suggests that a range adaptation after the first fraction could be beneficial to improve range accuracy. Future results from this clinical study will improve the understanding of proton range uncertainty and will be used to develop strategies to reduce it.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the National Cancer Institute Federal Share program income on C06-CA059267, and by the National Cancer Institute grants R01-CA229178 and U19-CA021239-35.