Room: Karl Dean Ballroom B1
Purpose: To present the commissioning of a full-scale prototype prompt gamma-ray spectroscopy system for in vivo range verification of proton beams.
Methods: We have developed a full-scale prototype system for the real-time measurement of the range of proton beams delivered to the patient. The system performs spectroscopy of prompt gamma-rays that are emitted during the decay of excited nuclei in the patientâ€™s tissue. These gamma-rays are collimated and detected by an array of 8 lanthanum(III) bromide scintillation detectors. The detection system is mounted on a robotic positioner for precise alignment with the patient. In the past year, extensive commissioning measurements in phantoms were performed to exercise the end-to-end clinical workflow, and to determine range verification performance in different situations. The measurements were performed using realistic treatment fields at standard clinical dose levels and dose rates. Various types of inhomogeneities tissue-equivalent materials were irradiated. Both the range of the pencil-beams and the elemental composition of the materials were determined from the spectroscopic measurements. We also analyzed the detectability of small range deviations that were introduced by placing thin strips of plastic in the beam path.
Results: The typical mean range verification precision was 0.8 mm standard deviation for proton pencil-beams delivering a dose of 1 Gy, when considering protons delivered within a 10 mm lateral radius. The mass concentration of the main elements in the materials was determined with a standard deviation of 0.05 g/cmÂ³. Both the location and magnitude of small 2 mm range deviations could clearly be identified in the beamâ€™s eye view.
Conclusion: Prompt gamma-ray spectroscopy provides a unique insight in the interaction of clinical proton beams with tissue and enables highly precise in vivo range verification. Our full-scale prototype system is now ready for clinical studies.