Purpose: Uncertainties in the in vivo proton range remain one of proton therapy’s most significant challenges. A variety of projects therefore aim to reduce range uncertainties or verify the proton range in vivo. Measuring prompt gamma-rays emitted during treatment allows for real-time proton range monitoring. We evaluated the performance of our prompt gamma spectroscopy detector prototype through range verification measurements using a variety of animal and tissue-mimicking samples.
Methods: Prompt gamma measurements were conducted for porcine brain and liver samples as well as water and three tissue-mimicking samples (muscle, spongiosa, and adipose). A 5x5x5 cm^3 target at a depth of 12.5-17.5 cm was irradiated with a dose of 1 Gy(RBE). Ground truth stopping powers were measured using a multi-layer ionization chamber. Ground truth mass compositions were either known from the used components or determined using combustion analysis. Ranges and compositions determined through prompt gamma measurements were compared to the ground truth.
Results: The mean range shift over all spots was ≤1.3 mm for all samples. The mean standard deviation over all samples was 0.8 mm (range: 0.4 mm to 1.6 mm). The largest standard deviation was observed for the brain sample, likely due to inhomogeneities in the form of microbubbles of air, which were specific to said sample. The mean difference between the ground truth and prompt gamma-based elemental concentrations was 0.06 g/(cm^3) (range: 0.00 g/(cm^3) to 0.14 g/(cm^3)).
Conclusion: We verified the performance of our prompt gamma detector prototype for real-time in vivo range verification. Measured proton ranges and tissue compositions were close to the ground truth. The detector system’s suitability for measurements in a variety of different tissues has therefore been established. The system’s reliable performance in animal tissues bridges the gap between previously-reported measurements in water and anthropomorphic phantoms and clinical in vivo patient measurements.
Funding Support, Disclosures, and Conflict of Interest: This work was in part supported by: - Federal Share of program income earned by Massachusetts General Hospital on C06-CA059267, Proton Therapy Research and Treatment Center, - National Cancer Institute grant R01-CA229178, Fast Individualized Delivery Adaptation in Proton Therapy. - A Monika Kutzner Foundation scholarship - DFG GRK2274
TH- External Beam- Particle/high LET therapy: Range verification (in vivo/phantom): prompt gamma/PET