Purpose: It is highly desirable to measure the proton beam-range (BR) with a fraction of therapeutic beams in a single fractionated treatment that will enable, if necessary, an adaptive delivery of the rest beams based on the measured range-shift to achieve the planned dose distribution. The success of such approach will be a paradigm-shift to establish a low-dose, intra-beam range measurement and range-guided adaptive beam delivery to substantially improve the therapy certainty and accuracy. This study aimed evaluating the capability and performance of a newly developed brain PET dedicated for such proposed on-line proton-induced positron activity-range (AR) measurement.
Methods: The prototype PET consists of 20 detector panels in a polygon configuration with ~32cm diameter and 6.4cm axial field-of-view, with two removable detector panels for beam passing. In the off-line study, Na-22 and F-18 sources were used to evaluate the system spatial resolution, sensitivity, positioning accuracy of reconstructed Na-22 point-source images and F-18 cylinder-source (12.4mm diameter 40.4mm length) images that mimicked the proton-induced positron activity distribution, and the impact of PET background activity, etc. In the initial on-line study, a Lucite cylinder phantom was irradiated by pencil-beam protons and imaged by the PET immediately after the radiation to evaluate the capability and accuracy of on-line AR measurement.
Results: Sensitivity and spatial resolution are ~3.0% (at field-of-view center) and ~2.8mm to ~5.5mm across the field-of-view, with <0.1mm difference between the physical and imaged source positions even without attenuation and scatter corrections. The mimicked activity distal edge falloff positions of the cylinder-source were measured from images with <0.2mm maximum difference from their physical positions. On-line study demonstrated that ARs can be measured with <1.5mm standard deviations by low-dose (~0.3Gy) and short-acquisition (60s) PET imaging.
Conclusion: The performance evaluation studies demonstrated the capable on-line PET imaging for potential proton intra-beam range verification.
Funding Support, Disclosures, and Conflict of Interest: This study was partially supported by NIH funding 1R01CA218402
TH- External Beam- Particle/high LET therapy: Range verification (in vivo/phantom): prompt gamma/PET