An Imaging System to Support Fast Neutron Therapy Quality Assurance (QA) of Intensity Modulated Neutron Therapy (IMNT)

The University of Washington Medical Center has clinically implemented intensity modulated neutron therapy (IMNT) as a novel, high linear energy transfer modality for palliative and curative treatments of certain cancers. Because of the destructive nature of fast neutrons to electronics, this required development of a novel patient specific quality assurance (QA) system. Therefore, we developed an in-house 2-D positron emission tomography (PET) system that images patient-specific QA fields by measuring induced 11C positron activity in polyethylene plates. The scanner is built around two parallel imaging panels of $2\times 16$ repurposed clinical PET detector modules. Images are reconstructed using focal plane tomography in a $14\times 16$ cm2 field of view. Standard metrics (gamma analysis) are used to compare images with simulated (MCNP6) fluence maps. Studies demonstrated a linear dose-response relationship and full system [x, y] spatial resolution of [ $5.2~\pm ~0.30$ , $5.3~\pm ~0.34$ ] mm2 with 1 mm-diameter point source. Final image spatial resolution is approximately 8.5 mm FWHM due to the geometry of the polyethylene plates. Energy resolution (FWHM) in the center crystals is $28~\pm ~3$ %. Assembly, characterization, and quantitative calibration of the neutron Positron Emission Portal Imaging (nPEPI) system was completed in 2022, and more than 100 patients have since completed QA.