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A Portal Imaging System for Intensity Modulated Neutron Therapy Quality Assurance

A Lehnert*, M Kranz , D DeWitt , W Hunter , R Emery , D Argento , R Stewart , R Miyaoka , University of Washington, Seattle, WA


(Thursday, 7/18/2019) 10:00 AM - 12:00 PM

Room: 301

Purpose: We have developed a quality assurance (QA) system for intensity modulated neutron therapy (IMNT) to support patient-specific comparisons of measured and expected 2D fluence maps. Because no commercially available system exists to measure high-resolution (< 2-4 mm) fluence maps for high-energy neutrons, we have developed an in-house portal imaging system based on parallel-plate positron emission tomography (PET).

Methods: In this system, a piece of plastic is irradiated (plate mounted on treatment head at reduced source to surface distance) to create patient-specific spatial patterns of induced C-11 and O-15 activity through (n, 2n) reactions. The induced-activity map is then read using a parallel-plate PET imaging system utilizing 32 repurposed PET detectors (Siemens ECAT EXACT HR+). Data acquisition is done with in-house electronics and custom software. Images are formed using focal plane tomography. System housing, as well as a fillable phantom for detector normalization, have been designed and constructed. We have completed tests of a small-scale prototype and are now constructing a full-size clinical system with a 10 cm x 10 cm field of view.

Results: All detector modules have been characterized (511-keV photon flood), and all crystals are discernable in the image maps. Early prototypes of the system demonstrated linear response with dose and no saturation for a clinically relevant dose range (few mGy up to multiple Gy). Partial system (8 module) coincidence measurements showed an average spatial resolution of 3.25 mm FWHM, improving to 2.5 mm after application of appropriate filters. Application of an analytical normalization further improved edge crystal spatial resolution by 5%.

Conclusion: Performance in partial-system testing is sufficient for verification of beam intensity and shape in future IMNT applications. Future testing will include full-scale imaging of patient-specific IMNT fields and comparisons to Monte Carlo simulated activity maps.


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