Proton range verification detectors

The PRECISE proton therapy research group at the University of Manchester and the Christie NHS Foundation Trust is developing a system to verify range during proton beam therapy treatments. Range uncertainty is arguably one of the biggest challenges in proton therapy. Range uncertainty arises from a number of sources: imaging, dosimetry, stopping powers, however, the largest uncertainty is always the patient. Patient setup, highly heterogeneous tissue, implants, or bone/tissue interfaces as well as anatomical changes during treatment can all influence proton range and thus, treatment outcomes. The full potential of proton beam therapy, particularly when there are organs-at-risk in the vicinity of the tumour, cannot be exploited unless these uncertainties are reduced or mitigated.

One possible method of determining proton range is through the detection of the prompt gamma-rays that are emitted naturally during therapy. It has been shown experimentally that the maximum intensity of these prompt gamma rays correlates well with the Bragg peak and end-of-range. By detecting these prompt gamma-rays and determining their origin the proton beam range could be established.

The system being developed is based on an array of scintillator detectors coupled with an image reconstruction algorithm based on gamma-ray coincidences. The detectors of choice are LaBr3 scintillators which exhibit good energy and timing resolution for the detection of the high energy gamma-rays emitted. The typical gamma-ray energy range of interest is 2 – 8 MeV so large crystal, 38.1 mm (1.5”) diameter and 50.8 mm (2”) long, detectors are required in order to obtain full energy photo peaks. As the reconstruction algorithm utilises gamma-ray coincidences, the detectors need to have an energy resolution of 3.5% or less at 662 keV and a coincidence resolving time of 0.5 ns or less. Ideally we are also looking for the detectors to have an anode pulse rise time of 0.8 ns or less and an electron transit time of 16 ns or less.