Proton therapy has the ability to deliver high dose to a tumour whilst sparing surrounding healthy tissue. This beneficial dose deposition profile is due to the interactions of protons with tissue, characterised by the Bragg peak. However, one of the major obstacles in the development and adoption of proton therapy is the uncertainty in the position of the distal dose fall-off. This range uncertainty is associated is a result of a number of factors, such as beam reproducibility, patient set-up, CT associated uncertainties, complex anatomical inhomogeneities, and variation in patient geometry over the course of treatment. Direct range verification is difficult since the proton beam stops inside the patient. This requires a method of indirect range verification, such as the detection of secondary radiation produced by the primary beam through nuclear interactions. One method to achieve in vivo range monitoring is to detect Prompt-Gamma (PG) rays emitted as a result of nuclear de-excitation from the atomic nuclei in the target region. PG emission occurs along the entire proton track. Interestingly, this emission is almost instantaneous and has a high production rate. The difficulty lies in detecting the PG rays, as they tend to be in the energy range of between 4-10 MeV. This gamma ray energy is too high to be measured by conventional diagnostic imaging detectors.
Miss Panaino project is to develop an optimised PG detector system with at least 2 mm resolution. This detector will be usable on-line during treatment to provide real time range verification. Proof of principle work is carried out in collaboration with the UK National Physical Laboratory (NPL) and the University of Birmingham. The system under evaluation (shown in figure) is based on the NANA (NAtional Nuclear Array) in NPL and is comprised of 16 symmetrically-spaced LaBr3(Ce) detectors. It does not need any collimation to maximise the PG signal. As a first step, the position reconstruction capability of the PG detector system has been examined by means of Monte Carlo Geant4 simulations.
The Prompt-Gamma (PG) detector system with an isotropic 60Co point source; geometry modelled with Monte Carlo Geant4 code.