Improved sub-milimeter range-verification method for proton therapy using a composite hadron tumour marker (HTM)

Objective. The results of a follow-up experiment investigating a novel method for sub-milimetre range verification (RV) in proton therapy (PT) are presented. Approach. The method consists of implanting a hadron tumour marker (HTM) near the planned treatment volume, and measuring the γ -ray signals emitted as a result of activation by the proton beam. These signals are highly correlated with the energy of the beam impinging on the HTM and can provide an absolute measurement of the range of the beam relative to the position of the HTM, which is independent of any uncertainties in beam delivery. Main results. Three candidate HTM materials were identified and combined into a single composite HTM, which makes use of the strongest reaction in each material. The setup of the previous experiment was improved on by using high-purity germanium detectors to measure the γ -ray signal with a higher resolution than was previously achieved. A PMMA phantom was also used to simulate the γ -ray background from tissue activation. HTM RV using the data collected in this study yielded range measurements whose average deviation from the expected value was 0.13(22)mm. Significance. Range uncertainty in PT limits the prescribed treatment plan for cancer patients with large safety margins and constrains the direction of the proton beam in relation to any organ at risk. The sub-milimetre range uncertainty achieved in this study using HTM RV, if implemented clinically, would allow for a reduction in the size of safety margins, increasing the therapeutic window for PT.