Range verification in heavy-ion therapy using a hadron tumour marker

Abstract

Objective. A new method to estimate the range of an ion beam in a patient during heavy-ion therapy was investigated, which was previously verified for application in proton therapy. Approach. The method consists of placing a hadron tumour marker (HTM) close to the tumour. As the treatment beam impinges on the HTM, the marker undergoes nuclear reactions. When the HTM material is carefully chosen, the activation results in the emission of several delayed, characteristic γ rays, whose intensities are correlated with the remaining range inside the patient. When not just one but two reaction channels are investigated, the ratio between these two γ ray emissions can be measured, and the ratio is independent of any beam delivery uncertainties. Main results. A proof-of-principle experiment with an 16O ion beam and Ag foils as HTM was successfully executed. The 107Ag(16O, x)112Sb and the 107Ag(16O, x)114Sb reaction channels were identified as suitable for the HTM technique. When only one γ-ray emission is measured, the resulting range-uncertainty estimation is at the 0.5 mm scale. When both channels are considered, a theoretical limit on the range uncertainty of a clinical fiducal marker was found to be ±290 μm. Significance. Range uncertainty of a heavy-ion beam limits the prescribed treatment plan for cancer patients, especially the direction of the ion beam in relation to any organ at risk. An easy to implement range-verification technique which can be utilized during clinical treatment would allow treatment plans to take full advantage of the sharp fall-off of the Bragg peak without the risk of depositing excessive dose into healthy tissue.