Construction and First Tests of an in-beam PET Demonstrator Dedicated to the Ballistic Control of Hadrontherapy Treatments With 65 MeV Protons

The construction and first proton beam tests of a demonstrator dedicated to the beam ballistic control in hadrontherapy cancer treatments are described. This cost-effective demonstrator, called large area pixelized detector, is a PET-like detector used for in-beam ballistic control. It was built to test the feasibility of monitoring in real time, during irradiation, the ion range in the patient through the measurement of the beam-induced β⁺ activity distribution. Achieving this goal necessitates to overcome several challenges. One of them is the rejection of the beam-induced background. Another one is the definition of fast event selection and reconstruction techniques so that real time monitoring is possible. Strategies employed to tackle these problems are presented and tested with the 65 MeV Medicyc proton beam of the cancer treatment center in Nice, France. In particular, an original fast reconstruction technique is presented. First performances obtained during irradiation of polymethyl methacrylate targets are described.

I. Introduction

Hadrontherapy is a cancer treatment technique based on the use of light ion beams under constant expansion and development worldwide [1]. Light ions lose most of their energy at the end of their path accross distances of the order of a few millimeters typically, in the so-called Bragg peak. This is illustrated in the case of protons with an energy of 62 MeV in Fig. 1. This property allows, after proper treatment planning, to increase the ratio between the dose deposited in the tumor and the dose deposited in the surrounding healthy tissues with respect to conventional radiotherapy. Hadrontherapy treatments are however, currently limited by the fact that the range of the ions in patients is not precisely known. Its uncertainty is approximately ±3% of the range [2]–[4]. For example, the uncertainty for protons of 200 MeV (corresponding to a range of approximately 22 cm in patients) is about 6.8 mm. Fig. 1.

Energy deposited (dashed-dotted line) and annihilation points (histograms) as a function of the penetration depth for 62 MeV protons on a PMMA target [5]. The points show experimental data from [6].