Prompt gamma imaging of a proton pencil beam at clinical current intensities: First test on a prototype and development of a full-size camera

Treatments delivered by proton therapy are affected by uncertainties on the range of the beam within the patient. To reduce these margins and improve feedback on treatment delivery, different projects are investigating real-time range control by imaging prompt gammas emitted along the proton tracks in the patient. This study supports the development of a prompt gamma camera using a knife-edge slit collimator to produce a reversed 1-dimensional projection of the beam path on a scintillation detector for treatments delivered in pencil beam scanning mode. The ability of this camera design to detect modifications of the beam penetration depth in a PMMA target was already demonstrated down to 1 mm accuracy for doses compatible with single pencil beams at low proton beam currents thanks to the HiCam photo-detection system. In order to fulfill the very demanding count rate capability required for prompt gamma imaging at clinical beam currents, a new, dedicated, cost-effective photo-detection system was designed. This 1-dimensional, high-energy gamma imaging device relies on two rows of 20 LYSO crystal slabs, directly coupled to SiPMs' arrays and readout by 40 independent acquisition channels in fast counting mode. A first prototype limited to 20 channels was implemented to benchmark the performances of various components and validate the adequate combination of crystal material, surface treatment, optical coupling and SiPMs. This prototype was tested during proton irradiation at the West German Proton Therapy Centre in Essen at clinical beam currents of several nA at nozzle exit.