Proton radiography with a commercial range telescope detector using dedicated post processing methods

Proton transmission imaging uses protons with high enough energy to fully traverse the phantom/patient and to be captured in a suitable detector placed behind it. The measured residual energy or residual range provide a direct estimate of the water equivalent thickness (WET) of the image volume. Requirements for proton imaging to be exploitable in clinical practice include: sufficient WET accuracy and integrability into the treatment room and the clinical workflow, as well as an acceptably low dose to the patient and a sufficient spatial resolution. In this work, we report on experiments performed at the Institut Curie-Proton therapy center in Orsay (IC-CPO), France, using a commercial range telescope commonly employed for quality assurance measurements. The purpose was to keep the experimental set-up as simple as possible and to achieve nonetheless high WET accuracy radiographies by developing and applying dedicated post processing methods. We explain these methods in detail and discuss their performance. We assess the WET accuracy based on two different reference phantoms: a CIRS electron density phantom with tissue equivalent inserts and a homogeneous step phantom. We find an agreement between the measured and the reference WET values of 0.2-0.5 mm. The lowest investigated dose was 10 mGy per acquisition. It could be lowered by modifying the irradiation plan and lowering the beam current, though the latter would impose slight optimisations of the detector hardware. Our work suggests that proton radiographies with good WET accuracy can be obtained with a reasonable experimental effort that would facilitate integration into clinical routine.