Processing of prompt gamma-ray timing data for proton range measurements at a clinical beam delivery

In proton therapy, patients benefit from the precise deposition of the dose in the tumor volume due to the interaction of charged particles with matter. Currently, the determination of the proton stopping point in the patient’s body during the treatment is not a clinical standard. The resulting range uncertainties cause broad safety margins around the tumor, which limit the actual potential of proton therapy.To overcome this obstacle, different methods are under investigation aiming at the verification of the proton range in real time during the irradiation.One approach is the Prompt Gamma-ray Timing (PGT) method, where the range of the primary protons is derived from the time-resolved emission profiles (PGT spectra)of promptly emitted gamma rays, which are produced along the particle track in the tissue. After verifying this novel technique in an experimental environment but far away from treatment conditions, the translation of PGT into clinical practice is intended. Therefore, new hardware was extensively tested and characterized in a close-to-clinical scenario using short irradiation times of 20 ms and clinical beam currents of 2 nA. Experiments were carried out in the treatment room of the University Proton Therapy Dresden. A pencil beam scanning plan was delivered to a target without and with embedded cylindrical air cavities of down to 5 mm thickness. The induced range shifts of the proton beam due to the material variation could be identified from the corresponding PGT spectra, comprising events collected during the delivery of a whole layer. Additionally, an assignment of the PGT data to the individual pencil beam spots allowed a spot-wise analysis of the variation of the PGT distribution mean and width indicating range shifts induced by the different air cavities. Furthermore, the paper presents a comprehensive software framework which standardizes future PGT analysis methods and calibration algorithms for technical limitations that have been encountered in the presented clinical-like experiments