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 beam range in the patient's body during the treatment is not a clinical standard. This lack causes broad safety margins around the tumor, which limits the 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 time-resolved profiles (PGT spectra) of promptly emitted gamma rays, which are produced along the particle track in 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 using short irradiation times of 70 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 cylindrical air cavities of down to 5 mm thickness. The range shifts of the proton beam induced due to the material variation could be identified from the corresponding PGT spectra, comprising events collected during the delivery of a whole energy 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, corresponding to range shifts produced by the different air cavities. Furthermore, the paper presents a comprehensive software framework which standardizes future PGT analysis methods and correction algorithms for technical limitations that have been encountered in the presented experiments.

Range assessment in particle therapy based on prompt γ-ray timing measurements.

Proton and ion beams open up new vistas for the curative treatment of tumors, but adequate technologies for monitoring the compliance of dose delivery with treatment plans in real time are still missing. Range assessment, meaning the monitoring of therapy-particle ranges in tissue during dose delivery (treatment), is a continuous challenge considered a key for tapping the full potential of particle therapies. In this context the paper introduces an unconventional concept of range assessment by prompt-gamma timing (PGT), which is based on an elementary physical effect not considered so far: therapy particles penetrating tissue move very fast, but still need a finite transit time--about 1-2 ns in case of protons with a 5-20 cm range--from entering the patient's body until stopping in the target volume. The transit time increases with the particle range. This causes measurable effects in PGT spectra, usable for range verification. The concept was verified by proton irradiation experiments at the AGOR cyclotron, KVI-CART, University of Groningen. Based on the presented kinematical relations, we describe model calculations that very precisely reproduce the experimental results. As the clinical treatment conditions entail measurement constraints (e.g. limited treatment time), we propose a setup, based on clinical irradiation conditions, capable of determining proton range deviations within a few seconds of irradiation, thus allowing for a fast safety survey. Range variations of 2 mm are expected to be clearly detectable.