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

Abstract

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.

On the Role of Single Particle Irradiation and Fast Timing for Efficient Online-Control in Particle Therapy

“…This is the strategy used for the PGPI technique and in the development of PGT at Oncoray. However, in the latter case, this bunch length is the main limitation of the accuracy of PGT [20,21] at clinical beam intensities. The reduction of the ToF resolution down to 100 ps rms would translate to a PG vertex position determination of 1-2 cm rms resolution (observation at 90 • ), proportional to the proton velocity: indeed, 1 cm rms holds for β = v/c = 0.3, i.e., 3 cm from the end of the proton range.…”

“…They estimated the achievable probability with 95% confidence level to detect a 3mm thickness variation of an air cavity in a PMMA phantom with 10 8 incident protons and a single detector having a detection efficiency of 1.5 × 10 −3 . In order to illustrate the asset of such 100 ps time-resolution in single proton counting mode, Figure 1 compares the results derived from PGT with 162 MeV proton beams at clinical intensities (in bunch-counting mode with >10 2 protons/bunch, where the ToF is measured between a detected PG relative to the accelerator HF signal) from Werner et al [21] and those obtained in Marcatili et al [24] with 68 MeV protons in single incident particle regime (i.e., the ToF is measured relative to the arrival of the single proton that induced the PG). In the first case, a 2-cm air cavity is inserted in a PMMA phantom at 9 cm depth.…”

“…In addition, the authors of ref. [21] mentioned that the large statistical fluctuations observed were caused by the limited statistics available for a single beam spot. Single counting mode makes it possible to improve the statistics by reducing dead-time and improving the detection solid angle with closer detectors.…”

Compact Method for Proton Range Verification Based on Coaxial Prompt Gamma-Ray Monitoring: A Theoretical Study

“…The choice of LaBr 3 as scintillation material is motivated by its excellent energy resolution and short decay time of 16 ns. The first trait is convenient for the compensation of gain fluctations in the Photomultiplier Tube (PMT) as a function of count rate [75]. The second is extremely important in order to minimize the pile-up probability.…”

Positron emission tomography for quality assurance in proton therapy

“…The latests results from the prompt gamma ray timing experiments (Werner et al, 2019) show that the device has been tested at short irradiation times of 70 ms and clinical beam currents of 2 nA. However, the method is sensitive to the unstable relation between the phase of the accelerator RF and the delivery of the proton bunches.…”