A full-scale clinical prototype for proton range verification using prompt gamma-ray spectroscopy

Abstract

We present a full-scale clinical prototype system for in vivo range verification of proton pencil-beams using the prompt gamma-ray spectroscopy method. The detection system consists of eight LaBr scintillators and a tungsten collimator, mounted on a rotating frame. Custom electronics and calibration algorithms have been developed for the measurement of energy- and time-resolved gamma-ray spectra during proton irradiation at a clinical dose rate. Using experimentally determined nuclear reaction cross sections and a GPU-accelerated Monte Carlo simulation, a detailed model of the expected gamma-ray emissions is created for each individual pencil-beam. The absolute range of the proton pencil-beams is determined by minimizing the discrepancy between the measurement and this model, leaving the absolute range of the beam and the elemental concentrations of the irradiated matter as free parameters. The system was characterized in a clinical-like situation by irradiating different phantoms with a scanning pencil-beam. A dose of 0.9 Gy was delivered to a [Formula: see text] cm target with a beam current of 2 nA incident on the phantom. Different range shifters and materials were used to test the robustness of the verification method and to calculate the accuracy of the detected range. The absolute proton range was determined for each spot of the distal energy layer with a mean statistical precision of 1.1 mm at a 95% confidence level and a mean systematic deviation of 0.5 mm, when aggregating pencil-beam spots within a cylindrical region of 10 mm radius and 10 mm depth. Small range errors that we introduced were successfully detected and even large differences in the elemental composition do not affect the range verification accuracy. These results show that our system is suitable for range verification during patient treatments in our upcoming clinical study.

Results from the experimental evaluation of CeBr scintillators for He prompt gamma spectroscopy

“…We observed eight additional discrete lines below this energy, three of which were related to reactions on the O target. In particular, the prompt gamma emission lines following reactions on O nuclei were: The possibility to detect the reactions in (3) has the benefit of increasing by three the number of prompt gamma lines to be used for PGS, additionally to the seven in use by the MGH system 20 . This increases the number of independent variables by over +40% promising an improvement in the accuracy in the detection of the ion range for future clinical applications.…”

“…The possibility to detect the reactions in (3) has the benefit of increasing by three the number of prompt gamma lines to be used for PGS, additionally to the seven in use by the MGH system. 20 This increases the number of independent variables by over +40% promising an improvement in the accuracy in the detection of the ion range for future clinical applications. However, the correlation of the cross sections for the reactions in (3) with the residual beam energy, that is, the Bragg peak position, has to be verified.…”

“…The second prototype is a promptgamma spectroscope developed at the MGH † that measures the absolute deviations with respect to the plan and it is foreseen for application in a upcoming clinical study. 20 It exploits another fundamental property of the prompt gamma radiation, that is, their discrete spectral energy lines associated with specific nuclear transitions. One can correlate the intensity of the several spectral lines with the absolute range provided an accurate knowledge of the energy-dependent cross sections.…”

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

“…It is common practice in Pencil Beam Scanning (PBS) to aggregate data from neighboring spots [20], [21], [42] in order to achieve around N p ∼ 10 8 protons per cluster. Since the lateral spread of the pencil beam close to the proton range is often larger than the lateral separation between pencil beams, an improvement in statistical precision can be achieved without significant loss in lateral resolution.…”

“…The second technique, proposed in 2003 [17], has shown promising advances in recent years [18]. First clinical tests were done with a knife-edge slit PGI camera [19], [20], and a clinical study with a Prompt Gamma-ray Spectroscopy (PGS) camera [21] is foreseen. Both camera prototypes rely on a passive collimation of prompt γ-rays (with energies up to ∼6 MeV) and are able to detect the proton range in clinical conditions with a few millimeters precision.…”