Microdosimetric characterization of a clinical proton therapy beam: comparison between simulated lineal energy distributions in spherical water targets and experimental measurements with a silicon detector.

Objective. Treatment planning based on computer simulations wasproposed to account for the increased relative biological effectiveness (RBE) of proton radiotherapy beams near to the edges of the irradiated volume. Since silicon detectors could be used to validate the results of these simulations, it is important to explore the limitations of this comparison.Approach. Microdosimetric measurements with a MicroPlus Bridge V2 silicon detector (thickness = 10µm) were performed along the Bragg peak of a clinical proton beam. The lineal energy distributions, the dose-mean values, and the RBE calculated with a biological weighting function were compared with PHITS simulations (microdosimetric target = 1µm water sphere), and published clonogenic survivalin vitroRBE data for the V79 cell line. The effect of the silicon-to-water conversion was also investigated by comparing three different methodologies (conversion based on a single value, novel bin-to-bin conversions based on SRIM and PSTAR).Main results. Mainly due to differences in the microdosimetric targets, the experimental dose-mean lineal energy and RBE values at the distal edge were respectively up to 53% and 28% lower than the simulated ones. Furthermore, the methodology chosen for the silicon-to-water conversion was proven to affect the dose-mean lineal energy and the RBE10up to 32% and 11% respectively. The best methodology to compensate for this underestimation was the bin-to-bin silicon-to-water conversion based on PSTAR.Significance. This work represents the first comparison between PHITS-simulated lineal energy distributions in water targets and corresponding experimental spectra measured with silicon detectors. Furthermore, the effect of the silicon-to-water conversion on the RBE was explored for the first time. The proposed methodology based on the PSTAR bin-to-bin conversion appears to provide superior results with respect to commonly used single scaling factors and is recommended for future studies.

Modeling the radiation-induced cell death in a therapeutic proton beam using thermoluminescent detectors and radiation transport simulations.

Changes in the relative biological effectiveness (RBE) of the radiation-induced cell killing of human salivary glands (HSG) were assessed along the Bragg peak of a 60 MeV clinical proton beam by means of coupling biophysical models with the results of Monte Carlo radiation transport simulations and experimental measurements with luminescent detectors. The fluence- and dose-mean unrestricted proton LET were determined along the Bragg peak using a recently developed methodology based on the combination of the response of 7LiF:Mg,Ti (MTS-7) and 7LiF:Mg,Cu,P (MCP-7) thermoluminescent detectors. The experimentally assessed LET values were compared with the results of radiation transport simulations using the Monte Carlo code PHITS, showing a good agreement. The cell survival probabilities and RBE were then calculated using the linear-quadratic model with the linear term derived using a phenomenological LET-based model (Carabe A et al 2012 Phys. Med. Biol. 57 1159) in combination with the experimentally-assessed or PHITS-simulated dose mean proton LET values. To the same aim, PHITS simulated microdosimetric spectra were used as input to the modified microdosimetric kinetic model (modified MKM, (Kase et al 2006 Radiat. Res. 166 629-38)). The RBE values calculated with the three aforementioned approaches were compared and found to be in very good agreement between each other, proving that by using dedicated pairs of thermoluminescent detectors it is possible to determine ionization density quantities of therapeutic proton beams which can be applied to predict the local value of the RBE.