Relative biological effectiveness in a proton spread-out Bragg peak formed by pencil beam scanning mode

Abstract

In recent years, there is an increased interest in using scanning modes in proton therapy, due to the more conformal dose distributions, thanks to the spot-weighted dose delivery. The dose rate in each spot is however much higher than the dose rate when using passive irradiation modes, which could affect the cell response. The purpose of this work was to investigate how the relative biological effectiveness changes along the spread-out Bragg peak created by protons delivered by the pencil beam scanning mode. Cell survival and micronuclei formation were investigated in four positions along the spread-out Bragg peak for various doses. Monte Carlo simulations were used to estimate the dose-averaged linear energy transfer values in the irradiation positions. The cell survival was found to decrease the deeper the sample was placed in the spread-out Bragg peak, which corresponds to the higher linear energy transfer values found using Monte Carlo simulations. The micronuclei frequencies indicate more complex cell injuries at that distal position compared to the proximal part of the spread-out Bragg peak. The relative biological effectiveness determined in this study varies significantly and systematically from 1.1, which is recommended value by the International Commission on Radiation Units, in all the studied positions. In the distal position of spread-out Bragg peak the relative biological effectiveness values were found to be 2.05 ± 0.44, 1.85 ± 0.42, 1.53 ± 0.38 for survival levels 90, 50 and 10%, respectively.

In Vitro Comparison of Passive and Active Clinical Proton Beams

“…The apoptosis level and the number of BNC containing MN have been found to be almost identical for the two modes. In terms of micronuclei frequencies and the 5-Gy-dose point, the frequencies are higher for the peak position in comparison to the proximal position for both modes, which is in agreement with the expected more complex DNA damage at the peak position (see [23]). The micronuclei frequencies appear to be almost the same for the 3-Gy-dose point and slightly higher for PBS for the 5-Gy-dose point, but no statistically significant differences have been found using the t-test in both cases.…”

Proton beam therapy delivered using pencil beam scanning vs. passive scattering/uniform scanning for localized prostate cancer: Comparative toxicity analysis of PCG 001-09

“…The two PBT delivery techniques are currently assumed to be radiobiologically equivalent, with a constant relative biological effectiveness (RBE) of 1.1 [11]. However, a number of preclinical studies have indicated subtle differences between linear energy transfer (LET) and, therefore, RBE, with the two proton delivery techniques, and the clinical relevance of these findings has not been explored [12], [13], [14].…”

Physical and biological impacts of collimator‐scattered protons in spot‐scanning proton therapy

“…The protons lose energy when hitting the collimator, and their linear energy transfer (LET) increases accordingly. A higher proton LET generates a greater biological effect, as observed in both in vivo and in vitro experiments and even in the clinical outcome . In the previously mentioned study, van Luijk et al estimated the increase of the biological damage based on a simple assumption that the protons with energy above 40 MeV have a relative biological effectiveness (RBE) of 1, while for those below 40 MeV have RBE of 1.2.…”