Abstract
A new phantom was designed for in vitro studies on cell lines in horizontal particle beams. The phantom enables simultaneous irradiation at multiple positions along the beam path. The main purpose of this study was the detailed dosimetric characterization of the phantom which consists of various heterogeneous structures. The dosimetric measurements described here were performed under non-reference conditions. The experiment involved a CT scan of the phantom, dose calculations performed with the treatment planning system (TPS) RayStation employing both the Pencil Beam (PB) and Monte Carlo (MC) algorithms, and proton beam delivery. Two treatment plans reflecting the typical target location for head and neck cancer and prostate cancer treatment were created. Absorbed dose to water and dose homogeneity were experimentally assessed within the phantom along the Bragg curve with ionization chambers (ICs) and EBT3 films. LETd distributions were obtained from the TPS. Measured depth dose distributions were in good agreement with the Monte Carlo-based TPS data. Absorbed dose calculated with the PB algorithm was 4% higher than the absorbed dose measured with ICs at the deepest measurement point along the spread-out Bragg peak. Results of experiments using melanoma (SKMel) cell line are also presented. The study suggested a pronounced correlation between the relative biological effectiveness (RBE) and LETd, where higher LETd leads to elevated cell death and cell inactivation. Obtained RBE values ranged from 1.4 to 1.8 at the survival level of 10% (RBE10). It is concluded that dosimetric characterization of a phantom before its use for RBE experiments is essential, since a high dosimetric accuracy contributes to reliable RBE data and allows for a clearer differentiation between physical and biological uncertainties.
Highlights
Radiobiological research has become an integral part of particle beam therapy since its beginning (Wilson 1946)
Absorbed dose to water in the proximal target region determined by the PinPoint ionization chambers (ICs) was in good agreement with the treatment planning system (TPS) data for both Monte Carlo (MC) and Pencil Beam (PB) algorithms
Doses measured with the PinPoint ICs were up to 4% smaller at the last measurements point of spread-out Bragg peak (SOBP) than those calculated by the TPS
Summary
Radiobiological research has become an integral part of particle beam therapy since its beginning (Wilson 1946). The aim of this research is twofold: first to investigate the underlying mechanisms of particle beams on biological tissue and second to compare these mechanisms with those observed in photon beam therapy. Particle beams can be several times more efficient in causing the damage in a tissue than the photon beams. The elevated radiobiological effect of different particle types is quantified by the relative biological effectiveness (RBE) (Karger and Peschke 2018). The RBE represents the basis for the estimation of the biologically weighted dose in clinical particle beam therapy (IAEA and ICRU 2008; Ödén et al 2017; Jones 2017; Jones et al 2018)
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