Microdosimetric measurements as a tool to assess potential in- and out-of-field toxicity regions in proton therapy.

In this study, we exploited microdosimetry for characterizing radiation quality in proton therapy in-field, and off-beam at 20 (beam-edge), 50 (close-out-of-field) and 100 (far out-of-field) mm from beam center. We measured lineal energy y spectra in a water phantom irradiated with 152 MeV protons, from which beam quality as well as the physical dose could be obtained. Taking advantage of the Linear Quadratic (LQ) and a modified version of the Microdosimetric Kinetic (MKM) models, the microdosimetric data were combined with radiobiological parameters (α and β) of Human Salivary Gland (HSG) tumor cells for assessing cell survival RBE and RBE-weighted dose. The results indicate that if a dose of 60 Gy is delivered to the peak, the beam-edge receives up to 6 Gy while the close and far out-of-field regions on the order of 10<-3> Gy and 10<-4> Gy, respectively. The RBE estimate in-beam shows large variations, ranging from 1.0±0.2 at the entrance channel to 2.51±0.15 at the tail. The beam-edge follows a similar trend but the RBE calculated at the Bragg peak depth is 2.27±0.17, i.e. 2 times higher than in-beam (1.05±0.15). Out-of-field, the estimated RBE is always significantly higher than 1.1 and increases with increasing lateral distance, reaching the overall highest value of 3.4±0.3 at a depth of 206 mm and later distance of 10 mm. The combination of RBE and dose into the biological dose points at the beam-edge and at the end-of-range in-beam as the areas at th...
Source: Physics in Medicine and Biology - Category: Physics Authors: Tags: Phys Med Biol Source Type: research