Introduction: The aim of the present study was to examine the RBE for early damage in an in vivo mouse model, and the effect of the increased linear energy transfer (LET) towards the distal edge of the spread-out Bragg peak (SOBP). Method: The lower part of the right hind limb of CDF1 mice was irradiated with single fractions of either 6 MV photons, 240 kV photons or scanning beam protons and graded doses were applied. For the proton irradiation, the leg was either placed in the middle of a 30-mm SOBP, or to assess the effect in different positions, irradiated in 4 mm intervals from the middle of the SOBP to behind the distal dose fall-off. Irradiations were performed with the same dose plan at all positions, corresponding to a dose of 31.25 Gy in the middle of the SOBP. Endpoint of the study was early skin damage of the foot, assessed by a mouse foot skin scoring system. Results: The MDD₅₀ values with 95% confidence intervals were 36.1 (34.2–38.1) Gy for protons in the middle of the SOBP for score 3.5. For 6 MV photons, it was 35.9 (34.5–37.5) Gy and 32.6 (30.7–34.7) Gy for 240 kV photons for score 3.5. The corresponding RBE was 1.00 (0.94–1.05), relative to 6 MV photons and 0.9 (0.85–0.97) relative to 240 kV photons. In the mice group positioned at the SOBP distal dose fall-off, 25% of the mice developed early skin damage compared with 0–8% in other groups. LET_d,z = 1 was 8.4 keV/μm at the distal dose fall-off and the physical dose delivered was 7% lower than in the central SOBP position, where LET_{d,z =1} was 3.3 keV/μm. Conclusions: Although there is a need to expand the current study to be able to calculate an exact enhancement ratio, an enhanced biological effect in vivo for early skin damage in the distal edge was demonstrated.

Significant progress has been made in the technological and physical aspects of dose delivery and distribution in proton therapy. However, mode of cell killing induced by protons is less understood in comparison with X-rays. The purpose of this study is to see if there is any difference in the mode of cell-killing, induced by protons and X-rays in an ex vivo human peripheral blood lymphocyte (HPBL) model. HPBL were irradiated with 60 MeV proton beam or 250-kVp X-rays in the dose range of 0.3-4.0 Gy. Frequency of apoptotic and necrotic cells was determined by the Fluorescein (FITC)-Annexin V labelling procedure, 1 and 4 h after irradiation. Chip-based DNA Ladder Assay was used to confirm radiation-induced apoptosis and necrosis. Chip-based DNA Ladder Assay was used to confirm radiation-induced apoptosis. Ex vivo irradiation of HPBL with proton beams of 60 MeV or 250 kVp X-rays resulted in apoptotic as well as necrotic modes of cell-killing, which were evident at both 1 and 4 h after irradiation in the whole dose and time range. Generally, our results indicated that protons cause relatively higher yields of cell death that appears to be necrosis compared to X-rays. The analysis also demonstrates that radiation type and dose play a critical role in mode of cell-killing. Obtained results suggest that X-rays and protons induce cell-killing by different modes. Such differences in cell-killing modes may have implications on the potential of a given therapeutic modality to cause immune modulation via programmed cell death (X-rays) or necrotic cell death (proton therapy). These studies point towards exploring for gene expression biomarkers related necrosis or apoptosis to predict immune response after proton therapy.