Tissue-level RBE determination and normal lung responses along the proton SOBP

BACKGROUND: To determine proton relative biological effectiveness (RBE) across the spread-out Bragg peak (SOBP) in rat lung using a histological endpoint, with emphasis on changes associated with increasing linear energy transfer (LET). Histological alterations in alveolar epithelial cells (ATI and ATII) and inflammatory responses involving [Formula: see text] and mast cells were also evaluated. METHODS: The rat lung was irradiated with single-dose proton irradiation delivered to four different regions (dose-mean LET 1.89-3.68 keV/μm) along a 9 cm SOBP or with 6 MV photon irradiation as a reference. Lung tissues were harvested 4 weeks after irradiation and subjected to histological analysis. Lung injury was scored using the American Thoracic Society-recommended system (endpoint [Formula: see text]), from which [Formula: see text] (effective dose for 50% response) and RBE values were derived. Cellular and inflammatory responses were evaluated at dose levels of 16 and 20 Gy using immunohistochemical staining for AQP5 (ATI), SFTPC (ATII), and [Formula: see text] as an inflammatory marker, while mast cells were identified by toluidine blue staining. RESULTS: Along the SOBP, the RBE values increased from [Formula: see text] to [Formula: see text] as [Formula: see text] values decreased from [Formula: see text] Gy to [Formula: see text] Gy, respectively. All irradiated groups showed altered ATI-positive cell proportion, [Formula: see text] expression, and mast cell number compared with controls. In the low-LET regions (proximal and central SOBP), these changes were comparable to those in the photon group, whereas the distal high-LET SOBP region exhibited more pronounced alterations, including reduced ATI-positive cell proportion and increased [Formula: see text] expression and mast cell accumulation. Although ATII-positive cell proportion was reduced in all irradiated groups, no regional differences were observed along the SOBP; however, morphological alterations were evident in the distal region. CONCLUSIONS: Tissue-level RBE calculations revealed an increase toward the distal region of the proton SOBP. Proton-induced lung injury was spatially heterogeneous along the SOBP, with distal high-LET regions showing the most severe epithelial damage and inflammation. These findings suggest that the use of a constant generic RBE may underestimate biological effects, highlighting the need for LET-informed proton therapy planning.