Microdosimetric characterization of proton relative biological effectiveness in pancreatic ductal adenocarcinoma cell lines

Purpose Pancreatic ductal adenocarcinoma (PDAC) is among the most radioresistant malignancies. Proton therapy assumes a uniform relative biological effectiveness (RBE) of 1.1 regardless of radiation quality. We characterized proton RBE as a function of dose-mean lineal energy ( ȳ D ) in PDAC cell lines. Methods Proton RBE was assessed in Panc-1 and COLO 357 FG cells using a high-throughput 96-well irradiation platform with an 81.4 MeV spot-scanning proton beam. Cells were irradiated at 12 discrete depths along the pristine Bragg curve, spanning ȳ D values of 1.9 to 20.0 keV/μm. Dose was varied by adjusting spot-scanning repaintings. Parallel 6 MV photon irradiations ( ȳ D = 0.8 keV/μm) served as reference. Cell viability was assessed by crystal violet assay and fitted with the linear-quadratic (LQ) model to extract RBE at 10% viability. Results Both cell lines demonstrated RBE near 1.0 at entrance energies ( ȳ D < 10 keV/μm). Beyond the Bragg peak, RBE rose sharply: for Panc-1, RBE reached 3.0 ± 0.3 at ȳ D = 18.7 keV/μm and 4.6 ± 0.4 at ȳ D = 20.0 keV/μm; for COLO 357 FG, RBE reached 3.6 ± 0.2 and 6.1 ± 0.5 at the same positions. For Panc-1, LQ fits were good across all conditions. In contrast, COLO 357 FG exhibited a linear dose-response (β≈0) for protons despite a clear LQ response to photons, suggesting proton-induced damage complexity exceeds the repair capacity of this cell line. Conclusions The clinical RBE = 1.1 assumption substantially underestimates proton effectiveness in the distal Bragg peak region for PDAC. This microdosimetric RBE dataset provides a quantitative basis for improving proton treatment planning in radioresistant PDAC.