Cell survival and DNA damage along the distal edge of the proton Bragg peak

Proton therapy employs a spread‑out Bragg peak (SOBP) to cover the tumor. Individual protons can reach high linear energy transfer (LET) near the end of their tracks across the SOBP, while the dose‑averaged LET remains low to moderate. Mean LET or a constant relative biological effectiveness (RBE) may therefore give an oversimplified description of biological responses. This study investigated how LET along a proton Bragg peak affects clonogenic survival and DNA double‑strand breaks (DSBs).

Monolayer cultures of T98G human glioblastoma and A549 human lung carcinoma cells were irradiated with 220 kV X‑rays at five depths in a monoenergetic proton Bragg peak (LET = 5.1, 27.1, 34.4, 38.4, 41.9 keV/μm). Clonogenic survival was assessed by colony formation, and DSBs 0.5 h and 24 h post‑irradiation were quantified by γH2AX flow cytometry. Response data were analyzed by summary statistics or linear and linear-quadratic model fitting.

Low‑LET protons (≤30 keV/μm) produced survival and DSB levels comparable to X‑rays, but RBEs reached up to around 2 and were significantly greater than 1 at the highest LETs. Despite differences in intrinsic radiosensitivity, T98G and A549 showed similar RBE-LET trends. RBE at 10% survival for LET of 41.9 keV/μm was 2.64 ± 0.13 (T98G) and 2.69 ± 0.18 (A549). RBE calculated from DNA DSBs 24 h post irradiation at 4.5 Gy was around 1 for LET < 35 keV/μm but increased significantly for 38.4 and 41.9 keV/μm.

Protons at the highest LET (41.9 keV/μm) yielded high RBE values in both cell lines across survival and DSB endpoints, suggesting that high‑LET protons near the end of their range can have a disproportionate biological impact.