Relative biological effectiveness of a high energy modulated proton beam using a spontaneous murine tumor In vivo

Abstract

The relative biological effectiveness (RBE) for proton beams produced by 160 MeV Harvard University synchrocyclotron was studied in C3Hf/Sed mouse fibrosarcoma (FSa-II). Reference radiation was Cobalt-60 γ-ray. Cell survival was determined by 2 types of lung colony assays. Type A assays involved irradiation of solid tumors followed by immediate preparation of single cell suspension and the intravenous injection of the suspension. In type B assays, intravenous injection of single cell suspension, prepared from non-treated tumors, was followed by mouse thorax irradiation. The type B assay was used to determine survival of aerobic cells while the type A assay was used to analyze the survival of acutely hypoxic cells. Tumors were irradiated with 1 to 10 doses in the center of spread-out Bragg peak (SOBP) or in an incident plateau area. Survival curves were analyzed by alpha-beta (linear quadratic) model and RBE was determined at various survival levels. The RBE in SOBP was found to be independent of fraction size although 2 % of the proton beam was reported to be a result of high linear energy transfer (LET) events of greater than 100 KeV/μ. The RBEs at survival levels of 0.5, 0.1, and 0.01 were 1.16 ± 0.12, 1.16 ± 0.06 and 1.17 ± 0.05 respectively. The RBE in the incident plateau was not significantly different from that in SOBP.