RELATIVE BIOLOGICAL EFFECTIVENESS OF INTERMEDIATE-ENERGY PROTONS - COMPARISONS WITH CO-60 GAMMA-RADIATION USING 2 CELL-LINES

Range modulated proton beams are used for radiotherapy of malignant tumours at several accelerator laboratories with the aim of introducing proton therapy as a clinical hospital-based therapy modality. Due to the finite range and the sharpness of the dose gradients, the dose to well defined target volumes can be raised without excessive irradiation of non-target tissue. The prescribed proton doses are determined in part on the basis of the relative biological effectiveness (RBE) of the particular radiation quality. In this study, RBE values were determined for a proton beam with a maximal range of 33 mm, which corresponds to an energy of approximately 67 MeV. The range modulated depth-dose distribution, with a 20 mm extended Bragg peak, was mainly designed for high precision treatment of small targets such as uveal melanomas. The tested cell lines, LS-174T and V79-379A, were chosen because of their suitability for clonogenic assays. The cells were irradiated with single doses in the range 2-10 Gy at different depths in the extended peak region of the range modulated proton beam. RBE values were determined by comparing the doses needed to obtain the same reduction in colony formation (0.5, 0.1 and 0.01) as with the reference Co-60 gamma source. The mean RBE value was 1.22 with a standard deviation of 0.08. The variations depended on both cell type and on the survival levels considered. The clonogenic cell survival showed only a very small decrease with depth when cells were placed in different positions, along the central axis of the range modulated depth-dose distribution, indicating only a small increase in RBE with depth. The RBE of the proton beam was greater than unity indicating that proton doses should be somewhat reduced in comparison with conventional radiation when irradiation of critical normal tissues is unavoidable.