Purpose: In clinical proton beam radiation therapy, an RBE of 1.1 relative to megavoltage X-rays is currently being employed at most treatment centers. This RBE pertains to radiation in the spread out Bragg-peak (SOBP) for all tissue systems, all dose levels per fraction and all proton beam energies. As the number of centers and treatment sites for which proton beam therapy continues to increase and additional experimental data is accrued, a re-assessment of the justification for a generic RBE is warranted. In this paper we address: (1) the constancy of the RBE along the central axis from the plateau entrance to the distal SOBP (upstream of the distal edge); (2) RBE as a function of dose (or cell survival level); and (3) the target cell or tissue (alpha/beta) dependency of the RBE. This analysis pertains to modulated proton beams of initial energies of approximately 70-200 MeV and SOBPs of approximately 2-10 cm, respectively.
Results and conclusions: With exceptions, the available experimental data indicate that the RBE of SOBP protons increases with decreasing dose or dose per fraction and increasing depth in the SOBP, with the magnitude of both effects likely being dependent on the alpha/beta ratios of the target cells or tissues. The use of a generic RBE of 1. for all tissues, especially those exhibiting low alpha/beta values such as CNS, may be too low, especially at dose levels of < or = 2 Gy/fraction. Systematic determination of the RBE values dependent upon the three interdependent variables identified in this manuscript (beam depth, dose size and target tissue) will provide an enhanced data base for detailed treatment planning and institutional trial comparisons, thereby maximizing the therapeutic benefit of proton beams.