TU‐A‐BRE‐01: The Relative Biological Effectiveness of Proton Beams Relative to Photon Beams

Abstract

Proton therapy patients receive a 10% lower physical dose than the dose administered using photons, i.e. the proton relative biological effectiveness (RBE) is 1.1 in comparison to high‐energy photons. The use of a generic, spatially invariant RBE within tumor targets and normal tissue structures disregards a large body of evidence indicating that proton RBE tends to increase with increasing linear energy transfer (LET). Because the doseaveraged proton LET in the distal edge of a spread out Bragg peak (SOBP) is larger than the LET in the plateau region or proximal edge of a SOBP, the use of a spatially invariant RBE is not well justified from a mechanistic point of view. On the other hand, the available clinical data on local tumor control rates and early or late side effects do not provide strong evidence against the continued use of a constant and spatially invariant clinical RBE. The only potential downside to the ongoing use of a constant RBE of 1.1 seems to be that we are missing a potential opportunity to enhance the therapeutic ratio, i.e., design proton therapy treatments in ways that exploit, rather than mitigate, spatial variations in proton RBE. Speakers in this symposium will:1‐review the laboratory and clinical evidence for and against the continued use of a spatially invariant RBE of 1.1,2‐examine some of the putative mechanisms connecting spatial variations in particle LET to estimates of the proton RBE at the molecular, cellular and tissue levels3‐assess the possible clinical significance of incorporating models for spatial variations in proton RBE into treatment planning systems.4‐discuss treatment planning and delivery techniques that will exploit the spatial variations of RBE within proton beams.Learning Objectives: To review laboratory and clinical evidence for and against the continued use of a constant RBE of 1.1 To understand major mechanisms connecting proton LET to RBE at the molecular, cellular and tissue levels. To quantify the potential opportunities and potential pitfalls of neglecting spatial variations in proton RBE in treatment plans To understand the minimum requirements required to design proton treatment plans that benefit from a spatially changing RBE and discuss some of the technological challenges they may require.