Report of the AAPM TG-256 on the relative biological effectiveness of proton beams in radiation therapy

AAPM task group report on the relative biological effectiveness (RBE) of proton beams. It defines RBE as the ratio of reference-radiation and proton doses producing the same biological effect, notes the ICRU recommendation that RBE-weighted dose be the product of absorbed dose and RBE (unit Gy(RBE)), and summarizes that RBE increases with LET and therefore with depth across a spread-out Bragg peak (on average about 1.1 at entrance, 1.15 at the center, 1.3 at the distal edge and 1.7 in the distal fall-off, for 2 Gy fractions and in vitro cell survival). It concludes that it is premature to adopt a variable RBE model clinically and that a constant RBE of 1.1 should in general be maintained, and lists planning methods commonly used to limit end-of-range RBE and range effects, such as beam angle selection to avoid ranging directly into an adjacent critical structure. Read in Europe PMC (PMC9559855).

The AAPM position on proton RBE: how RBE varies with LET, dose and tissue, why a constant 1.1 is still recommended clinically, and how planners handle end-of-range uncertainty.