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).
LET & RBE
Linear energy transfer describes how densely a particle deposits energy along its track; relative biological effectiveness compares the dose of one radiation with the dose of a reference radiation that has the same effect. Together they explain why protons are planned in Gy(RBE).
What is it?
Linear energy transfer (LET) is a physical quantity. AAPM TG-256 defines it as the average energy locally transferred to the material by a charged particle per unit length of its track, usually given in keV/µm. It describes the quality of the radiation: low-LET radiation such as megavoltage photons deposits its energy sparsely, high-LET radiation densely. Proton RBE literature such as TG-256 uses the dose-averaged LET (LETd).
Relative biological effectiveness (RBE) is a biological ratio. It compares the absorbed dose of a reference radiation (such as cobalt-60 gamma rays or megavoltage x-rays) with the dose of another radiation — protons, for example — that produces the same biological effect under otherwise the same conditions. An RBE above 1 means the test radiation needs less dose for the same effect.
The two are linked: TG-256 summarizes that proton RBE increases with increasing LET.
Why does it matter?
Megavoltage photons are themselves the usual reference radiation — clinically, Paganetti notes, the RBE of interest is relative to 6 MV photons — so photon plans need no RBE correction. For protons, RBE is built into every prescription. Proton prescriptions and organ constraints are based on physical dose multiplied by a constant RBE of 1.1, and the ICRU recommends defining this RBE-weighted dose as absorbed dose × RBE, with the unit Gy(RBE).
The constant is a simplification. RBE also varies with dose, the biological endpoint and the tissue's radiosensitivity (α/β), and with depth: LET rises as protons slow down at the end of their range. For a typical spread-out Bragg peak (SOBP) at 2 Gy per fraction, TG-256 gives average in vitro RBE values of about 1.1 at entrance, 1.15 in the center, 1.3 at the distal edge and 1.7 in the distal fall-off. Paganetti notes that the rise in LET and RBE extends the biologically effective range by 1 to 2 mm.
Even so, TG-256 concludes that it is premature to adopt a variable RBE model clinically, and that in general the constant RBE of 1.1 should be maintained in current clinical practice. The concern is concentrated at the end of range — the higher RBE at the distal end of a beam matters most where it lands in a critical normal structure, particularly one with a maximum tolerance dose such as the brainstem, spinal cord or optic pathway.
LET also changes the oxygen effect: as LET increases, the oxygen enhancement ratio decreases.
How is it evaluated?
- Check which dose you are reading. In a proton plan, check whether a value is physical dose (Gy) or RBE-weighted dose (Gy(RBE)), and which RBE the planning system applied. Prescriptions and constraints for protons are written in RBE-weighted dose.
- Look at where each beam stops. TG-256 lists methods commonly used when designing and evaluating proton plans to limit end-of-range effects, including selecting beam angles that avoid ranging directly into a critical normal structure adjacent and distal to the target, using more beam angles to spread the high-RBE end of each beam, and using lateral beam fall-off to spare nearby structures.
Constant or variable RBE, and any adjusted constraints, are institutional and protocol decisions. DoseLab does not give values for them.
Example
RBE-weighted dose [Gy(RBE)] = absorbed dose × RBE
Illustrative arithmetic only. With the clinical constant RBE of 1.1, a fraction of 1.82 Gy of physical proton dose is reported as 1.82 × 1.1 ≈ 2.0 Gy(RBE).
The schematic shows the other side of that simplification: across an SOBP the physical dose is flat, but the LET — and with it the RBE — climbs toward the distal end of the beam.
Authoritative resources
Review of proton RBE. It explains that proton prescriptions and organ constraints are based on physical dose multiplied by a generic constant RBE of 1.1, derived from early in vivo measurements; that RBE varies with LET, dose, biological endpoint and tissue radiosensitivity; and gives typical dose-averaged LET values across a spread-out Bragg peak, rising from about 0–3 keV/µm up to the center to 9–15 keV/µm in the dose fall-off, with the rise in LET and RBE extending the biologically effective range by 1 to 2 mm. Read in Europe PMC (PMC6200410).
Open-access review of tumor hypoxia in radiotherapy. It traces the history from the early 1900s to Gray's work in the 1950s, describes chronic (diffusion-limited) and acute (fluctuating blood flow) hypoxia, reviews attempts to modify hypoxia — including nitroimidazole radiosensitizers, where nimorazole in head and neck cancer is the only one incorporated into standard treatment, and only in Denmark — and notes that although OER falls as LET rises, the oxygen effect only disappears at around 500 keV/µm or greater, beyond what can be achieved clinically. Full text read in Europe PMC (PMC4990104).
TPS guidance
Step-by-step guidance planned. Added only once verified.
Step-by-step guidance planned. Added only once verified.