# Relative biological effectiveness (RBE)

Source: https://onco.cc/terms/relative-biological-effectiveness/  
OnCo record `relative-biological-effectiveness` (Term). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

How many grays of ordinary X-rays you would need to match the damage from one gray of this radiation. Protons are treated as 1.1 times as damaging as X-rays; that number is a convention, and the true value is higher where the beam stops.

## Summary

Relative biological effectiveness is the photon dose divided by the particle dose that produces the same biological effect. Clinical proton therapy reports dose as Gy(RBE) using a constant 1.1 (ICRU 78). Laboratory and modelling work show RBE rising above 1.1 at the distal edge, where linear energy transfer is highest, which can put extra biological dose in tissue just beyond the target. Carbon-ion therapy uses higher, model-dependent RBE (often around 2 to 3; local effect model or microdosimetric kinetic model). RBE depends on endpoint, dose per fraction, and the tissue's alpha/beta ratio: late-responding tissues with low alpha/beta are more sensitive to high-LET increments. Constant 1.1 is a reporting rule, not a measurement.

## Fields

- Kind: Term
- Last checked: 2026-09-17
- Also known as: RBE; relative biological effectiveness; RBE 1.1; variable RBE; Gy(RBE)
- Tags: radiation-wave5

## Sources

- Wikipedia: https://en.wikipedia.org/wiki/Relative_biological_effectiveness
- Wikipedia: https://en.wikipedia.org/wiki/Relative_biological_effectiveness
- Paganetti, RBE values for proton beam therapy (IJROBP 2014): https://doi.org/10.1016/j.ijrobp.2014.07.001

## Connected records

- fronts: [Radiation Therapy](https://onco.cc/fronts/radiation/)
- technologies: [Carbon-ion therapy](https://onco.cc/technologies/carbon-ion/), [IMRT / IGRT (modern external beam)](https://onco.cc/technologies/imrt-igrt/), [Pencil-beam scanning and intensity-modulated proton therapy](https://onco.cc/technologies/intensity-modulated-proton-therapy/), [Proton therapy](https://onco.cc/technologies/proton-therapy/), [The linear-quadratic model and fractionation](https://onco.cc/technologies/linear-quadratic-model/)
- terms: [Alpha vs beta emitters](https://onco.cc/terms/alpha-vs-beta/), [Alpha/beta ratio](https://onco.cc/terms/alpha-beta-ratio/), [Biologically effective dose (BED) and EQD2](https://onco.cc/terms/biologically-effective-dose/), [Bragg peak](https://onco.cc/terms/bragg-peak/), [Linear energy transfer (LET)](https://onco.cc/terms/linear-energy-transfer/), [Oxygen enhancement ratio](https://onco.cc/terms/oxygen-enhancement-ratio/)
- trials: [PARTIQoL](https://onco.cc/trials/partiqol/), [RADCOMP](https://onco.cc/trials/radcomp/)
- key papers: [Paganetti 2014: proton RBE is not a fixed 1.1](https://onco.cc/key-papers/paper-paganetti-proton-rbe-ijrobp-2014/)
- pairings: [Caution: proton therapy vs IMRT](https://onco.cc/pairings/proton-vs-imrt/)
- ideas: [Pooled coverage-with-evidence for proton therapy across all centres](https://onco.cc/ideas/idea-fund-proton-coverage-with-evidence/), [Use LET, RBE and alpha/beta to decide when protons beat IMRT](https://onco.cc/ideas/idea-bio2-let-rbe-ab-selects-protons/)
- roadmaps: [Radiotherapy roadmap: X-rays → shaped beams → fewer fractions, particles and FLASH](https://onco.cc/roadmaps/radiation-roadmap/)

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