# Linear energy transfer (LET)

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

## TL;DR

How densely a radiation track dumps energy along its path. Sparse tracks (X-rays, most of a proton beam) are easier to repair; dense tracks (the proton's stop-point, carbon ions, alpha particles) punch clustered holes in DNA.

## Summary

Linear energy transfer is the energy a charged particle deposits per unit length of track, usually in keV per micrometre. Photons and electrons are low-LET. Protons are mostly low-LET along the entrance plateau and rise at the Bragg peak and just beyond it, which is why a constant relative biological effectiveness of 1.1 is only a reporting convention. Carbon ions and alpha particles (around 100 keV per micrometre) are high-LET: they cause clustered double-strand breaks, work in hypoxia, and are less sensitive to fraction size. LET is the physical input to relative biological effectiveness; the two are not the same number.

## Fields

- Kind: Term
- Last checked: 2026-09-17
- Also known as: LET; linear energy transfer; high-LET; low-LET; keV/µm; keV per micrometre
- Tags: radiation-wave5

## Sources

- Wikipedia: https://en.wikipedia.org/wiki/Linear_energy_transfer
- Wikipedia: https://en.wikipedia.org/wiki/Linear_energy_transfer
- 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: [Boron neutron capture therapy](https://onco.cc/technologies/bnct/), [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/), [Targeted alpha therapy](https://onco.cc/technologies/targeted-alpha-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/), [Bragg peak](https://onco.cc/terms/bragg-peak/), [Oxygen enhancement ratio](https://onco.cc/terms/oxygen-enhancement-ratio/), [Relative biological effectiveness (RBE)](https://onco.cc/terms/relative-biological-effectiveness/)
- 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/)
- key papers: [Paganetti 2014: proton RBE is not a fixed 1.1](https://onco.cc/key-papers/paper-paganetti-proton-rbe-ijrobp-2014/)
- roadmaps: [Radiotherapy roadmap: X-rays → shaped beams → fewer fractions, particles and FLASH](https://onco.cc/roadmaps/radiation-roadmap/)
- trials: [PARTIQoL](https://onco.cc/trials/partiqol/), [RADCOMP](https://onco.cc/trials/radcomp/)

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