# Use LET, RBE and alpha/beta to decide when protons beat IMRT

Source: https://onco.cc/ideas/idea-bio2-let-rbe-ab-selects-protons/  
OnCo record `idea-bio2-let-rbe-ab-selects-protons` (Idea). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Protons cost more than shaped X-ray beams. They are worth it when the extra punch where the beam stops, and the missing exit dose, widen the gap between killing the tumour and harming late-reacting tissue: an alpha/beta and RBE question, not a machine question.

## Summary

Adult proton versus photon randomised trials have mixed toxicity results and scarce survival differences. Plans are still reported at a constant RBE of 1.1. The biological reason protons could win is site-specific: a low-alpha/beta tumour next to a low-alpha/beta late organ, plus high linear energy transfer at a distal edge that can be kept inside the target and off the organ. That predicted window should be written down before the trial reads out. The coverage-with-evidence idea asks payers to fund the trials; this idea asks the trials to test the radiobiology, not only the machine.

## Fields

- Kind: Idea
- Last checked: 2026-09-08
- Tags: radiation-wave5
- Hypothesis: Pre-specifying tissue alpha/beta and a variable-RBE (LET-weighted) proton plan predicts which anatomies will show less late toxicity at equal tumour control in proton-versus-IMRT randomised trials. A constant 1.1 RBE plan will not.
- Rationale: Constant RBE 1.1 is a reporting convention. Variable RBE rises at the distal edge and is larger for low-alpha/beta late tissues. If that physics does not show up in PARTIQoL, RADCOMP and the NRG oesophageal and lung comparisons, the adult proton case rests only on integral-dose and second-cancer arguments, not on a wider therapeutic window for the index tumour.
- Proposed test: On PARTIQoL (prostate), RADCOMP (breast), and the NRG/Lin oesophageal proton-versus-photon trials, lock an alpha/beta pair and an LET-weighted RBE model before unblinding the late-toxicity analysis, and ask whether the predicted window matches the result. A miss is as informative as a hit.
- Maturity: preclinical-evidence
- Actor: research

## Sources

- Paganetti, RBE values for proton beam therapy (IJROBP 2014): https://doi.org/10.1016/j.ijrobp.2014.07.001
- PARTIQoL (NCT01617161): https://clinicaltrials.gov/study/NCT01617161
- RADCOMP (NCT02603341): https://clinicaltrials.gov/study/NCT02603341

## Connected records

- ideas: [Pooled coverage-with-evidence for proton therapy across all centres](https://onco.cc/ideas/idea-fund-proton-coverage-with-evidence/)
- pairings: [Caution: proton therapy vs IMRT](https://onco.cc/pairings/proton-vs-imrt/)
- cancers: [HR-positive / HER2-negative breast cancer](https://onco.cc/cancers/breast-hr-positive/), [Non-small-cell lung cancer](https://onco.cc/cancers/nsclc/), [Oesophageal cancer](https://onco.cc/cancers/esophageal/), [Prostate cancer](https://onco.cc/cancers/prostate/)
- 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/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/), [Relative biological effectiveness (RBE)](https://onco.cc/terms/relative-biological-effectiveness/)
- trials: [PARTIQoL](https://onco.cc/trials/partiqol/), [RADCOMP](https://onco.cc/trials/radcomp/)
- bottlenecks: [Surgery and radiotherapy cure most, get least](https://onco.cc/bottlenecks/b-surgery-radiation-innovation/)
- 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/)

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