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A proton beam stops at a depth that cannot be predicted exactly, so planners deliberately aim it a few millimetres past the tumour. At one leading centre the allowance is 3.5 per cent of the beam's range plus 1 millimetre, which for a beam reaching 20 centimetres into soft tissue means overshooting by about 8 millimetres.
The main physical attraction of protons is that they stop: the dose peaks at the end of the track and falls almost to nothing beyond it. Where exactly the track ends depends on the stopping power of every tissue the beam passes through, which is estimated from the planning CT scan, and on set-up, anatomy on the day, beam delivery and the dose calculation. Each of these carries an error, and together they are called range uncertainty.
Planners protect the target by extending the beam's range by a margin. Paganetti's 2012 review gives the recipes in use at the time: 3.5 per cent of the range plus 1 millimetre at the Massachusetts General Hospital, 3.5 per cent plus 3 millimetres at MD Anderson, Loma Linda and the University of Pennsylvania, and 2.5 per cent plus 1.5 millimetres at the University of Florida. The MGH recipe goes back to estimates made with early-1980s CT. The review calls the resulting 8 millimetre overshoot for a 20 centimetre field 'quite substantial', and estimates that Monte Carlo dose calculation could reduce range uncertainty by several millimetres in complex geometry.
The margin matters because it spends part of the advantage protons are chosen for. Tissue just beyond the target receives dose the plan was built to avoid, and the biological effect at the end of the track is itself higher than the standard conversion factor assumes (see relative-biological-effectiveness).
Showing the technology this term belongs to: Proton therapy.
The physical advantage of protons did not, in this trial, translate into less lung damage or better control. It used an older proton technique without pencil-beam scanning, so it does not settle the question for current proton treatment.
A proton plan that looks safe on a 1.1 RBE map can still over-dose a late-responding organ sitting on the distal edge. Comparing protons with IMRT without an LET-weighted view asks the wrong physical question.
The millimetres added for range uncertainty are healthy tissue irradiated to protect the target, and they are a large part of why the theoretical advantage of protons over photons is smaller in practice.
Shares GTV, CTV and PTV (target volumes), Radiotherapy roadmap: X-rays → shaped beams → fewer fractions, particles and FLASH and the tags precision, radiation.
Shares GTV, CTV and PTV (target volumes) and the tags precision, radiation.
Shares GTV, CTV and PTV (target volumes) and the tags precision, radiation.
Shares GTV, CTV and PTV (target volumes) and the tags precision, radiation.
Shares GTV, CTV and PTV (target volumes) and the tags precision, radiation.
Shares the tags precision, radiation.
Shares the tags precision, radiation.
Shares the tags precision, radiation.
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