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Before a radiotherapy plan can be made, a doctor draws the tumour and the organs around it on the planning scan, and two doctors given the same scan do not draw the same outline. A 2016 review found 119 studies measuring this across every major cancer site, and only one in five checked what the differences did to the dose.
Every plan starts with outlines: the visible tumour, the region thought to hold microscopic spread, and the organs to protect. The outlines are drawn by hand or by software and approved by a clinician, and they are the one input every later step trusts. A margin added afterwards for set-up error and movement does not correct an outline that missed tumour or included healthy tissue: the same outline is used at every session, so its error is repeated each time.
Vinod and colleagues reviewed studies published from 2000 to 2014 that compared outlines drawn by more than two observers. They found 119, covering all major tumour sites, most often of the clinical and gross target volumes, with a median of seven participants. The studies used such varied measures that their results could not be pooled; 68 per cent reported a statistical analysis and 21 per cent reported what the variation did to the dose. The review proposed a common framework for future studies.
Approaches to reducing it include contouring guidelines, peer review of outlines before treatment, imaging the target with MRI or PET alongside CT, and AI auto-contouring, which NICE allowed into NHS use in 2023 on condition that a professional reviews every outline and evidence continues to be collected.
Showing the technology this term belongs to: AI auto-contouring and adaptive planning.
The clearest randomised evidence that putting more dose where imaging shows the tumour, rather than spreading it evenly, improves control. It depends on seeing and outlining the lesion on MRI, and on respecting the limits of the bladder and rectum first.
Variation between clinicians in drawing the target is one of the largest errors in radiotherapy and one of the least measured in terms of what it does to patients. The field has studied whether outlines differ far more than whether the differences matter.
In the recipe a systematic error, one repeated at every session, costs about three and a half times as much margin as a random error of the same size, so reducing systematic error is the larger lever for shrinking the margin.
Shares GTV, CTV and PTV (target volumes), IMRT / IGRT (modern external beam) and the tags precision, radiation.
Shares GTV, CTV and PTV (target volumes), IMRT / IGRT (modern external beam) and the tags precision, radiation.
Shares GTV, CTV and PTV (target volumes), Radiotherapy roadmap: X-rays → shaped beams → fewer fractions, particles and FLASH and the tags precision, radiation.
Shares IMRT / IGRT (modern external beam) and the tags precision, radiation.
Shares the tags precision, radiation.
Shares the tags precision, radiation.
Shares the tags precision, radiation.
Shares the tags precision, radiation.
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