{"entity":{"id":"paper-van-herk-ptv-margin-recipe-ijrobp-2000","kind":"paper","name":"The probability of correct target dosage: dose-population histograms for deriving treatment margins in radiotherapy","aka":[],"tldr":"The paper behind the margin most radiotherapy departments add around a target. It showed that errors repeated every day matter far more than errors that vary day to day, and turned that into a formula: a margin of 2.5 times the systematic error plus 0.7 times the random error.","summary":"Van Herk and colleagues described mathematically what set-up and movement errors do to the dose a target receives across a population of patients. They separated errors made once, in preparing the treatment (systematic), from errors that vary at each session (random). Random errors blur the dose a little for many patients; the same size of systematic error shifts the whole dose distribution and leaves some patients badly underdosed.\n\nFrom this they derived a recipe. To give at least 95 per cent of the prescribed dose to the whole clinical target volume in 90 per cent of patients, the margin should be 2.5 times the standard deviation of systematic errors plus 1.64 times that of random errors combined with the beam penumbra, which for a typical penumbra simplifies to 2.5 Sigma plus 0.7 sigma. The authors call it a lower limit, because it leaves out rotations and changes of shape.","asOf":"2026-10-20","links":[{"label":"IJROBP 2000","url":"https://doi.org/10.1016/S0360-3016(00)00518-6"},{"label":"PubMed 10863086","url":"https://pubmed.ncbi.nlm.nih.gov/10863086/"}],"tags":["precision","radiation"],"related":[],"cancers":[],"sections":["radiation"],"technologies":["imrt-igrt"],"targets":[],"drugs":[],"companies":[],"institutions":[],"pathways":[],"terms":["planning-target-volume","target-delineation-variability"],"trials":[],"people":[],"bottlenecks":[],"keyPapers":[],"journals":["ijrobp"],"dependsOn":[],"notes":[],"journal":"International Journal of Radiation Oncology, Biology, Physics","year":2000,"doi":"10.1016/S0360-3016(00)00518-6","pmid":"10863086","authors":"van Herk M, Remeijer P, Rasch C, Lebesque JV.","paperType":"methods","findings":["To give at least 95 per cent of the dose to the clinical target volume in 90 per cent of patients, the margin is 2.5 times the standard deviation of systematic (preparation) errors plus 1.64 times that of random (execution) errors combined with the penumbra, minus 1.64 times the penumbra's standard deviation.","For a penumbra standard deviation of 3.2 mm the recipe simplifies to 2.5 Sigma + 0.7 sigma.","The same level of systematic error leads to a much larger underdosage for some patients than random error does."],"whatItMeans":"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.","caveats":["The recipe excludes rotational errors and shape deviations, and the authors describe it as a lower limit for safe radiotherapy.","It is a model of dose to a population, not a trial of patient outcomes."],"changedPractice":true},"route":"/key-papers/paper-van-herk-ptv-margin-recipe-ijrobp-2000/","neighbours":{"section":[{"id":"radiation","kind":"section","name":"Radiation Therapy","route":"/fronts/radiation/"}],"technology":[{"id":"imrt-igrt","kind":"technology","name":"IMRT / IGRT (modern external beam)","route":"/technologies/imrt-igrt/"}],"term":[{"id":"planning-target-volume","kind":"term","name":"GTV, CTV and PTV (target volumes)","route":"/terms/planning-target-volume/"},{"id":"target-delineation-variability","kind":"term","name":"Variation between clinicians in drawing the radiotherapy target","route":"/terms/target-delineation-variability/"}],"journal":[{"id":"ijrobp","kind":"journal","name":"International Journal of Radiation Oncology, Biology, Physics","route":"/journals/ijrobp/"}]}}