Margins

Aspects of margin derivation come from the uncertainties involved in many parts of the radiotherapy process – not least, for image-guided radiotherapy (IGRT) and on-treatment verification imaging, the set-up errors involved interfractionally, internal organ motion and movement intrafractionally, uncertainties in outlining, contouring and the definition of both target and organ at risk (OAR) volumes. Perhaps the most important margin might be regarded as the clinical target volume (CTV)-planning target volume (PTV) margin, but it must not be forgotten that this depends on the assumed accuracy of the CTV itself ( McPartlin et al. 2016 ) and therefore margin reduction must always be exercised with caution. But the ideal is to have as small a margin as possible around the CTV, while (a) assuming the CTV itself is accurate and (b) ensuring the high-dose volume fully encompasses the CTV inter- and intrafractionally, in order to achieve the highest possible tumour control while (through small margins and as small and conformal a high-dose volume as possible) minimising the dose to nearby OARs and non-involved tissues. The smaller the margin, naturally, the greater the likelihood of reducing dose, and therefore toxicity, to non-involved tissues.