Margins in radiotherapy

As part of the treatment prescription procedure, one has to prescribe in anatomical terms any GTV (Gross Tumor Volume) according to the general TNM-rules as well as any other tissues that are to be treated for presumed subclinical diseases. These prescriptions of one or several Clinical Target Volumes (CTVs) are based on general oncological principles, and are not related to the treatment modality. If external beam radiotherapy is being used, one has to consider special problems related to different geometric inaccuracies and uncertainties, which can be both intrafractional and interfractional. Such inaccuracies and uncertainties are due to either the position, shape and size of the patient/tissues in relation to a fix point, or to variations in beam geometry. The two different types of variations may or may not co-variate. A margin or margins for these uncertainties has to be included in the dose planning procedure, which then will evaluate a static situation, representing the CTV(s) plus geometric ''safety'' margin(s) (Planning Target Volume, PTV) and in fact not the true clinical situation. The dose distribution arrived at for this static representation will however have to be considered as representative for the CTV. This presents of course a dilemma, but one has to accept a reasonable compromise. There is no general rule on the size of the different geometric uncertainties or how they should be added up. Individual evaluations are needed. (author)