The width of margins in radiotherapy treatment plans

Publication of ICRU Reports 50 and 62 has highlighted the need to devise pr otocols for the process of drawing the planning target volume (PTV) around the clinical target volume (CTV). The margin surrounding the CTV should be wide enough to account for all geometric errors so that no part of the CTV accumulates a dose less than, for instance, 95% of that prescribed. One app roach to the problem has been to draw a margin around the CTV delineated at the treatment preparation stage which is sufficiently wide that the mean p osition of the CTV will be encompassed in a specific percentage of cases, f or example 90%. This accounts for the systematic errors. A further margin i s then drawn to account for random set-up and organ-motion uncertainties du ring treatment. The width of this second margin has previously been shown to be 1.64(sigma - sigma (p)). Here sigma, a vector quantity, is the standard deviation whic h results from convolving the penumbra spread function of standard deviatio n sigma (p) with the Gaussian distributions of the daily positional uncerta inties of organ motion and set-up error. However, it is shown in this paper that the calculation should take into account the beam configuration of th e treatment plan. In a typical coplanar multibeam plan, usually in the tran sverse plane, any given edge of the target volume is normally defined by a single beam or two parallel and opposed beams. However, because of the pres ence of the other beams, the effect of the blurring of the edge-defining be am(s) is reduced, which changes the value of the required margin to beta(si gma - sigma (p)) where, for example, beta can be as low as 1.04 in the tran sverse plane of a three-beam plan. The width of the required margins is calculated for up to six beams and pre sented in a table. It is shown that, while the table was derived using an i dealized plan of equally weighted plane beams irradiating a spherical targe t, it is also valid for non-uniform beam weightings, wedged-beam plans, tar get volumes of general shape and intensity-modulated radiotherapy (IMRT).