PROPOSAL FOR THE SO-CALLED 'MARGIN' OF RADIATION FIELD : ANALYSIS OF A MODEL OF AN OPPOSED TWO FIELDS TECHNIQUE USING A DOSE-VOLUME HISTOGRAM

Abstract

The methods for determining the margin of a radiation field using the opposed two fields irradiation technique are evaluated. Conventionally, the prescribed distance of the margin is determined by only one side projection of the two ports (method A). There is another method in which the gantry is rotated to the other side and the margins are determined for both sides projection (method B). Differences in the irradiated volumes are analyzed using a dose-volume histogram between method A and B. In the target, irradiated volumes of the 100% dose range are not significantly different for either method. In the phantom, however, irradiated volumes of each dose range differ according to the location of target by method A. In contrast, those in method B are not changed. It is considered that taking the exact distance from the target as a margin is theoretically acceptable for determining the size of the field.