High-dose areas are unintentionally created as a result of gap shifts when the prescribed dose in the two adjacent areas are different

Abstract

A large number of methods have been described for matching adjacent fields to prevent over- or underdosage at a field junction. Most authors recommend that the location of field junctions be shifted to different locations during the course of the treatment in an effort to “smear out” any dose inhomogeneity regardless of which field matching technique is used. This report focuses on the considerable variation in dose that can occur when the dose in the two adjacent treatment areas is different at the time of the gap shift. In a typical cranio-spinal treatment we found a 16% dose increase over the prescribed central axis dose.