EFFECT OF SET-UP UNCERTAINTIES ON THE DOSE DISTRIBUTION IN THE MATCH REGION OF SUPRACLAVICULAR AND TANGENTIAL BREAST FIELDS

Background: The objective of this paper is to illustrate the effect of a gap of 5 mm, an overlap of 5 mm and a perfect match on the dose dis tribution across the junction of tangential breast fields and adjacent supraclavicular and axillary fields. Materials and methods: For this purpose film dosimetry was applied to measure relative dose distributi ons in two sagittal planes in an anthropomorphic breast phantom having cork lungs, simulating a radiation therapy treatment of the breast an d adjacent supraclavicular lymph nodes. Two different treatment techni ques, an SSD match technique and a geometrically exact isocentric matc h technique, as routinely applied in the two institutes were examined. The three-dimensional treatment planning system of each institute was used to calculate the dose distribution In the match region of the su praclavicular fields and the two opposing tangential fields. The measu red and calculated dose distributions were evaluated and compared alon g lines in two sagittal planes from the supraclavicular fields down to the tangential fields crossing the match planes. These dose distribut ions in the match region were extremely dependent on the set-up of the fields. Results: Although the reproducibility of the film measurement s was within 2%, it became clear that the set-up of the fields to achi eve a gap of 5 mm, a perfect match or an overlap of 5 mm required a lo t of attention, even when using a phantom, Conclusions: It can be conc luded that in clinical practice, these set-up difficulties do influenc e the dose distribution in the match region much more than the systema tic uncertainties in the dose calculation algorithms of the treatment planning systems and the type of treatment technique. (C) 1998 Elsevie r Science Ireland Ltd.