IN-VIVO DETERMINATION OF THE ACCURACY OF FIELD MATCHING IN BREAST-CANCER IRRADIATION USING AN ELECTRONIC PORTAL IMAGING DEVICE

The purpose of this study was to investigate the accuracy of field mat ching in patients treated by irradiation of the breast and adjacent ly mph nodes. Field matching is performed by the radiographers during eac h session on a match line drawn on the patient's skin, Field edge posi tions were assessed in the cranial match plane of tangential breast fi elds and supraclavicular-axillary fields using an electronic portal im aging device and match line markers placed on the skin of the patients . The mean gap/overlap of the four fields for individual patients duri ng each treatment session, derived from 374 marker projections, was +0 .5 mm indicating that no systematic gap or overlap was observed, The u ncertainty in the position of the four fields with respect to the matc h plane ranges from 3.1 to 5.1 mm (1 SD) for the individual patients, Gaps and overlaps between fields were also related to an absolute matc h line position, found by comparison of simulator and portal images, s howing a small systematic uncertainty of 2.4 mm and a standard deviati on of 3.3 mm, It can be concluded that the use of an electronic portal imaging device in combination with match line markers is a good metho d to quantify the accuracy of field matching in vivo. The results show ed good stability and reproducibility in the field matching region for this treatment technique of breast cancer irradiation.