Accuracy of alignment in breast irradiation: a retrospective analysis of clinical practice

The objective of the study was to determine the accuracy of patient positio ning in radiotherapy for breast cancer. Portal images were obtained using a fast electronic megavoltage radiotherapy imaging system in 30 cases of bre ast cancer. Quantitative analysis of 530 megavolt portal images and compari son with 30 digitized simulation films were performed. Five linear measurem ents were taken for each simulation and verification film. Central lung dis tance (CLD) is the distance from the dorsomedial beam edge to the inner tho racic wall in the central plane of the beam. Cranial lung distance (CrLD) i s the distance from the dorsomedial beam edge to the inner thoracic wall in the plane of the beam at 4 cm from the central plane. Central beam edge to skin distance (CBESD) is the distance from the skin to the ventrolateral b eam edge in the central plane of the beam. The central irradiated width (CI W) is defined as the distance from the dorsomedial beam edge to the skin. T he craniocaudal distance (CCD) is defined as the distance from a particular landmark to the caudal field border. Concerning patient position in the fi eld, mean standard deviations of the difference between simulation and trea tment images were 3.9 mm for the CLD, 3.2 mm at +4 cm, 3.6 mm for the CIW, 3.3 mm for the CBESD, 3.8 mm for the CCD. In 90% of all set-up for treatmen t, errors were less than 1 cm. The variation of the CLD was the largest set -up error. This parameter is clinically the most significant. Future treatm ent delivery should be improved by introducing patient positioning devices such as thermoplastic shells. The electronic portal imaging device (EPID) a ppears to be an adequate tool to study the accuracy of treatment set-ups li ke this.