An analysis of anatomic landmark mobility and setup deviations in radiotherapy for lung cancer

Purpose: To identify thoracic structures that exhibit little internal motio n during irradiation and to determine setup variations in patients with lun g cancer. Methods and Materials: Intrafractional images were generated with an electr onic portal-imaging device from the AP fields of 10 patients, during severa l fractions. To determine the intrafractional mobility of thoracic structur es, visible structures were contoured in every image and matched with a ref erence image by means of a crosscorrelation algorithm. Setup variations wer e determined by comparing portal images with the digitized simulator films using the stable structures as landmarks. Results: Mobility was limited in the lateral direction for the trachea, tho racic wall, paraspinal line, and aortic notch, and in the craniocaudal dire ction for the clavicle, aortic notch, and thoracic wall. Analysis of patien t setup revealed random deviations of 2.0 mm (1 SD) in the lateral directio n and 2.8 mm in the craniocaudal direction, while the systematic deviations were 2.5 and 2.0 mm (1 SD) respectively. Conclusions: We have identified thoracic structures that exhibit little int ernal motion in the frontal plane, and recommend that these structures be u sed for verifying patient setup during radiotherapy. The daily variation in the setup of lung cancer patients at our center appears to be acceptable. (C) 1999 Elsevier Science Inc.