What margins are necessary for incorporating mediastinal nodal mobility into involved-field radiotherapy for lung cancer?

Purpose: The mobility of mediastinal nodes was studied on multiple CT scans of the thorax from patients with non-small-cell lung cancer. Patients and Methods: A total of 10 enlarged mediastinal nodes/masses were identified in 8 patients with non-small-cell lung cancer. Nodal locations were classified using the Naruke/ATS-LCSG system, and between 3 and 6 scans were available for each site. The CT data sets were coregistered, and the contoured nodes were automatically projected onto the initial planning CT scan. An encompassing nodal volume (ENV) of all contours of a particular node was manually contoured on all scans. Individual nodal volumes were expanded in three dimensions to establish additional margins required to encompass the ENV. Results: The mean volume of nodes studied ranged from 0.8 to 23.2 cc. The addition to individual nodes of a margin of 5 mm was found to result in a mean ENV coverage of ≥95% at all sites. For individual nodes at locations N4R, N5, and N6, however, the coverage ranged from 87.8% to 92.6%. Conclusion: The addition of a margin of 5 mm to individual mediastinal nodes seems to be adequate to account for variations in both contouring and mobility