Designing Targets for Elective Nodal Irradiation in Lung Cancer Radiotherapy: A Planning Study

Purpose: To assess doses received by mediastinal and hilar lymph node stations (LNS) delineated according to published recommendations when 'standard' two-dimensional (2D) elective fields are applied and to assess doses to critical structures when fields are designed using 2D and three-dimensional (3D) treatment planning for elective irradiation. Methods and Materials: LNS were delineated on axial CT scans according to existing recommendations. For each case and tumor location, 2D anteroposterior-posteroanterior (AP-PA) elective fields were applied using the AP-PA CT topograms. From the 2D portal fields, 3D dose distributions were then calculated to particular LNS. Next, 3D plans were prepared for elective nodal irradiation for tumors of different lobes. Doses for critical structures were compared for 2D and 3D plans. Results: LNS 1/2R, 1/2L, 3A, 3P, 5, 6, and 8 were not adequately covered in a substantial part of plans by standard 2D portals when guidelines for delineation were strictly followed. The magnitude of the lack of coverage increased with margin application. There was a trend for a higher yet probably still safe dose delivered to lung for 3D plans compared with 2D plans with a prescription dose of 45 Gy. Conclusions: 2D fields did not entirely cover LNS delineated according to the recommendations for 3D techniques. A strict adherence to these guidelines may lead to larger portals than traditionally constructed using 2D methods. Some modifications for clinical implementation are discussed.