A study of tumor motion management in the conformal radiotherapy of lung cancer

Purpose: To assess the benefit derived from the reduction of planning target volumes (PTVs) afforded by tumor motion management in treatment planning for lung cancer. Methods: We use a simple formula that combines measurements of tumor motion and set-up error for 7 patients to determine PTVs based on the following scenarios: standard uniform 15 mm margin, individualized PTVs (no gating), spirometry-based gating, and active breath-control (ABC). We compare the percent volumes of lung receiving at least 20 Gy (V20) for a standard prescription, and the maximum tolerated doses (MTDs) at fixed V20. In anticipation of improvements in set-up accuracy, we repeat the analysis assuming a reduced set-up margin of 3 mm. Results: Relative to the standard, the average percent reductions in V20 (+/- 1 standard deviation) for the ungated and gated scenarios are 17 +/- 5 and 21 +/- 8; the percent gains in MTD are 25 +/- 12 and 33 +/- 11, respectively. For the 3 mm setup margin, the corresponding results for V20 are 28 +/- 7 and 36 +/- 7, and for MTD are 57 +/- 23 and 79 +/- 31. Conclusions: Any form of motion management provides a benefit over the use of a standard margin. The benefit derived from gating compared to the use of ungated individualized PTVs increases with tumor mobility but is generally modest. While motion management may benefit patients with highly mobile tumors, we expect efforts to reduce set-up error to be of greater overall significance. The practical limit for lung PTV margins is likely around 4-5 mm, provided setup error can be reduced sufficiently. (C) 2007 Elsevier Ireland Ltd. All rights reserved.