Evaluation of a Motion-Encompassing Treatment Strategy for Treating Non-Small Cell Lung Cancer

Abstract

This study analyzes the setup error of a motion-encompassing treatment strategy for treating non-small cell lung cancer. Using data from patients who were treated with daily KV cone-beam CT (CBCT) image-guidance (IG) for setup correction, the residual setup errors for a hybrid off-line IG protocol were assessed. 22 patients with NSCLC were treated with a motion-encompassing daily CBCT IG strategy during 4/2006 to 3/2007. 4D-CT imaging was used for treatment planning. Patients were scanned in the supine position during normal relaxed free-breathing and were immobilized with a wing-board. The internal target volume (ITV) was determined using the maximum-intensity-projection (MiP) CT data to account for organ motion. A 5-mm PTV margin was used to account for setup error. On-line setup correction was based on the daily CBCT imaging prior to treatment, with an action level of 3 mm. The systematic and random errors were analyzed retrospectively. The results were then fit to a hybrid off-line IG strategy where the patient underwent CBCT-IG for the first 4 fractions. The treatment setup was corrected on-line if the setup error exceeded the action level of 3 mm. A mean shift was then applied on the 5th and the subsequent fractions to correct for the systematic error. The residual errors of the hybrid off-line protocols were assessed. The systematic and random errors were found to be 5.2 ± 3.2 mm, 4.4 ± 2.7 mm and 4.4 ± 3.4 mm for anterior-posterior (AP), medial-lateral (ML) and superior-inferior (SI) directions respectively. The 3D displacement vectors (DV) were 9.9 mm ± 4.7 mm (systematic ± random). The residual error was within 2 mm in all directions for the daily IG strategy. For the hybrid off-line IG strategy, the frequency of the residual errors is shown in the figure. The occurrence frequency was 61% for the 5-mm DV, indicating that the hybrid off-line IG strategy was inadequate. The use of a 5 mm PTV margin on the ITV determined by MiP dataset combined with daily CBCT on-line correction is adequate to avoid geometric miss of the tumor for NSCLC patients. However, the hybrid off-line IG strategy is inadequate unless a larger PTV margin is used.