| Purpose: Tumor size reduction has been observed for patients underwent lung stereotactic body radiation therapy (SBRT). Adaptive planning has the potential to reduce normal tissue toxicity and/or escalate dose to target in these patients. In this study, we evaluated the dosimetric effectiveness of adaptive planning in lung SBRT for patients who presented large target volume changes during the treatment. Methods and Materials: 20 out of 66 consecutive lung SBRT patients who showed largest percentage internal target volume (ITV) change throughout the treatment were included in the study. All patients went through 3D and 4DCT before treatment, and treatment on a Linear Accelerator machine equipped with kV imager, kV CBCT, and MV electronic portal imaging device (EPID). CBCT images were acquired at each fraction for patient positioning purpose. CBCT images for all fractions were used for contour and treatment planning. Adaptive plans were created on the CBCT images using the same planning parameters as the original CT-based plan, with the goal to achieve comparable conformality index (CI). For each patient, two cumulative plans, non-adaptive plan (PNON) and adaptive plan (P ADP), were generated and compared for the following organs-at-risks (OARs): cord, esophagus, chest wall, and the lungs. Correlations were evaluated between changes in dosimetric metrics induced by adaptive planning and potential impacting factors, including tumor-to-OAR distances (dT-OAR), initial ITV (ITV1), ITV change (ΔITV), and effective ITV diameter change (ΔdITV). Results: For the 20 selected patients, percentage ITV change ranged from 16.4% to 59.6%, with a mean (±SD) of 38.8% (±12.5%). CI of all plans ranged from 1.03 to 1.46, with small intra-subject variations (0.01–0.1). Compared to PNON, PADP resulted in significantly (p=0–0.02) lower values for all dosimetric metrics. ΔdITV/dT-OAR was found to correlate with changes in dose to 5cc (ΔD5cc) of esophagus (r=0.77) and dose to 30cc (ΔD30cc) of chest wall (r=0.72). Stronger correlations between ΔdITV/dT-OAR and ΔD30cc of chest wall were discovered for peripheral (r=0.82) and central (r=0.94) tumors, respectively. Conclusions: Adaptive lung SBRT planning can potentially reduce dose to adjacent OARs if patients present significant tumor volume shrinkage during the treatment. Dosimetric benefits of adaptive lung SBRT planning depend upon target volume changes and tumor-to-OAR distances. |
