Determining Radiation Doses of Critical Normal Tissues for Stereotactic Body Radiotherapy (SBRT) of Central Lung Tumors

Abstract

While respiratory motion is included in the definition of target volumes for lung SBRT, delineation of organs at risk (OARs) and dose calculation are typically performed on average intensity projection images (AIP). This approach often misses structural detail during contouring due to motion blurring on AIP. Including respiratory motion in the dose calculation for OARs will result in more accurate dose assessment, which is necessary to establish reliable dose-effect relationships, particularly for SBRT of central lung tumors with a high risk of critical side effects.For 17 centrally-located lung tumors, clinical 4DCT-based SBRT treatment plans (BED≥100 Gy in 76%) were created for both AIP and mid ventilation image (MidV, 30% breathing phase) and contour sets. Internal GTVs were produced via GTV propagation over all breathing phases. OARs were outlined on both AIP and MidV images according to RTOG 0813. Both plans were also recalculated on all 10 breathing phases and registered back deformably to the MidV to assess differences in the accumulated OAR doses over the full breathing cycle. MidV was selected over the AIP as the reference image for comparison as it does not suffer from motion blurring.Volumes were, on average, larger on AIP than MidV image sets for all OARs by 3-15% (P < 0.05 for heart, lungs, great vessels (GV)), except the esophagus. Mean differences ± SDs in the max doses (Gy) between respiration-averaged MidV and AIP plans were for proximal bronchial tree (PBT) 2.26 ± 2.11, heart 2.24 ± 2.89, GV 1.71 ± 1.12, esophagus 1.38 ± 1.1, spinal cord 1.11 ± 0.67, and trachea 0.71 ± 1.16. Doses were significantly different for heart Dmean/10cc/15cc (P = 0.03/0.03/0.04). Relative to MidV plans, AIP plans resulted in higher max doses for PBT in 11/17 (65%) plans, 9/17 (53%) for esophagus, GV and spinal cord, and 5/17 (29%) for heart and trachea. Max dose differences > 5 Gy were observed for GV and PBT in 2/17 (12%) and for heart in 3/17 (18%) of individual plans. Superior-inferior carina motion ranged from 0.32 to 0.73 cm. No significant correlations between dose differences and carina motion were identified.Though limited by a small sample size, this study represents a comprehensive assessment of OAR doses for SBRT planning of centrally located lung tumors including the effects of respiratory motion. We often observed only small differences in dose delivered to central OARs. However, clinically relevant differences ≥ 5 Gy were observed in individual patients for PBT, GV and heart, resulting in over- or underestimation of the actual OAR dose over the full breathing cycle when using AIP-based planning. We therefore recommend MidV image-based OAR dose assessment to develop reliable dose-effect relationships for central lung OARs.D.A. Cooper: None. L. Padilla: None. A. Watson: None. K. Neiderer: None. B. Smith: None. E. Weiss: Research Grant; NIH. royalties; UpToDate.