Dosimetric Impact of Deformable Image Registration-Based Target Volume Propagation in Stereotactic Body Radiotherapy of Ultracentral Lung Tumors

Abstract

In stereotactic body radiotherapy (SBRT) of non-small cell lung cancer (NSCLC), the use of steep dose gradients and reduced margins has improved local control while limiting toxicity. Clinical benefit of these techniques is dependent on accurate target definition. NSCLC patients are vulnerable to errors in target definition because lung tumors deform with respiratory motion. For ultracentral lung tumors, phantom studies suggest that auto-propagation of internal gross tumor volumes (IGTVs) can be inaccurate. In SBRT of ultracentral tumors, deformable image registration (DIR) is widely used for target volume propagation. We hypothesized that errors in DIR-based IGTV propagation could jeopardize SBRT target coverage for ultracentral tumors despite use of population-based safety margins.Nine patients with ultracentral NSCLC tumors appropriate for SBRT were studied. For each patient, (1) 4D CT was acquired into 10 discrete phase-based bins; (2) the GTV was manually segmented on all phases; and (3) DIR-based IGTV propagation was carried out for each manual GTV on every phase, resulting in 10 propagated IGTVs (P-IGTVs) per patient and 90 for the entire cohort. Ground truth IGTV (GT-IGTV) was defined as the union of all manual GTVs from all 10 phases. Dice coefficient was used to measure the similarity of P-IGTV relative to GT-IGTV. For each patient, SBRT plans were generated for P-IGTVs with greatest (Plan A) and least (Plan B) Dice coefficients using a PTV margin of 5 mm. Prescriptions were 60 Gy in 8 fractions with V60 Gy > 95% PTV. Dose constraints for organs-at-risk (OARs) were defined according to published prospective trials. For each patient, dosimetric differences between Plans A and B for the ground truth PTV (GT-IGTV + 5 mm margin) and OARs were examined.All tumors (6 in the right lung, 3 in the left) were within 2 cm of the proximal bronchial tree and at hilar level. Table 1 compares IGTVs and target coverage with median and range values. Plans A and B differed significantly in ground truth PTV (GT-PTV) coverage at both 99% and 95% of Rx dose (Kruskal-Wallis tests; P = 0.003). There were no significant differences between plans for dose to OARs. For Plan A, 7 out 9 plans were generated from GTVs at near-end of inhalation (respiratory phases 8 or 9); for Plan B, 7/9 plans were generated from GTVs at end of inhalation through mid-exhalation (phases 0 through 3).In SBRT of ultracentral tumors, patterns of error exist in DIR-based IGTV propagation that can compromise target coverage despite the use of population-based safety margins. These findings are important to enhance efforts for more robust and accurate targeting in lung SBRT.