Local Control in Thoracic Stereotactic Ablative Radiation Therapy: Analysis of Motion Management and Single-Fraction Dosimetry From the iSABR Trial.

Abstract

Thoracic stereotactic ablative radiation therapy (SABR) is an effective treatment for lung tumors. We evaluated the association between tumor control and (a) tumor respiratory motion and motion management approach and (b) single-fraction dose metrics in patients treated on a prospective clinical trial. We evaluated 235 patients with 277 thoracic tumors treated on the iSABR trial. Motion management approaches included motion inclusive (MI, 41%), MI with extreme breaths excluded (13%), expiratory gating (Exp Gating, 24%), and inspiratory breath hold (IBH, 22%). The association between tumor motion, motion management technique, and local recurrence (LR) was evaluated using Fine-Gray Analysis. Among the cohort of patients treated in a single fraction (150 tumors), we performed a tumor control probability analysis for dose to the gross tumor (GTV) and planning treatment volumes. There was no significant difference in LR by tumor motion when dichotomized to <1 cm or ≥1 cm (3-year LR of 5.8% vs 6.3%, P = .98). Similarly, there was no difference in LR between patients treated with MI, MI with extreme breaths excluded, Exp Gating, and IBH with 24-month estimates of 5.5%, 5.9%, 4.7%, and 3.7%, respectively (P = .75). For tumors treated with single-fraction SABR, GTV D_68.3% and planning treatment volume D_65.3% had the strongest correlation with local control (LC). Tumor control probability analysis demonstrated a statistically significant association with GTV D_99.7%, D_95%, and D_68.3%. Rates of LC at 3 years were greater than 90% with a GTV D_68.3% > 29.4 Gy and a GTV D_95% > 28.1 Gy. These findings suggest that motion management techniques including Exp Gating and IBH can adequately control respiratory motion. Furthermore, single-fraction SABR with 25 Gy resulted in high rates of LC for small tumors when using heterogeneous dosimetry, including 29.4 Gy to 68.3% of the GTV and 28.1 Gy to 95% of the GTV.