Dosimetric Outcomes of SBRT to Ultra-central Lung Tumors: Lessons from the SUNSET trial

PURPOSE: The Stereotactic Radiation Therapy for Ultra-Central Non-Small Cell Lung Cancer: Safety and Efficacy Trial (SUNSET) trial investigated the maximum tolerated dose for ultracentral lung tumors treated with stereotactic body radiation therapy. Here, we report a spatial and dosimetric secondary analysis of the treatment plans and assess relationships between doses to targets, organs at risk (OARs), and clinical outcomes. METHODS AND MATERIALS: Five institutions enrolled patients with ultracentral lung cancer, cT1-3N0M0, and all received 60 Gy in 8 fractions. Maximum dose was limited to 120% of prescription. Planning data sets and treatment plans were imported into a central repository. Univariable logistic and Cox proportional hazards regression modeling were performed to identify significant dosimetric predictors for related grade ≥2 adverse events, overall survival, and local control (LC). RESULTS: Thirty patients were included in this analysis. At median follow-up of 36.5 months, 11 patients experienced grade ≥2 toxicity. The planning target volume (PTV) overlapped with 58 central OARs of which airway was most common (n = 26). The mean maximum dose was 69.3 Gy (range, 63.8-72.0 Gy); all were within the internal target volume. Mean ± SD PTV D98 was 56.1 ± 7.8 Gy, whereas D0.1 cm³ of proximal bronchial tree, esophagus, and pulmonary artery were 53.1 ± 12.6 Gy, 26.5 ± 10.5 Gy, and 57.0 ± 8.6 Gy, respectively. On regression analysis, the combined overlap volume (cm³) of the 2 primary overlapping OARs with PTV was associated with inferior LC (hazard ratio [HR], 2.86; P = .012); however, it was not associated with increase in grade ≥2 adverse events (odds ratio, 1.17; P = .49). There was no association between OAR doses (D1 cm³ and D0.1 cm³) with toxicity. PTV undercoverage (D98) was not associated with worse LC (HR per 5 Gy, 1.54; P = .68); however, lower PTV coverage was significantly associated with reduced overall survival for D98 (HR, 0.65; P = .014) and D95 (HR per 5 Gy, 0.66; P = .035). CONCLUSIONS: Within the dose constraints used in the trial, there was no relationship identified between OAR doses and toxicity. LC decreased with increasing overlap of PTV with OARs; however, this was not associated with dosimetric undercoverage of the target.