Dosimetric Outcomes of Stereotactic Body Radiation Therapy to Ultracentral Lung Tumors: Lessons From the SUNSET Trial

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.

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).

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).

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.