Chest Wall Pain after Single-Fraction Thoracic Stereotactic Ablative Radiotherapy: Dosimetric Analysis from the Isabr Trial

Background and Purpose: Concerns over chest wall toxicity has led to debates on treating tumors adjacent to the chest wall with single-fraction stereotactic ablative radiotherapy (SABR). We performed a secondary analysis of patients treated on the prospective iSABR trial to determine the incidence and grade of chest wall pain and modeled dose -response to guide radiation planning and estimate risk. Materials and Methods: This analysis included 99 tumors in 92 patients that were treated with 25 Gy in one fraction on the iSABR trial which individualized dose by tumor size and location. Toxicity events were prospectively collected and graded based on the CTCAE version 4. Dose -response modeling was performed using a logistic model with maximum likelihood method utilized for parameter fitting. Results: There were 22 grade 1 or higher chest wall pain events, including five grade 2 events and zero grade 3 or higher events. The volume receiving at least 11 Gy (V 11Gy ) and the minimum dose to the hottest 2 cc (D 2cc ) were most highly correlated with toxicity. When dichotomized by an estimated incidence of >= 20 % toxicity, the D 2cc > 17 Gy (36.6 % vs. 3.7 %, p < 0.01) and V 11Gy > 28 cc (40.0 % vs. 8.1 %, p < 0.01) constraints were predictive of chest wall pain, including among a subset of patients with tumors abutting or adjacent to the chest wall. Conclusion: For small, peripheral tumors, single-fraction SABR is associated with modest rates of low-grade chest wall pain. Proximity to the chest wall may not contraindicate single fractionation when using highly conformal, image-guided techniques with sharp dose gradients.