286 - Dosimetric Outcomes of SBRT to Ultracentral Lung Tumors: Lessons from the SUNSET Trial

R. R. Salunkhe Sr¹, D. A. Palma², A. Warner³, H. Bahig⁴, J. M. Laba⁵, P. Lang⁶, G. Rodrigues², M. P. Campeau⁷, M. Duclos⁸, T. T. T. Vu⁷, B. H. Lok¹, S. Raman⁹, A. Louie¹⁰, A. Hope¹¹, A. Bezjak⁹, S. V. Bratman¹², A. Swaminath¹³, V. Kundapur¹⁴, R. Doucet¹⁵, B. M. Keller¹⁶, M. Wierzbicki¹³, L. Drever¹⁴, S. Gaede², J. P. Bissonnette¹⁷, and M. E. Giuliani¹; ¹Princess Margaret Cancer Centre, Toronto, ON, Canada, ²London Health Sciences Centre, London, ON, Canada, ³Department of Radiation Oncology, Western University, London Health Sciences Centre, London, ON, Canada, ⁴Centre Hospitalier de lUniversité de Montréal, Montréal, QC, Canada, ⁵Department of Radiation Oncology, Western University, London, ON, Canada, ⁶Department of Radiation Oncology, London Health Sciences Centre, London, ON, Canada, ⁷Centre Hospitalier de l’Université de Montréal, Montréal, QC, Canada, ⁸McGill University Health Centre, Montréal, QC, Canada, ⁹Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, ON, Canada, ¹⁰Sunnybrook Odette Cancer Centre, Toronto, ON, Canada, ¹¹Department of Radiation Oncology, University Health Network, Toronto, ON, Canada, ¹²Princess Margaret Cancer Centre, University Health Network, Toronto, ON, Canada, ¹³Juravinski Cancer Centre, McMaster University, Hamilton, ON, Canada, ¹⁴Saskatoon Cancer Centre, Saskatoon, SK, Canada, ¹⁵Centre Hospitalier de lUniversité de Montréal, Montreal, QC, Canada, ¹⁶Department of Medical Physics, Odette Cancer Centre, Sunnybrook Health Sciences Centre, University of Toronto, Toronto, ON, Canada, ¹⁷Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada

Purpose/Objective(s): The SUNSET phase I trial investigated the maximum tolerated dose for ultracentral (UC) lung tumors treated with stereotactic body radiotherapy (SBRT). Here we report a detailed spatial and dosimetric secondary analysis of the treatment plan and assess relationships between doses to targets, organs-at-risk (OARs) and clinical outcomes. Materials/

Methods: Five Canadian institutions enrolled patients with UC primary lung cancer, cT1-3N0M0, and all received SBRT to a dose of 60 Gy in 8 daily fractions. Maximum dose (Dmax) was limited to 120% of prescription. Targets were delineated as internal target volume (ITV) (combining gross tumor volume (GTV) across respiratory phases) expanded by 5 mm to form planning target volume (PTV). All OARs including great vessels and bilateral proximal bronchial tree up to segmental bronchus were contoured at baseline. To evaluate OAR and target doses, planning datasets and treatment plans were imported into a central repository. Descriptive statistics were generated for baseline characteristics and dosimetry. Univariable logistic and Cox proportional hazards regression modelling were performed to identify significant dosimetric predictors for related grade = 2 adverse events (CTCAE v4.0), overall survival (OS) and local control (LC).
Results: All 30 enrolled patients (13 males, 17 females) were included in this sub-study. At median follow-up of 36.5 months, 9 patients (30.0%) experienced grade 2 adverse events and 1 patient (3.3%) each with grade 3(dyspnea) and 5(infection) attributed to treatment. Median ITV and PTV sizes were 11.3 cc (IQR: 6.2-26.8) and 34.4 cc (IQR: 21.9-62.2), respectively. PTV overlapped with at least one OAR for all patients, most commonly the proximal bronchial tree (PBT) or trachea (27/30). Additionally, a second overlapping structure was identified in 26/30 patients, most commonly the pulmonary artery (14/26) and esophagus (9/26). The mean overlap of first and second OAR with PTV were 0.95 cc (range: 0-4.2) and 0.7 cc (range: 0-4.7), respectively. The mean Dmax was 69.4 Gy (range: 64-72.5 Gy); all were within the PTV. Mean(±SD) PTV D98 was 56.1±7.8Gy, while D0.1cc of PBT, esophagus and pulmonary artery were 53.1±12.6Gy, 26.5±10.5Gy and 57±8.6Gy, respectively. On analysis, combined overlap volume (cc) of the two primary overlapping OARs with PTV was associated with statistically significant inferior LC (hazard ratio [HR] per 1 cc increase: 2.86, p=0.012); however, overlap volume was not associated with an increase in grade = 2 adverse events (odds ratio [OR] per 1 cc increase: 1.17, p=0.49). There was no association between OAR doses (D1cc/D0.1cc) with toxicity. PTV under-coverage (D98) was not associated with worse LC (HR per 5 Gy increase: 1.54, p=0.68).
Conclusion: Within the dose constraints used in the trial, there was no relationship identified between OAR doses and toxicity. Local control decreased with increasing overlap of PTV with OARs, however, this was not associated with dosimetric under-coverage of the target.