Session Item

Results from Nordic-HILUS study indicate high dose radiotherapy may be associated with unacceptable high-grade toxicity for ultracentral tumors. We hypothesized that MR-guided SBRT (MRgSBRT) or hypofractionated RT (MRgHRT) for metastatic and primary ultracentral and central lung lesions would have excellent local control and minimal toxicity.

 Single institution retrospective review of patients with ultracentral (per HILUS) or central (per RTOG) lesions treated with MRgSBRT/MRgHRT with image-guided gating and/or adaptation. Survival was estimated using Kaplan-Meier and stratifications tested using log rank test. Associations between incidence of toxicities and other patient factors were tested using Mann Whitney U test for continuous variables and Kendall’s tau-b for categorical variables. Other statistics analyzed descriptively.

Between 2/2019-3/2021, 29 patients were treated with MRgSBRT/MRgHRT (median age 66.2 years, range 32.3-80.7) with a median follow-up of 18.1 months (95% CI 14.7-21.5). Sixty-two percent (n=18) had lung metastases and the remainder had primary lung cancers. All patients had central lesions per RTOG and 86% (n=25) fit criteria for Group A per HILUS trial; median distance from the proximal bronchial tree (PBT) was 0 mm (interquartile range [IQR] 0-6.5mm). The median GTV/PTV volumes were 11.1cc (IQR 6.6-20.2) and 21.8cc (IQR 14.8-42.5), respectively. The median biologically equivalent dose (BED; α/β=10) was 105 Gy (range 75-151.2), with the most common schedule being 7.5 Gy x 8 fractions (45%, n=13). A majority (55%) had prior systemic therapy, 45% (n=13) had immunotherapy, 17% (n=5) had thoracic RT, and 17% (n=5) had concurrent therapy with MRgSBRT, with 2 patients getting SBRT to another area of the lungs. Eight patients (27%) underwent daily adaptation with a median of 94% of fractions adapted (range 75-100%). Adaptive treatment course was shorter than non-adaptive (median 9 v. 13 days, p=0.013) and trended towards higher BED (105 Gy v. 100 Gy, p=0.053). Median OS was 24.8 months (95% CI 17.8-31.8), median PFS was 13.3 months (6.9-19.6), median LC was not reached. Five patients had pre-existing cough or dyspnea. No Grade 4 or 5 toxicities were seen. Single Grade 3 toxicity was esophagitis in one patient and persisted >3 months. Twelve Grade 1 toxicities and 8 Grade 2 toxicities were noted in 12 patients. No significant associations were found between toxicity and HILUS Group, BED, PBT distance, GTV/PTV volumes, prior therapies, concurrent therapy, or adaptive MRgRT.