Nima Aghdam, MD, MS
Purpose/Objective(s): SBRT is an effective treatment for ES-NSCLC. Central tumors however pose a therapeutic challenge due to proximity to critical organs such as central airways. Five fraction approaches have been advocated to limit treatment induced toxicity. In this report, we evaluate the dosimetric parameters of central airways and identify GED as a predictor of poor overall survival in a diverse cohort of patients.
Materials/Methods: Medically inoperable ES-NSCLC patients were treated with robotic SBRT per institutional protocol. The majority (n=41) underwent bronchoscopy for mediastinal staging, biopsy, fiducial placement and identification of GED. Proximal bronchial tree (PBT) was contoured based on the RTOG atlas, and included trachea (T), mainstem bronchus (MB) and secondary bronchi (SB). Dosimetry for PBT and each of the three subvolumes was analyzed. An additional structure, eval-PBT, was created as the subtraction of GTV from PBT.
Results: From December 2010 to December 2015, 50 patients with biopsy proven ES-NSCLC (stage I - 31; stage II – 19) with median age of 75 were treated (50 Gy in 5 fractions). At median follow up of 36 months, OS did not differ between peripheral (n=39) and central tumors (n=11) (54% vs 46%; p=0.41). Local control for peripheral tumors was significantly better than central tumors (94% vs. 60% p=0.018). Of 41 patients who underwent bronchoscopy, five central tumors were found to have GED involving the RUL (n=2), RLL (n=1), LUL (n=1) and LLL (n=1). LC and OS for tumors with GED was 66% and 20%. Cox regression analysis identified GED as a predictor of OS (HR: 4.216, p=0.009). Three patients with GED likely experienced grade 5 bronchial strictures. Median Dmax to PBT was 62.3 Gy (33.7-68) in central tumors. This represented the Dmax to SB, however median Dmax to T and MB were 12.1 Gy (1.1-37.1) and 30.8 Gy (15.2-68). Median Dmax to PBT in central tumors with GED was higher than those without GED (64.8 vs 59.1 p=0.17). Dmax to eval-PBT was lower 60.2 Gy (56.3-66.9). Patients who likely experienced grade 5 bronchial strictures had higher median Dmax to PBT and eval-PBT (67.3 and 66.9 Gy) than other central tumors (62.3 Gy). Presence of GED was associated with higher V18 (16.7cc vs 4.1 cc p=0.03) and V16.5 (20.7cc vs. 5.6 cc, p=0.05).
Conclusion: ES-NSCLC with GED predicts for worse OS following treatment with 5-fraction SBRT. Despite high Dmax delivered to PBT, LC remained relatively poor for central tumors. Furthermore, it is likely that high Dmax delivered to PBT resulted in Grade 5 toxicity for central tumors with GED. Future ES-NSCLC SBRT trials should require pretreatment bronchoscopy for central tumors to confirm these preliminary findings and to hone PBT dosimetric constraints.
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