2130 - Stereotactic Ablative Radiotherapy for Ultra-Central Lung Tumors: Local Control and Toxicity from a Seventeen-Year Single-Institution Experience

A. H. Safavi¹, A. T. Santiago², C. Torres³, S. Raman³, B. H. Lok², A. J. Hope³, A. Bezjak³, A. Y. Sun^2,3, G. Wu³, J. P. Bissonnette³, B. C. J. Cho³, and M. E. Giuliani³; ¹University of Toronto, Toronto, ON, Canada, ²Radiation Medicine Program, Princess Margaret Cancer Centre, University Health Network, Toronto, ON, Canada, ³Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, ON, Canada

Purpose/Objective(s): Stereotactic ablative radiotherapy (SABR) for ultra-central (UC) lung tumors can be associated with morbidity and mortality. We report local control (LC) and toxicity following SABR (=8 daily fractions) with planning target volumes (PTV) overlapping trachea, proximal bronchial tree (PBT), esophagus, and/or pulmonary artery and vein (PA/PV). Materials/

Methods: One hundred thirty-four of 1771 (7.6%) patients receiving lung SABR (January 2006-May 2023) were included from a prospective institutional database. In total, 141 UC lung tumors (75% primary NSCLC, 25% metastases, no nodes) were analyzed. Primary outcomes were LC and incidence of grade (G) 3-5 CTCAE V5.0 toxicity. Secondary outcomes were overall survival (OS), progression-free survival (PFS), regional (RC) and distant control (DC), and incidence of G2+ pneumonitis (RP).

Results: Median follow-up was 23.0 months (2.0-165.0). Median tumor size was 2.1 cm (0.8-6.0). Sixty-five (62.5%) patients with primary NSCLC were medically inoperable. PTV overlapped trachea in 7.1%, PBT 67.4%, esophagus 2.1%, PA 47.5%, and PV 12.8%. Tumor abutted PBT in 36.2% and PA/PV in 29.8%. No endobronchial invasion was noted. The most common prescription (89.4%) was 60 Gy in 8 fractions (BED₁₀ 59.5-123.6 Gy). Prescription isodose line (median: 87% [68.0-99.0]) was =90% for 96 tumors (68.1%). Median PTV D99 and D95 were 96.2% (38.1-99.2) and 100.0% (70.9-108.4). PTV D99=90% was achieved for 87.9% of tumors. PTV D95=100% was achieved for 66.7% of tumors. Median PTV average dose was 109.2% (100.2-125.0). PTV D_max (median: 119.8% [104.8-149.2]) was =120% for 72 tumors (51.1%). Median D_{max,EQD2(a/b=3)} (Gy) for trachea was 9.6 (0.1-138.5), PBT 124.6 (0.8-178.9), esophagus 22.2 (3.0-92.2), and PA/PV 135.6 (72.7-143.7). Median lung V20 was 8.8% (1.0-21.0). Five-year LC was 91.9% (95%CI:86.3-97.8). Tumor size (subdistribution hazard ratio (sHR)=1.50; 95%CI:1.01-2.23; p=.044), PTV volume (sHR=1.01; 95%CI:1.01-1.02; p<.001), and PET SUV_max (sHR=1.07; 95%CI:1.01-1.14; p=.021) predicted local failure. Two patients (1.5%) experienced G3-5 toxicities: 1) one with G3 pneumothorax/pneumopericardium, G3 RP, and G5 aspiration post 60 Gy in 8 fractions, 2) another with G5 heart failure exacerbation post 24 Gy in 1 fraction. Median OS was 46.0 months (95%CI:35.0-67.0). For patients with primary NSCLC, median PFS was 75.0 months (95%CI:35.0-NR) and 5-year RC and DC were 70.7% (95%CI:60.2-83.0) and 80.9% (95%CI:72.5-90.3). Incidence of G2+ RP was 12.9%. Multiple tumors (sHR=6.13; 95%CI:2.37-15.8; p<.001), lung V20 (sHR=1.12; 95%CI:1.01-1.23; p=.025), and systemic therapy use (sHR=5.70; 95%CI:1.88-17.3; p=.002) predicted G2+ RP.

Conclusion: SABR, planned over 8 fractions with limited organ-at-risk hotspots and moderate PTV D_max, has high LC and low toxicity for well-selected patients with UC lung tumors.