Purpose: Current standard treatments for Stage IV polymetastatic non-small cell lung cancer(NSCLC) using immunotherapy and/or systemic chemotherapy result in low durable response rates. Recent studies suggest that low dose radiotherapy(RT) may improve immunotherapy response. In this work, we developed a workflow/planning strategy to deliver low dose(1Gy) RT to all sites of polymetastatic disease every 4-6 weeks(aligned with immunotherapy administration) using: (1)conventional planning/delivery with Halcyon; (2)the AI-based Ethos online adaptive RT platform.
Methods: Ten patients with polymetastatic NSCLC patients with available PET/CT scans were used for planning gross tumor volumes (GTVs) and were delineated using PET standardized-uptake-value(SUV) thresholding(normal liver SUV mean+2σ). Planning target volumes(PTVs) were generated with a 1cm expansion of GTVs to account for setup uncertainty and organ motion. The number of targets per patient varied from 9-52(median=15). An algorithmic approach defining 2-4 isocenters(depending on target distribution) was developed. Dosimetric objectives were: Dmin,0.03cc≥95%(acceptable variation(Δ)≥90%), V100%≥95%(Δ≥90%), and Dmax,0.03cc≤120%(Δ≥125%). Additionally, online adaptation was simulated for a subset of patients(n=5) within Ethos(using subsequent diagnostic CT when available or a deformed CT from the initial PET/CT).
Results: Initial plans generated both for Halcyon and Ethos achieved clinical goals within acceptable variation. For 200 PTVs across 10 patients: Dmin,0.03cc≥95%, V100%≥95%, and Dmax,0.03cc≤120% goals were achieved for 84.8%/99.5%, 97.7%/98.7%, 97.4%/92.3%, in Halcyon/Ethos plans, respectively. The ratio of 50% isodose volume to PTV volume(R50%), maximum dose at 2cm from PTV(D2cm), and the ratio of the 100% isodose volume to PTV volume (conformity index) were 7.9/5.9, 102.3%/88.44%, and 0.99/1.01 in Halcyon/Ethos plans, respectively. Adapted plans in Ethos maintained PTV coverage whereas non-adapted plans often resulted in geographic misses due to changes in tumor size, patient position, and body habitus.
Conclusion: This study demonstrates feasibility of conventional planning/treatment for Halcyon and automated planning/online-adaptive treatment with Ethos for immunostimulatory low dose RT conformally delivered to all sites of polymetastatic disease.
Funding Support, Disclosures, and Conflict of Interest: Study supported by Varian Medical systems
Radiation Therapy, Treatment Planning, FDG PET