Individualized VMAT lattice radiotherapy on a standard flattening-filter linear accelerator for advanced bulky tumors: a retrospective feasibility and early clinical experience.

Background: Advanced bulky tumors cause pain, hemorrhage, and functional impairment, but their volume, hypoxic core, and proximity to organs at risk limit dose escalation with conventional radiotherapy. Lattice radiotherapy (LRT) delivers ablative doses to intratumoral vertices while sparing peripheral tissue. We evaluated personalized LRT on standard flattening-filter (FF) linacs, alone or with radiotherapy. Methods: Seven patients (8 sites) with advanced bulky tumors treated February-November 2025 using 6 MV VMAT in FF mode were retrospectively reviewed. LRT (1-5 fractions of 6-15 Gy) was individualized by performance status, prior therapy, and tumor geometry; additional radiotherapy was at physician discretion. Co-primary endpoints were symptom relief (NRS, analgesic use, tumor-related symptoms) and local disease control; acute toxicity was secondary. Results: Median age was 69 years, median ECOG 2 (1-3), and mean tumor volume 370.7 cc (101.8-792.4 cc). All patients completed planned LRT. Symptom improvement occurred in 6 of 8 sites (75%), usually within two weeks. At median follow-up of 8 months (4-12), partial response (PR) was achieved in 5 of 8 sites (62.5%), stable disease (SD) in 2 (25.0%), and progressive disease (PD) in 1 (12.5%); the disease control rate (DCR = CR + PR + SD) was 7/8 (87.5%), with no complete response (CR) observed. Acute toxicity occurred in 3 patients (all grade 1-2, mainly fatigue and mild dermatitis); no grade ≥3 events occurred. Conclusion: Personalized LRT on FF linacs was feasible and well tolerated, with encouraging early palliative benefit in this small, heterogeneous cohort, supporting prospective evaluation with standardized symptom-assessment tools.