3577 - Clinical Application of Simultaneous Integrated Boost in Stereotactic Body Radiotherapy for Large-Volume Tumors: A Single-Center Retrospective Study

Purpose/Objective(s): Stereotactic body radiation therapy (SBRT) for large-volume tumors remains technically challenging due to excessive normal tissue irradiation. The simultaneous integrated boost (SIB) technique utilizing a frameless robotic radiosurgery system with a multi-leaf collimator (MLC) offers a promising approach to ensure efficacy while reducing side effects. This study evaluates the clinical outcomes and safety of this dose-escalation strategy.

Materials/Methods: From June 2023 to October 2024, 31 consecutive patients with large-volume tumors (lung: n=20; bone/soft tissue: n=5; others: n=6) were treated using SBRT-SIB. A central boost target (GTV_CENTER) was delineated within the gross tumor volume (GTV) based on hyperenhanced CT/MRI or hypermetabolic PET regions, adjusted for tumor volume and proximity to organs at risk The median prescription doses were 40 Gy (IQR: 27–40 Gy) for PTV and 50 Gy (IQR: 44–55 Gy) for GTV_CENTER, delivered in 3 (n=11) or 5 fractions (n=24). The most common regimen was GTV_CENTER 50 Gy/5F and PTV 40 Gy/5F (n=7). The median isodose line was 74.0% (IQR: 70–80%). Biological effective doses (BED) were 72 Gy (IQR: 43.2–72 Gy) for PTV and 100.0 Gy (IQR: 100–115 Gy) for GTV_CENTER. Follow-ups occurred every 1–3 months to assess ORR, DCR, PFS, OS, and toxicity (CTCAE v5.0) .

Results: With a median follow-up of 10.2 months (IQR: 5.8–14.1), the cohort had a median age of 62 years (IQR: 54–73), and 90.3% were stage III/IV (III: 8; IV: 20). Tumor characteristics included a median maximum diameter of 106.0 mm (IQR: 71.8–149.7), PTV volume of 287 cc (IQR: 105.8–530.5), and GTV_CENTER volume of 32.1 cc (IQR: 11.9–92.6), with GTV_CENTER occupying 9.7% (IQR: 5.8–18.9) of PTV. Among 27 evaluable patients, ORR was 59.3% (16/27; 95% CI: 40.7–75.5) and DCR was 100.0% (27/27; 95% CI: 87.5–100.0). Median OS and PFS were 14 months (95% CI: 10.0–18.0) and 10.5 months (95% CI: 5.7–15.3), respectively. The 1-year OS and PFS rates were 63.4% (95% CI: 46.2–80.6) and 47.2% (95% CI: 26.8–67.6). Safety analysis revealed no acute grade = ? toxicities. Late toxicities included grade ? radiation pneumonitis (n=2) and skin reactions (n=2) near superficial targets.

Conclusion: SBRT-SIB with a frameless robotic radiosurgery system achieved safe dose escalation (GTV_CENTER BED =100Gy) for large-volume tumors, demonstrating high local control (DCR 100%) and acceptable toxicity. These findings support further prospective validation of long-term outcomes.