Author: James Knight II, Joshua Misa, Damodar Pokhrel, William St Clair, Eddy S Yang ๐จโ๐ฌ
Affiliation: University of Kentucky, Department of Radiation Medicine, University of Kentucky, Radiation Medicine ๐
Purpose: Highly-heterogenous sieve-like dose distribution via SFRT to large/bulky tumors (โฅ6 cm) could enhance tumor cell-death via both direct/indirect cell-kill mechanisms. Adding highly-conformal SBRT dose post-SFRT could further increase therapeutic-ratio and reduce pain. We present a novel SBRT scheme to debulk unresectable larger tumors immediately post-SFRT.
Methods: Utilizing same-day image-guided MLC-based SFRT, we have treated 11 extracranial patients with large/bulky unresectable tumors of different histology using single-dose of 15Gy (6/10MV/AcurosXB). These patients also received highly-conformal VMAT-SBRT (6MV-FFF; 30-40Gy/5-fraction, every-other day) 2-3 days post-SFRT. Average SFRT tumor (GTV) was 354.4cc. SBRT plans were generated for PTV=GTV+5mm, resulting large PTV:683.5+/-434.8(144.5โ1361.0)cc. For accurate dose-assessment, novel voxelized spatial-biological effective dose (s-BED)/spatial-EQD2 scripts were developed. Treatment delivery efficiency/accuracy was assessed. Tumor response, pain control, and toxicity profiles was assessed by follow-up exams/imaging study in 3-month intervals.
Results: SFRT plans had average peak-to-valley-dose-ratio=GTVD10%รทGTVD90%, GTV(V7.5Gy) and mean GTV dose were 3.0, 50.5%, and 7.8Gy. For highly-conformal SBRT plans (CI=1.04+/-0.03), mean and maximum s-BED10 dose to PTV were 69.0+/-9.7(61.4โ98.6)Gy and 94.2+/-8.8(79.9โ119.7)Gy, enhancing tumor-dose via SBRT. EQD2 maximum and 1 cc of skin were 61.9Gy and 39.5Gy; other adjacent OAR were spared (bowel 71.9Gy). Monte-Carlo second-check for both plans agreed within ยฑ3.3%. Average patient-specific QA result was 97.8% for 2%/2mm ฮณ-criteria. Both IGRT plans were delivered in <15 min via 6DOF couch corrections. Sarcoma was common treated histology, seen in 4(40%) patients. Median follow-up was 3-month (3-12 months). One patient lost follow up. 5/10 patients (50%) demonstrated tumor local-control on follow-up imaging and 7(70%) reported clinical pain relief. No patient reported grade 2+ toxicities.
Conclusion: This novel and clinically useful SFRT plus SBRT scheme provided short course of rapid, safe, and effective debulking for unresectable large tumors while sparing OAR. Longer clinical follow-up result with larger cohort is warranted. Adapting SBRT fractions for anatomical changes post-SFRT delivery is underway.