Reported Early Clinical Outcomes of Forward-Planned Multileaf Collimator-Based 3-Dimensional Conformal Spatially Fractionated Radiation Therapy Technique for Large and Bulky Tumors

Purpose: Conventionally fractionated radiation therapy for large, bulky (≥8 cm), unresectable tumors has been hampered by radiation-induced morbidity, but application of spatially fractionated radiation therapy (SFRT) for both palliative and curative intent has been increasingly accepted. We report our clinical use of novel 3‐dimensional conformal multileaf collimator (MLC)-based SFRT with same-day computed tomography simulation and forward-planning method, providing a safe, rapidly efficacious reduction in disease burden and pain, with minimal normal-tissue toxicity. Methods and Materials: Patients with large, unresectable bulky tumors received 15 Gy in 1 fraction to the gross tumor volume (GTV) within an hour of computed tomography simulation, using a forward-planned MLC-based SFRT technique. All patients subsequently received either 30 Gy in 10 fractions, generally 2 days after SFRT for palliative intent, or site-specific, full-prescription doses starting 2 to 3 days after SFRT for curative intent. Patients underwent follow-up examinations and imaging in 3-month intervals to assess tumor response, pain control, and radiation-associated toxicity. Results: Between November 2019 and January 2024, 24 large tumors in 23 patients were analyzed. Median follow-up was 6 months (range 3-36 months). After SFRT, 16 patients (69.5%) proceeded with palliative-intent radiation therapy, 6 patients (26.0%) underwent curative-intent radiation therapy, and 1 patient (4.3%) declined further radiation therapy. Seven patients (30.4%) reported acute radiation-associated toxicities. A total of 3 acute grade ≥3 toxicities (13.0%) were reported, but no grade 5 toxicities occurred. Complete or partial response was seen in 14 of 24 (58%) tumors; clinical benefit rate was 79.2%. Twenty of 23 patients (86.9%) reported pain relief from tumor burden. Conclusions: Same-day 3‐dimensional MLC-based SFRT method provides fast, safe, and effective management of large, bulky, unresectable tumors for both palliative and therapeutic intents across a wide range of tumor sites and histologies, reducing tumor burden and improving patient comfort and compliance. This method could be useful for rapid SFRT including adaptive treatment. We recommend commissioning and validating this method at other institutions, including community cancer centers, to expand the access of efficient, high-quality SFRT treatment to underserved patient cohorts.