Grid Spatially Fractionated Radiation Therapy for Bulky Tumors: A Large Single Institution Experience.

Abstract

Spatially fractionated radiation therapy, specifically grid radiation therapy (grid-RT), is a treatment option for bulky tumors (≥6 cm). However, its modern use for locally advanced or metastatic disease remains limited. We evaluated outcomes in patients treated with grid-RT at our institution. We retrospectively reviewed patients with locally advanced or metastatic bulky tumors treated with grid-RT from December 2013 to June 2022. Patient and treatment characteristics, response rates, survival outcomes, and toxicity were analyzed. Eighty patients underwent 82 grid-RT treatments. Median age was 58.5 years. The head and neck region was the most frequently treated site (40/82, 49%). The most common grid-RT dose was 20 Gy in a single fraction (78/82, 95%; range, 15-20). Median gross tumor volume was 364 cm³ (range, 46-3860). Most treatment courses (51/82, 62%) used a Cerrobend grid block, whereas 31 of 82 (38%) treatments used intensity modulated radiation therapy (IMRT) via tomotherapy. Median follow-up was 8 months (range, 1-100). Seventeen patients were lost to follow-up before imaging response assessment. Among 65 evaluable treatments, 53 demonstrated an objective response (82%). Definitive treatment intent and definitive RT dose were associated with improved response. Cumulative locoregional recurrences at 1, 2, and 3 years were 21% (95% CI, 11.1%-31.4%), 24% (95% CI, 12.7%-34.4%), and 26% (95% CI, 14.5%-37.9%), respectively. Median time to progression was 11.7 months (IQR, 5.9-35.8 months). Median progression-free survival was 5.7 months (95% CI, 3.3-6.9). Median overall survival was 7.9 months (95% CI, 5.1-11.6). Common acute toxicities included grade-2 radiation dermatitis (14/102, 14%) and grade-2 oral mucositis (13/102, 13%). Late toxicities (>30 days post-RT) included grade-1 fatigue (4/43, 10%), grade-2 pain (5/43, 12%), and grade-2 dry mouth (4/43, 10%). Grid block was associated with higher acute xerostomia than IMRT. Grid-RT appears to be an effective treatment for bulky tumors, either standalone or as a boost to palliative or definitive radiation. Grid block and IMRT have comparable short- and long-term toxicity profiles.