2627 - Spatially Fractionated Radiotherapy GRID for Bulky Tumor: A Large Single Institution Experience

K. Wang¹, J. J. Marrufo², C. V. Cothran², P. D. McClain¹, G. D. Lewis³, P. Sabouri⁴, A. R. Wolfe¹, and F. Xia¹; ¹Department of Radiation Oncology, University of Arkansas for Medical Sciences, Little Rock, AR, ²College of Medicine, University of Arkansas for Medical Sciences, Little Rock, AR, ³Department of Radiation Oncology, Mays Cancer Center, UT Health San Antonio, San Antonio, TX, ⁴Department of Radiation Oncology, UAMS Winthrop P. Rockefeller Cancer Institute, University of Arkansas for Medical Sciences, Little Rock, AR

Purpose/Objective(s): Spatially fractionated radiotherapy (i.e GRID radiotherapy) is utilized to treat bulky (>=6cm) tumors. Reports on the modern use of GRID radiotherapy for the treatment of locally advanced or metastatic bulky tumors are limited. In this study we analyzed clinical and treatment outcomes in patients treated with GRID at our institution. Materials/

Methods: Patients with locally advanced or metastatic bulky tumors treated with GRID radiotherapy from December 2013 to June 2022 were retrospectively reviewed. Consolidative external beam radiation therapy (EBRT) was delivered at physicians’ discretion. Patient and treatment characteristics, treatment response (symptom/imaging improvement, locoregional control) and toxicity data were collected.
Results: The study cohort consisted of 80 patients treated with 82 GRID treatments. Median age at treatment was 58.5 years. Head and neck was the most frequently treated site (n=40, 49%), followed by thorax (n=17, 21%), pelvis (n=16, 19.5%), extremities (n=7, 8.5%), and abdomen (n=2, 2.4%). Treatment intent was 50% definitive and 50% palliative. The most prescribed GRID dose was a single fraction of 20 Gy (n=78, 95%, range: 15-20 Gy). The majority of patients (n= 62, 76.5%) were treated with a GRID block and 19 patients (23.5%) were treated with GRID IMRT using the IMRT system. EBRT was delivered post-GRID in 62 (75.6%) courses with a median time interval from GRID to EBRT of 1 day (range: 1-47 days), and EBRT was delivered pre-GRID in 6 (7.3%) courses, and the remaining 14 (17.1%) courses were delivered with GRID alone. Median EBRT dose and fractionation was 46.3 Gy (range: 8-70 Gy) and 25 fractions (range: 1-35 fractions), respectively. Concurrent systemic therapy was administered in 36 treatment courses (43.9%), including 26 (31.7%) concurrent chemotherapy and 10 concurrent immunotherapy (12.2%). Median follow-up was 8 months (range: 1-100 months). 54 patients (87%) had disease response (either symptomatic response, imaging response, or both), 8 (13%) patients had either no improvement or disease progression after treatment. Median progression free survival was 3 months (95% confidence interval: 1.8-4.1), and median overall survival was 4.7 months (95% confidence interval: 0.8-8.6). There are a total of 101 acute toxicities recorded, including 32 (31.7%) grade 1, 51 (50.5%) grade 2, 18 (17.8%) grade 3 and 0 grade 4. The most common acute toxicities were radiation dermatitis (13.86%, grade 2) and oral mucositis (12.87%, grade 2). There are a total of 41 late toxicities recorded, including 16 (39%) grade 1, 21 (51.2%) grade 2, 4 (9.8%) grade 3 and 0 grade 4. The most common late toxicities are fatigue (9.76%, grade 1), pain (12.2%, grade 2) and dry mouth (9.76%, grade 2).
Conclusion: Our study is among the largest single-institution study to date on GRID radiotherapy outcomes and toxicities. Our results suggest GRID combined with EBRT provides a high response rate and tolerable toxicity profile for bulky locally advanced or metastatic tumors.