Minibeam radiotherapy valley dose determines tolerance to acute and late effects in the mouse oral cavity.

Purpose
Minibeam radiotherapy (MBRT) is an innovative strategy to improve normal tissue sparing by delivering alternating, submillimeter-wide regions of high "peak" and low "valley" doses. The purpose of this work is to characterize both acute and late MBRT-induced normal tissue toxicities and determine the dosimetric parameters that dictate toxicity.Methods And Materials
Mice were stratified by weight and randomized to receive a single dose of conventional radiotherapy (uniform open field) or MBRT (0.5 mm-wide minibeams spaced 1.1 mm center-to-center) to the oral cavity. The conventional RT groups (n=4 per group) received 16 or 20 Gy, while the MBRT groups (n=5 per group) received peak:valley doses of 48:8, 72:12, 96:8, 96:16 or 152:8 Gy. Acute toxicity (≤3 weeks) was evaluated using changes in weight and mucosal histology. Late effects on bone and dentition were evaluated using microscopic computed tomography (microCT).Results
Animals irradiated with 16 Gy (n=1), 20 Gy (n=4), and 96:16 Gy (n=5) reached acute toxicity endpoint (≥20% weight loss) between 9-11 days post-radiation and exhibited histological changes indicative of mucositis. No animals in the other MBRT groups reached acute toxicity endpoint. While 96:8 Gy induced marked mucosal damage in peak regions, the spared tissue in the valley regions enabled restoration of mucosal integrity within two weeks post-MBRT. MicroCT of surviving mice 12 months post-radiation revealed an alternating pattern of decreased bone volume consistent with the MBRT pattern. The upper incisors of most animals were shortened or completely missing. The mice receiving 16 Gy and 48:8 Gy exhibited the most and least dental damage, respectively.Conclusion
This preliminary work emphasizes that normal tissue sparing by MBRT, as determined by the valley dose, significantly ameliorates dose-limiting toxicities and enables escalation to MBRT peak doses up to an order of magnitude greater than conventional RT doses.