Dose response of rat retinal microvessels to proton dose schedules used clinically: a pilot study

This preclinical rat pilot study quantifies retinal microvessel, endothelial, and pericyte population changes produced by proton irradiationThe left eyes of rats were irradiated with single doses of 8, 14, 20, and 28 Gy protons; right eyes, with two fractions. Animals were euthanized, and eyes were removed; elastase digests were prepared, and cell populations were counted in sample fields. Results were compared with unirradiated controls.Progressive time- and dose-dependent endothelial cell loss occurred following all schedules. Cell loss was significantly different from control values (p0.001) following 28 Gy and following 20 Gy (p0.05) in a single dose. Endothelial cell loss was the same for single- and split-dose schedules. Progressive endothelial cell loss produced vessel collapse and acellular vessel strands. Endothelial cells were in the G(0) phase of the mitotic cycle. 28 Gy produced photoreceptor cell loss.The retinal digest is an elegant bioassay to quantify the microvessel population response. Single- and split-dose schedules appear to yield similar outcomes, in terms of endothelial cell density.