Purpose: To explore the effects of microbeam radiation (MR) on vascular biology, we used the chick chorioallantoic
membrane (CAM) model of an almost pure vascular system with immature vessels (lacking periendothelial
coverage) at Day 8 and mature vessels (with coverage) at Day 12 of development.
Methods and Materials: CAMs were irradiated with microplanar beams (width, 25 mm; interbeam spacing,
200 mm) at entrance doses of 200 or 300 Gy and, for comparison, with a broad beam (seamless radiation
[SLR]), with entrance doses of 5 to 40 Gy.
Results: In vivo monitoring of Day-8 CAMvasculature 6 h after 200 GyMRrevealed a near total destruction of the
immature capillary plexus. Conversely, 200 Gy MR barely affected Day-12 CAM mature microvasculature. Morphological
evaluation of Day-12 CAMs after the dose was increased to 300 Gy revealed opened interendothelial
junctions, which could explain the transient mesenchymal edema immediately after irradiation. Electron micrographs
revealed cytoplasmic vacuolization of endothelial cells in the beam path, with disrupted luminal surfaces;
often the lumen was engorged with erythrocytes and leukocytes. After 30 min, the capillary plexus adopted a striated
metronomic pattern, with alternating destroyed and intact zones, corresponding to the beam and the interbeam
paths within the array. SLR at a dose of 10 Gy caused growth retardation, resulting in a remarkable
reduction in the vascular endpoint density 24 h postirradiation. A dose of 40 Gy damaged the entire CAM
Conclusions: The effects of MR are mediated by capillary damage, with tissue injury caused by insufficient blood
supply. Vascular toxicity and physiological effects ofMRdepend on the stage of capillary maturation and appear in
the first 15 to 60 min after irradiation. Conversely, the effects of SLR, due to the arrest of cell proliferation, persist
for a longer time.