Early response of microvessels to low doses of x-irradiation in vivo

The effects of single doses of x-irradiation ranging from 20-200 cGy in 20 cGy increments were studied by direct morophometry in vivo of the mature stable microvasculature in the rabbit ear chamber. Reproducible measurements of vascular parameters and computer data analysis using methods previously described revealed a significant and early reduction of >50% of the capillaries with doses below 400 cGy. This radiosensitive population shows an early functional and morphological response, which is completed in three days. The functional breakdown results in increased permeability, massive extravasation, microthrombi formation and changes in wall thickness. This is quantitated by transfer of fluorescent plasma (labelled intravenously with Acridine Red and Acridine Orange and illuminated with Xenon light) into perivascular spaces. The morphological vascular damage is quantitated at days 1, 3 and 5 post treatment by in vivo digitized morphometry of high magnification photomicrographs. For purpose of analysis, the microvessels are divided into four categories according to diameters: ≤10 μm, 11-20μm, 21-30μm, and >30μm. A program was developed to evaluate the dose survival relationship of this target population in relation to the linear quadratic survival model