Neutrons: Early response of microvasculature in vivo

The mature microvasculature of modified Sandison Clark ear chambers implanted in ten white New Zealand rabbits was irradiated with various dose levels of x-rays or neutrons. Vascular parameters including microvascular length, surface area, volume and intervessel distance were determined to provide an in vivo end point of vascular effects as a function of dose and time. Effects of the two modalities were scored in vivo using morphological post irradiation changes in the microvasculature. X-irradiation was delivered to an area of 5.5 x x 5.5 cm on the dorsum of the ear containing the chamber from 250 kV maxitron at 80 rads per minute and 50 cm source to skin distance. The DD reaction neutrons at a median energy of 8.3 MeV were produced from a in-hospital cyclotron at 105 cm source to skin distance. Data for morphometry were obtained by serial photomicrography of each chamber at 160 X magnification. The same 36 sites in the chamber (+-10 microns accuracy) were photographed before and on days 1, 5 and 10 following treatment. Vessels smaller than 10 microns (capillaries) and vessels bigger than 10 microns were scored separately. Data were analyzed as a comparison between the two radiation modalities and expressed as fraction of control. An RBE of 4-5 is suggested for neutron-photon comparison