Ultrastructural morphometric analysis of peripheral nerves after intraoperative irradiation

Intraoperative irradiation (IORT) is used to enhance local tumour control by using large, single doses while removing critical structures from the treatment field. Peripheral nerve remains a dose-limiting normal tissue that often cannot be removed from the field. To assess ultrastructural changes in canine sciatic nerve after IORT, computerized morphometric analysis of plastic sections and electron micrographs of nerve cross-sections was used. Surgically exposed sciatic nerves were irradiated with 6 MeV electrons to 12, 20 or 28 Gy. Twelve months after treatment dogs were killed humanely and the nerves from three dogs per dose group, including non-irradiated controls, were analyzed. Twelve months after 28-Gy IORT a significant decrease in nerve fiber density occurred. Nerve fiber loss was particularly prominent in the central portion of the nerve predominantly among large nerve fibers. Other nerve fiber parameters including fiber and axon area, diameter and perimeter, myelin thickness, form factor (measure of roundness), and G ratio (axon diameter/fiber diameter) did not show significant, dose-related changes. An increase in microtubule and neurofilament density in irradiated nerve axons was found. These changes are suggestive of radiation-induced hypoxia (damage to microvasculature) resulting in axon damage and subsequent nerve fiber loss as a possible mechanism of late radiation injury to peripheral nerve.