Incidence of radiation myelitis of the cervical spinal cord at doses of 5500 cGy or greater.

The incidence of permanent damage to the spinal cord as a complication of radiation therapy generally correlates positively with total radiation dose. However, several reports have indicated that fraction size is also an important factor in the development of late damage in normal tissue. To determine the effect of fraction size on the incidence of radiation-induced spinal cord damage, the authors reviewed 176 cases of head and neck cancer treated at their department between 1980 and 1990 with radiation doses of 5500 cGy or greater to a portion of the cervical spinal cord. Majority of these patients received 6000 cGy or greater with fraction size ranging from 150 to 200 cGy. Seventy-two of 176 patients have been observed for 2 years or more. More than one third (26 patients) of these received greater than 6000 cGy with fraction sizes of 157 to 170 cGy. Four of 72 (5.6%) patients had experienced permanent cervical spinal cord damage. The results of this study suggest that radiation damage to the cervical spinal cord correlates not only with total radiation dose, but also with fraction size. Low fraction sizes appear to decrease the incidence of such damage.