Radiation myelopathy of the thoracic spinal cord in long term survivors treated with radical radiotherapy using conventional fractionation.

Previous reports on radiation myelopathy of the thoracic spinal cord have attempted to predict tolerance doses as a function of overall treatment time and number of fractions by extrapolation of data from collected myelopathy cases having markedly heterogeneous treatment parameters. A review of long term survivors receiving radical radiotherapy with conventional fractionation for lung and esophageal cancer was undertaken to determine if the proposed tolerance levels were consistent with clinical results in a series of patients constituting the group at risk for developing myelopathy. Spinal cord doses were determined in 58 patients who survived greater than or equal to 18 months following radiotherapy. All patients were treated with megavoltage equipment at dose rates of 200 rads/day continuously (53 patients) or 250 rads/day using split course (5 patients). Tumor doses ranged from 4500 rads to 7000 rads. Seventeen patients (29%) received less than 5000 rads to the thoracic cord, 6 patients (10%) received 5000 rads, and 35 patients (61%) received greater than 5000 rads. There were two patients who developed myelopathy. Review of our data, the clinical data of others and recent experimental results suggest that the slope of tolerance curves for thoracic spinal cord commonly utilized to express time-dose-fractionation relationships may be inappropriate. It is suggested that cord tolerance to well fractionated radiation has been previously underestimated.