THE INFLUENCE OF FIELD SIZE ON THE LATE TOLERANCE OF THE RAT SPINAL-CORD TO SINGLE DOSES OF X-RAYS

Varying lengths of the cervical spinal cord of rats were irradiated with single doses of X rays. Dose-related changes in the latency for the development of paralysis, of for the presence of histological lesions in the spinal cord, indicated a dependency on the length of spine irradiated. The dose associated with a specified latency increased as the field size was reduced from 16 mm to 4 mm. A more precise indication of the importance of field size came from a determination of the ED50 values for rats developing paralysis in < 30 weeks of irradiation or from those that showed neurological signs, or histological evidence, or irradiation damage, occuring after latent periods of > 30 weeks. These end-points were primarily related to white-matter necrosis and gross vascular damage respectively. For paralysis in > 30 weeks the ED50 increased markedly from 21.5 ± 0.3 Gy for a 16 mm field to 50.98 ± 2.28 Gy when a 4 mm length of cord was irradiated. For vascular lesions only a small change in ED50 value was found, from 20.0 ± 0.5 Gy to 25.58 ± 2.78 Gy for 16 mm and 4 mm fields respectively. At ≤ ED25 no evidence for a field-size effect was seen for this late lesion. These results were examined in the light of some old and some more recently analysed clinical data for radiation myelopathy. It is hoped that this will initiate other investigations in this important area of radiotherapy practice.