THE RADIATION RESPONSE OF THE CERVICAL SPINAL-CORD OF THE PIG - EFFECTS OF CHANGING THE IRRADIATED VOLUME

Purpose: An investigation of the field size effect for the cervical spinal cord of the pig after single doses of gamma-rays. In this study, clinically relevant volumes of the spinal cord were irradiated. Methods and Materials: The effects of the local irradiation of different lengths of the spinal cord (2. 5 cm, 5.0 cm, and 10.0 cm) have been evaluated in mature pigs (37-43 weeks). Single doses of 25-31 Gy were given using a Co-60 gamma-source, at a dose rate of 0.21-0.30 Gy/min. The incidence of radiation-induced paralysis was used as the endpoint. The data were analyzed using probit analysis and a normal tissue complication probability (NTCP)-model. Results: Twenty-five animals out of a total of 53 developed paralysis, with histological evidence of parenchymal and vascular changes in their white matter. The slope of the dose-response curves decreased with the decrease in field size; however, there was no significant difference at the radiation dose associated with a 50% incidence of paralysis (ED(50)) irrespective of the method of analysis. The ED(50) values +/- standard errors (+/- SE) were 27.02 +/- 0.36 Gy, 27.68 +/- 0.57 Gy, and 28.28 +/- 0.78 Gy for field lengths of 10, 5, and 2.5 cm, respectively. Analysis of the data with a normal tissue complication probability (NCTP) model gave similar results. The latent period for paralysis was 7.5-16.5 weeks with no significant differences between dose and field size. Conclusion: No significant field size-related differences in response were detectable in the cervical spinal cord of mature pigs after single dose irradiations, specifically at a clinically relevant level of effect (< ED(10)).