Objective To compare the spine intensity modulated radiation therapy (IMRT) and the conventional radiation therapy on the beagle spinal cord neurons,in order to prove the biological safety of IMRT of the spinal cord.Methods Twelve selected purebred beagles were randomly divided into 2 groups.A beagle clinical model of tumor was mimiced in the ninth and tenth thoracic vertebrae.Then the beagles were irradiated by 2 different models of intensity modulated radiotherapy and conventional radiation therapy,with the total irradiation doses of 50 and 70 Gy.The samples of spinal cord were taken out from the same position of the nine and tenth thoracic vertebrae at the third month after radiation.All the samples were observed by the electron microscope,and the Fas and HSP70 expression in spinal cord neurons were evaluated by immunohistochemistry method.Terminal deoxynucleatidyl transferase mediated dUTP nick and labeling (TUNEL) technique was used to examine the apoptotic cells in the spinal cord.Results The neurons in the spinal cord of IMRT group were mainly reversible injury,and those in the conventional radiation therapy were mainly apoptosis.Compared with the conventional radiation therapy group [50 Gy group,(7.3±1.1)% ;70 Gy group,(11.3 ±.4)%],the apoptosis rate of the spinal cord neurons of the intensity modulated radiotherapy group[50 Gy group,(1.2 ±0.7)%;70 Gy group(2.5 ±0.8)%] was much lower[(50 Gy group,t＝0.022,P＜0.05;70 Gy group,t＝0.017,P＜0.05)].The expression levels of Fas in the IMPT group(50 Gy group,4.6 ±0.8;70 Gy group,7.4 ±1.1)were also much lowerthan those in the other group(50 Gy group,15.1 4-6.4;70 Gy group,19.3 ±7.6.50 Gy group,t＝0.231,P＜0.05;70 Gy group,t:0.457,P＜0.05),while the expression levels of HSP70 in the IMPT group(50 Gy group,9.1 ±0.8;70 Gy group,7.3±1.4)were much higher than those in the conventional radiation therapy group(50 Gy group,2.1 ±0.9;70 Gy group,1.7±0.3;50 Gy group,t＝0.153,P＜0.05;70 Gy group,t＝0.223,P＜0.05).The expression of Fas/HSP70 was positively correlated with the apoptosis rate of the spinal cord neurons(r＝0.996,t＝1.14.P＜0.05).Conclusions The late radiotherapy response of the spinal cord neurons was obviously observed on the third month.Comparison of the morphology and apoptosis rate of the spinal cord neurons after radiotherapy,the expression of Fas and HSP70 indicated that the biological safety in the 1MRT might be much better than that the conventional radiation therapy.