Abstract: Objective    To observe the occurrence of spontaneous tumors in SD rats, and to provide the background data for carrying out new drug induction trials. Methods    In a carcinogenicity test, 120 normal SD rats (SPF-grade) at 4 weeks of age were divided into male and female with the same number. All rats were observed for one week before the experiment. After 104 weeks of conventional feeding, the sedation was performed. The dead rats during the experiment and the sacrificed rats were dissected. The organs such as heart, lung, liver, spleen, kidney, adrenal gland, thymus, brain, testis and epididymis (or uterus) were weighed, and the types and incidence of spontaneous tumors in each organ were observed by an optical microscope after routine pathology. Results    A total of 113 SD rats were observed (7 rats were lost during the test), the tumor-bearing rats accounted for 66.4% (75/113), of which the benign tumors accounted for 48.7% (55/113), and the malignant tumors accounted for 17.7% (20/113). Tumors occurred in 49.1% (28/57) of male rats, of which 50.9% (26/57) were benign tumors, and 3.5% (2/57) were malignant tumors; the benign tumors were mainly pituitary adenomas (36.8%) and adrenal medulla tumor (7.0%), the malignant tumors were mainly thyroid adenocarcinoma (1.8%) and fibrosarcoma (1.8%). Female rats had tumors of 83.9% (47/56), of which the benign tumors accounted for 51.8% (29/56), and the malignant tumors were 32.1% (18/56); the benign tumors were mainly pituitary adenomas (50.0%), breast fibrous glands tumors (12.5%) and breast adenomas (12.5%), the malignant tumors were mainly breast adenocarcinoma (25.0%) and fibrosarcoma (3.6%). Conclusion    The incidence of spontaneous tumors was higher in more than 2 years old SD rats, and the incidence of benign tumors was higher than that of malignant tumors, which may be related to aging, tumors and individual differences. 

Key words: Spontaneous tumor, Pathological observation, SD rats