The age-related tumor incidence in Wistar-Imamichi rat.

The age-related incidences and types of spontaneous tumors were studied in 320 male and 354 female Wistar-Imamichi rats reared up to 130 weeks of age. The tumor incidence increased abruptly between 53 and 78 weeks of age, and 170 males and 269 females were shown to bear tumors during the observation period. Multiple tumors occurred in 62 males and 115 females. Most common were hypophyseal (male 69: female 224), mammary gland (29:99), testicular (74:0) and adrenal gland (26:21) tumors. The incidences of hypophyseal and testicular tumors increased with age. Tumors occurred in later stages of the observation period in the ovary, uterus and vagina (0:27), thyroid and parathyroid (13:12), skin (8:8), spleen and lymph nodes (8:7), pancreas (9:5), liver (3:7), lung (3:3), thymus (1:4), omentum and mesenterium (2:3), stomach and intestine (2:2), kidney and urinary bladder (1:2), right femur (0:1), and brain (0:1).