How common are mammary and pituitary tumors?
Unspayed female rats are prone to developing mammary tumors, pituitary tumors and other estrogen-dependent tumors as they age. At the time of menopause (450-540 days) mammary tumor incidence increases sharply and pituitary tumors begin to appear. A second increase in mammary and pituitary tumors occurs around the end of the second year (from 600-800 days) (Durbin 1966). Most of the tumor susceptibility research in unspayed rats has been done on different strains of laboratory rats. Strains and populations differ in how susceptible the females are to developing tumors. Susceptibility may vary widely. Below are some studies that examine the percentages of female rats of a particular strain that develop mammary tumors. Where reported, the percentage of benign vs. cancerous mammary tumors is included below: • Wistar rats: • 25.3% of females developed benign mammary fibroadenomas, 13.1% developed mammary carcinomas (Walsh and Poteracki 1994) • 36% of females developed benign mammary fibroad
