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Development of Spontaneous Mammary Tumors over the Life-Span of the Female Charles River (Sprague-Dawley) Rat: The Influence of Ovariectomy, Thyroidectomy, and Adrenalectomy-Ovariectomy¹

Division of Biology and Medicine, Lawrence Radiation Laboratory, University of California, Berkeley, California

Several groups of virgin female cesarean-originated, barrier-sustained Charles River CD rats were observed for incidence of mammary tumors over their life-span. Maximum life-span ranged from 885 to 1040 days. Crude MT³ incidence for all groups was 61.7%. When a life table method was used to correct for extraneous deaths, the mean MT incidence for all groups was 71.5 ± 5.7%. The age at onset of the median MT was 671 ± 41 days. Cumulative MT incidence agreed closely among control groups early and late in life, with a greater variability appearing from the 500th to the 850th day of age.

In normally aging rats there were at least 2 age-related changes leading to abrupt increases in the rate of development of MT—one occurring at about the 500th and th eother at about the 660th day of life.

Both total MT incidence and MT incidence rate were the same for uniparous rats as for virgin controls. Thyroidectomy did not reduce the life-span incidence of MT, but did postpone slightly those MT's arising late in life. Ovariectomy nearly eliminated MT development; only 6.7% of ovariectomized rats developed MT over a maximum life-span of 1295 days. No MT developed in adrenalectomized-ovariectomized rats although they lived long enough (maximum life-span 1110 days) to have permitted observation of late-appearing MT's.

The proportion of MT's diagnosed as carcinomas in the individual control groups was highly variable, 18.6 ± 12.1% In these studies carcinomas did not appear to arise earlier than benign fibroadenomata. Other superficial tumors arising in tissues different from the breast are tabulated.

The results of these studies are compared with those from other laboratories using the same or a related strain of rat. The relationships between development of spontaneous MT and the age-related changes in the ovary and pituitary gland are discussed.

¹ Work performed under auspices of the U. S. Atomic Energy Commission.

³ Abbreviations used in text: MT = Mammary Tumor; FA = Fibroadenoma; MC = Mammary Carcinoma; UP = Uniparous; TX = Thyroidectomized; OX = Ovariectomized; ADX-OX = Adrenalectomized-Ovariectomized; T_a = Age at onset of first mammary tumor.

² Present address: Kansas City General Hospital, Kansas City, Missouri.