Morphologically normal lobules and atypical lobules postulated precancerous to ductal carcinoma were transplanted to test their biological behavior. Supravital staining disclosed the 1 to 4-mm microorgans. “Cleared” mammary fat pads of nude mice were optimal transplantation sites. Of the total of 217 transplants from 19 cancer-associated and 13 non-cancer-associated breasts, 151 survived after 2 to 27 weeks. Of 61 surviving normal-appearing lobules from cancer-associated breasts transplanted without prior in vitro maintenance, 20 (30%) dedifferentiated, and of 48 surviving lobules from noncancerous breasts, 11 (20%) dedifferentiated. Fifteen of 28 histologically normal-appearing lobules (60%) obtained from cancer-associated breasts after age 50 dedifferentiated. Thirty of 36 atypical lobules isolated from the breast tissue were obtained from cancer-associated breasts and the 5 of those that dedifferentiated came from cancer-associated breasts. Twenty of 22 (90%) dedifferentiating transplants from cancer-associated breasts showed a vascularization response, whereas 3 of 7 (43%) from non-cancer-associated breasts did so. If dedifferentiation in this experimental setting is indicative of a precancerous potential, the data on normal-appearing lobules obtained from cancer-associated breasts from women over age 50 suggest that these lobules carry the greatest precancerous potential. Such lobules probably belong to a type persisting after menopause and they may be hormonally autonomous. Such lobules might undergo further atypia in vivo and, eventually, cancerous transformation.

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Presented at the Conference “Early Lesions and the Development of Epithelial Cancer,” October 21 to 23, 1975, Bethesda, Md. Supported by USPHS Contract PHS NO1-CB-43908 from the Breast Cancer Task Force of the National Cancer Institute.

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