Coexistent lobular carcinoma in situ and intraductal carcinoma in a single lobular-duct unit.

Current classifications of mammary carcinoma generally divide noninvasive disease into two seemingly distinct categories: in situ lobular carcinoma and intraductal carcinoma. The two are considered to be independent entities which may sometimes coexist separately in the same breast. Three cases illustrated in this report each had classical histological changes of noninvasive duct and lobular carcinoma merging in single lobular-duct systems. The morphological separation of lobular carcinoma in situ and intraductal carcinoma may not be clear-cut in all cases. The existence of combined patterns may explain why some patients with untreated in situ lobular carcinoma subsequently develop invasive duct carcinoma.