Adenoid cystic carcinoma of the breast may or may not belong to carcinoma in situ, which needs to be determined according to the specific lesion. Carcinoma in situ is determined according to the stage of breast cancer development, indicating that the cancer cells exist inside the ducts and have not yet broken through the ductal basement membrane, while adenoid cystic carcinoma of the breast is categorized according to the pathology of the breast cancer, and the two criteria are not the same, so there is no way to determine whether it indicates carcinoma in situ. If the primary lesion is inside the breast and the lesion is confined, it belongs to breast carcinoma in situ; however, if the primary lesion is in other parts of the body and the breast adenoid cystic carcinoma has metastasized to the breast from other parts of the body, it belongs to secondary breast cancer, not breast carcinoma in situ.