Approximately 95% of breast cancers are adenocarcinomas that start in the glandular tissue. Breast adenocarcinomas are further described as invasive or in situ. Invasive or infiltrating tumors have penetrated through the basement membrane into surrounding tissues. In situ tumors are malignant cells whose location is limited either to the ducts or lobules of the breast by the basement membrane.
Ductal carcinoma in situ (DCIS) is an important subtype that is most typically identified by calcifications detected as density on mammography examination. Without treatment, DCIS can spread or invade the breast ducts or lobules to involve large portions of the breast.
Another important subtype is lobular carcinoma in situ (LCIS), which often affects young women and is present in both breasts in 20-40% of cases. LCIS is typically detected on incidental biopsy and usually does not present by calcifications or densities on mammography.
Invasive breast carcinomas are detectable by a palpable mass that is radio dense on mammography. The axillary lymphatics (lymph nodes) can exhibit metastases (tumor spreading).
Metaplastic breast carcinoma, shown in the image, includes a wide variety of relatively rare tumor types (less than 1% of cases) such as squamous cell and spindle cell breast carcinomas. While these rare tumors typically do not involve lymph node metastases, they may be ER-negative, PR-negative, HER2-negative, or triple-negative (ER-, PR-, and HER2- are all negative), with poor prognosis.