The most common breast cancers, making up about 95% of cases, are called adenocarcinomas. These cancers develop from the epithelial cells of the mammary gland, according to the National Cancer Institute. There are several types of breast cancer, each differing based on where they start and how far they have spread. The grade of a cancer is determined by evaluating three factors: the tumor's structure, the shape and size of the cell nuclei, and the number of dividing cells, which indicates how quickly the cancer is growing. This results in a score ranging from I to III, based on the Elston-Ellis histoprognostic grade. A score of 3 to 5 is classified as grade I (least aggressive), 6 to 7 as grade II (intermediate), and 8 to 9 as grade III (most aggressive). The best treatment plan for a cancer depends on both how far the tumor has spread, as determined by the TNM classification, and its aggressiveness, which is identified through histopathological analysis. The TNM classification considers three factors: the tumor's size and infiltration (T), whether it has spread to nearby lymph nodes (N), and the presence of metastases (M). Tumor size is classified from T0 (not palpable) to T4 (spreading to the chest wall or skin). Lymph node involvement ranges from N0 (no spread) to N2-3 (spread to nearby nodes). Metastases are either present (M1) or absent (M0). Based on this system, doctors classify cancer into stages from 0 (in situ, or non-invasive) to 4 (widespread metastasis). In situ carcinoma refers to tumors where cancer cells remain confined to the ducts or lobules where they started, without invading surrounding tissues or spreading. These tumors do not form metastases. The most common type is ductal carcinoma in situ (DCIS), which accounts for eight to nine out of ten in situ breast cancers. Lobular carcinoma in situ (LCIS) is rarer, making up 10 to 15% of in situ cases. LCIS is considered a risk factor for developing breast cancer, but it is not a direct precursor. Paget's disease is a type of high-grade in situ ductal carcinoma. Invasive carcinoma occurs when cancer cells break through the basement membrane of the milk ducts or lobules, allowing them to spread into the deeper layers of the breast tissue and potentially reach the lymph nodes or other parts of the body. Invasive ductal carcinoma is the most common type, while invasive lobular carcinoma is less frequent. Sarcomas and lymphomas are extremely rare forms of breast cancer that arise from non-glandular tissues such as fat or blood vessels. Metastatic breast cancer refers to cancer that has spread to other parts of the body, forming secondary tumors. According to Dr. Sophie Klinger from the Léon Bérard Cancer Fight Center, metastases can occur in any type of breast cancer and commonly affect the bones, liver, lungs, brain, and lymph nodes. Despite the advanced stage, modern treatments can often manage the disease effectively. Rare forms of breast cancer include medullary, mucinous, tubular, papillary, and inflammatory carcinomas, each with specific characteristics and prevalence rates. Hormone-dependent or hormone-sensitive breast cancers have receptors for estrogen and progesterone, making them ER+ and PR+. These cancers account for about 80% of all breast cancers and generally have a better prognosis. In contrast, triple-negative breast cancer lacks these hormone receptors and does not overexpress the HER2 protein. These cancers are more common in younger women and those with a genetic predisposition. They are often aggressive and classified as high-grade. HER2 overexpressed breast cancer has a high number of HER2 protein receptors on its cells, making it an aggressive cancer. However, immunotherapy has proven to be an effective treatment. This type of cancer affects 12 to 20% of breast cancer cases and is more likely to spread than cancers that are HER2 negative.