All breast cancers have this in common: They begin in breast tissue. Beyond that, they aren't all the same. Doctors use these differences to decide on the most effective treatment plan for women diagnosed with the disease.
Seventy percent of breast cancers begin in cells lining a breast duct. This common type is called ductal carcinoma. Ducts are thin tubes that deliver milk to the nipple. They are connected to lobules—sections of the breast that contain milk-producing glands.
Breast cancer can also start in the lobules. Known as lobular carcinoma, this type is found in about 10% of women with breast cancer. Some women may even develop a combination of both: cancerous cells in the ducts and lobules.
Other types of breast cancer, such as inflammatory breast cancer, are less common. Inflammatory breast cancer usually causes no single tumor. Instead, skin on the breast becomes red, warm, and swollen. Another rare form is Paget disease. It's diagnosed when the cancer has spread from a duct to the nipple and the areola, the dark-colored skin surrounding the nipple.
Breast cancers also differ in how far they have spread in the body. This information is important in deciding the most appropriate treatment. Doctors use a staging system to describe the extent of the cancer. Various tests, such as a biopsy or chest X-ray, help with this classification. Doctors then label the cancer with Roman numerals I through IV and further refine those categories with A, B, and C.
Stage I cancers are early forms of the disease. They are smaller-sized tumors that haven't yet spread outside the breast. But they may be in nearby lymph nodes. Stage IV cancers are the most advanced. They have spread to other parts of the body, such as the lungs or bones. Cancers in between are stage II or III, depending on certain characteristics, including the size of the tumor.
In addition to staging, a cancer can also have a certain grade—G1, G2, or G3. Grade refers to how the cancer cells look under a microscope. A cancer's appearance can foretell how fast it will grow. Cells that look fairly normal (G1) tend to grow slowly. More abnormal looking ones (G3) may spread quickly.
Breast cancers can have other defining qualities, too. For instance, some breast cancer cells contain too much of a growth protein called HER2. These HER2-positive cancers often spread rapidly. Certain drugs, such as trastuzumab, can stunt their growth.
Genetic testing helps scientists to further differentiate the types of breast cancer. A study in the journal Nature identified 4 genetic subtypes of the disease. This work may one day help to develop more effective treatment options.
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A pathology report provides detailed information about a cancer. It helps a doctor better plan treatment. The report is put together after a biopsy—a procedure that removes a sample of cells from the breast tissue. A pathologist analyzes those cells in a lab.
The report generally includes:
Type of breast cancer, such as ductal or lobular carcinoma
Grade of the cells
Size of the tumor
Number of lymph nodes involved, if any
The extent the cancer has spread—called invasive or infiltrating carcinoma