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About 15%-25% of breast cancers are HER2-positive. Because the treatment for HER2-positive breast cancers is different, all breast cancer tissue is tested for the presence of HER2. This is done on the surgically removed tissue sample, which is tested for hormone receptor status (estrogen and progesterone receptors), as well.
HER2-positive tumors tend to grow faster than tumors that do not express the HER2 protein. However, recurrence rates vary and depend upon more than simply the HER2 status of the tumor. Like other breast cancers, recurrence rates depend upon the extent of spread of the tumor at the time of diagnosis (stage) of the tumor along with other characteristics of the tumor. The development of anti-HER2 therapies has significantly improved the outlook for patients with HER2-positive breast cancer.
Metastatic Breast Cancer
When any type of breast cancer spreads into the surrounding tissues, it is termed an infiltrating cancer. Cancers spreading from the breast’s milk ducts into adjacent spaces are termed infiltrating ductal carcinomas. Cancers spreading from the breast’s lobules are infiltrating lobular carcinomas.
The most serious and dangerous breast cancers – wherever they arise or whatever their type – are metastatic cancers. Metastasis means that the cancer has spread from the place where it started into other tissues distant from the original tumor site.
The most common place for breast cancer to metastasize is into the lymph nodes under the arm or above the collarbone on the same side as the cancer. Other common sites of breast cancer metastasis are the brain, the bones, and the liver.
Cancers which have spread only into the lymph nodes under the arm may still be cured. Those which spread to more distant lymph nodes or other organs are not usually curable with available treatments today. Treatments can extend lives for years even in these cases.
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"HER2 Testing for Breast Cancer." Cancer.net. <http://www.cancer.net/research-and-advocacy/asco-care-and-treatment-recommendations-patients/her2-testing-breast-cancer>.
United States. National Cancer Institute. "General Information About Breast Cancer." Sept. 25, 2014. <http://www.cancer.gov/cancertopics/pdq/
United States. National Cancer Institute. "SEER Stat Fact Sheets: Breast Cancer." <http://seer.cancer.gov/statfacts/html/