Halsted radical mastectomy: See mastectomy.

Hematologist: A physician who specializes in diagnosis and treatment of conditions which occur in the blood and blood-forming tissues, including bone marrow.

Hematoma: A collection of blood outside a blood vessel caused by a leak or injury. Hematomas that occur in the breast after injury or after surgery may feel like a lump. As with other breast lumps, it’s important to have this checked to be sure that it is indeed a hematoma and not a symptom of a more serious problem.

HER2 or HER-2/neu: Human epidermal growth factor receptor 2; a protein receptor found on the surface of cells. HER2 is a key component in regulating cell growth. When the HER2 gene is altered, extra HER2 receptors may be produced. This over-expression of HER2 causes increased cell growth and reproduction, often resulting in more aggressive tumor cells.  Advanced breast cancer patients who over-express the HER2 gene may be treated with the drug, Herceptin. See also Herceptin.

Herceptin (generic name, trastuzumab): A drug used to treat advanced breast cancer patients whose tumors over-express the HER2 growth factor.  See also HER2 or HER-2/neu. 

Hereditary cancer syndrome: Conditions associated with cancers that occur in multiple family members, because of an inherited, mutated gene.

High risk: Having a higher risk of developing cancer, compared with the general population. See also risk factor.

Histologic grading: The pathologist will assign a histologic grade to a breast tumor upon examination that helps to identify the type of tumor present and the patient’s prognosis.  See also grade.

Histologic section: The preparation of tissue specimens for microscopic examination. 

Histology: The anatomical study of the microscopic structure of tissues, including cellular structure and function. See also histologic sectioning and histologic grading.

Hormone: A chemical substance released into the body by the endocrine glands, such as the thyroid, adrenal, or ovaries. The substance travels through the bloodstream and controls the actions of certain cells of organs in the body. For example, prolactin, which is produced in the pituitary gland, begins and sustains the production of milk in the breast after childbirth.

Hormone receptor assay: A test to see whether a breast tumor is likely to be affected by hormones or if it can be treated with hormones. See also estrogen receptor assay, progesterone receptor assay.

Hormone replacement therapy (HRT): The use of estrogen and/or progesterone from an outside source after the body has stopped producing these hormones. See estrogen replacement therapy for a more detailed explanation.

Hormone therapy: Treatment with hormonal drugs that interfere with hormone production or hormone action, or surgical removal of hormone-producing glands to kill cancer cells or show their growth. The most common hormonal therapy for breast cancer is the drug tamoxifen. Other hormonal therapies include megestrol, aminoglutethimide, androgens and surgical removal of the ovaries (oophorectomy). See also tamoxifen.

Hyperplasia: An abnormal increase in the number of cells in a specific area, such as the lining of the breast ducts or the lobules. By itself, hyperplasia is not cancerous, but when the proliferation (rapid growth) is marked and/or the cells are atypical (unlike normal cells), the risk of cancer developing is greater.

Hypertrophic scarring: This type of scar can occur after a surgical incision.  A very severe form of a scar, it actually grows into normal uninvolved skin and does not resolve over a period of time.  Also called keloid.