Glossary — M

Magnetic resonance imaging (MRI) is a diagnostic imaging technique that uses powerful magnets and radio waves — not radiation — to produce detailed, cross-sectional images of organs and soft tissues. It provides information that X-rays and CT scans often cannot, making it particularly valuable for imaging the brain, spine, breasts, and pelvic organs.

Malignant describes a tumor or growth made up of cancer cells that can invade nearby tissue and travel to distant parts of the body through the blood or lymphatic system. Unlike benign growths, malignant tumors do not stay contained and can cause serious harm if untreated.

A mammogram is the actual X-ray picture taken of your breast tissue during a mammography exam. Radiologists study these images for signs of cancer, calcifications, or other changes. Mammograms are one of the most effective tools available for finding breast cancer early, when it is most treatable.

Mammography is the process of using low-dose X-rays to image the inside of the breast. It is the gold-standard screening tool for detecting breast cancer early and is also used to evaluate suspicious areas found during a clinical or self-exam. The images produced are called mammograms.

In cancer surgery, a margin is the ring of normal tissue surrounding the tumor that is removed along with it. A pathologist measures and examines the margins to determine whether cancer cells extend to the edges of what was taken out. Clear margins mean no cancer was found at the outer edges; positive margins mean cancer cells were detected at the boundary.

A mastectomy is an operation to remove all or most of the breast tissue. It is performed to treat breast cancer when breast-conserving surgery is not appropriate, or to significantly reduce risk in people with a high genetic risk such as BRCA mutations. Several types of mastectomy exist, ranging from simple removal to skin-sparing and nipple-sparing techniques.

Mastitis is an inflammation of breast tissue that often involves a bacterial infection. It most commonly affects women who are breastfeeding, causing breast pain, swelling, warmth, and redness, sometimes accompanied by fever and flu-like symptoms. It is treated with antibiotics and continued breastfeeding or milk expression.

Menopause is defined as the point when a woman has gone 12 consecutive months without a menstrual period, marking the end of reproductive years. It typically occurs in the late 40s to early 50s and is a natural biological transition, though the years leading up to it — perimenopause — can bring a wide range of symptoms caused by shifting hormone levels.

Metastasis is the process by which cancer cells break away from the original tumor, travel through the blood or lymphatic system, and form new tumors in distant organs or tissues. A cancer that has metastasized is called metastatic. Common sites for breast cancer metastasis include the bones, lungs, liver, and brain.

Microcalcifications are very small calcium deposits in breast tissue — too small to feel — that show up as tiny white specks on a mammogram. Their size, shape, and clustering pattern help radiologists determine whether they are likely benign or warrant further investigation. Certain patterns are associated with early breast cancer, particularly DCIS.

Multiple sclerosis (MS) is a chronic autoimmune disease in which the immune system mistakenly attacks myelin, the protective sheath covering nerve fibers in the brain and spinal cord. This disrupts the electrical signals nerves use to communicate, causing a wide range of neurological symptoms. MS affects approximately 2.8 million people worldwide and is more common in women.

Myelin is a fatty, white substance that wraps around the axons of nerve cells, forming a sheath that speeds up and protects electrical nerve signals. In multiple sclerosis, the immune system attacks and destroys myelin, disrupting communication between the brain and the rest of the body. Myelin can regenerate to some extent, but repeated damage can lead to permanent nerve fiber loss.