Mucinous carcinoma of the breast — sometimes called colloid carcinoma — is a rare form of invasive ductal carcinoma (cancer that begins in the milk duct and spreads beyond it into nearby healthy tissue). In this type of cancer, the tumor is made up of abnormal cells that “float” in pools of mucin, a key ingredient in the slimy, slippery substance known as mucus.
Normally, mucus lines most of the inner surface of our bodies, such as our digestive tract, lungs, liver, and other vital organs. Many types of cancer cells — including most breast cancer cells — produce some mucus. In mucinous carcinoma, however, mucin becomes part of the tumor and surrounds the breast cancer cells. Under a microscope, it looks like the cancer cells are scattered throughout pools of mucus.
Research suggests that only about 2-3% of invasive breast cancers are “pure” mucinous carcinomas — meaning that this is the only type of cancer present within the tumor. Mucinous tumor cells tend to behave less aggressively than more typical kinds of invasive ductal cancer. About 5% of invasive breast cancers appear to have a mucinous component within them, with other types of cancer cells present as well. Mucinous carcinoma is extremely rare in men.
Like other types of breast cancer, mucinous carcinoma of the breast may not cause any symptoms at first. Over time, a lump may grow large enough to be felt during breast self-exam or examination by a doctor. As with the other less common types of breast cancer, diagnosing mucinous carcinoma takes special skill and may be detected by mammogram, ultrasound, MRI or biopsy.
Although mucinous carcinoma can be diagnosed at any age, it tends to affect women after they’ve gone through menopause. Some studies have found that the average age at diagnosis is in the 60s or early 70s.