An Overview of Mucinous Carcinoma of the Breast

Mucinous breast cancer, also called colloid breast cancer, is a rare kind of invasive ductal breast cancer. Like other types of invasive ductal cancer, it starts in a milk duct of the breast and spreads to nearby healthy tissues. With mucinous carcinoma, the jelly-like tumor is made of abnormal cells floating in pools of mucin, a key ingredient in the slippery compound called mucus. 

Most mucinous carcinomas of the breast are estrogen- and progesterone-receptor positive. This type of breast cancer rarely spreads to the lymph nodes. It is also very treatable, and the outlook is usually good with early diagnosis and treatment.

Prevalence

Mucinous carcinoma of the breast usually appears in older women. It accounts for 1 to 7 percent of invasive breast cancers (cancers that start in the milk ducts and invade healthy tissues), according to a report in Baylor University Medical Center Proceedings. Women under age 35 are on the lower end of this spectrum, while women over age 75 are on the higher end.

Symptoms

A gelatinous tumor of mucinous carcinoma of the breast will feel like a slightly bumpy water balloon, similar to harmless fluid-filled cysts. Smaller tumors may be too little to detect with touch, but larger tumors may press on surrounding breast tissue and cause the area to feel tender.

Additional signs and symptoms may include:

• A lump under the arm that is soft to the touch
• Thickening or swelling of the breast
• Change in the size or shape of the breast
• Nipple inversion (pulling in of the nipple into the breast)
• Nipple discharge, sometimes bloody
• Changes to skin of the breast or nipples, including dimpling (having the texture of an orange peel), irritation, redness, peeling, or scaling
• Pain in the breast or nipple (rare)

Causes

Researchers don’t yet know what causes mucinous carcinoma. However, they suspect that hormonal influences (such as menopausal hormone therapy) and certain gene variants, such as BRCA1 and BRCA2, may play a part. These factors are not specific to mucinous carcinoma, however, and apply to all types of breast cancer.

Some studies suggest that HER2 gene mutations play a role in the development of mucinous carcinoma. Human epidermal growth factor receptor 2 (HER2) is associated with the promotion of cancer cell growth. 

Additionally, common risk factors associated with all types of breast cancers include:

• Being female, although men may also develop breast cancer
• Family history of breast cancer
• Giving birth for the first time after age 30
• Prior radiation therapy to the chest
• An unhealthy diet
• Being overweight and/or having a sedentary lifestyle
• Using tobacco products 

Diagnosis

Mucinous breast cancer cells are easily distinguishable from normal cells under a microscope. That said, there are some realities that prompt the need for several tests:

• Mucinous carcinoma may be found near or mixed with other more common types of breast cancer cells. Sometimes, ductal carcinoma in situ (DCIS), a cancer that has not spread outside the milk duct, is found near mucinous carcinoma cells.
• Mucinous carcinoma tumors may have areas that contain invasive ductal carcinoma (IDC) cells. If the IDC cells make more than 10 percent of the tumor, the cancer is called a mixed mucinous carcinoma. A pure mucinous tumor has at least 90 percent mucinous cells.
• Mucinous carcinoma is sometimes mistaken for a mucinous disorder called mucocele-like tumor (MLT), which is often associated with atypical ductal hyperplasia (ADH) and DCIS.

Because of this, testing for mucinous breast cancer may include:

• Physical examination: Your healthcare provider will check both breasts and the lymph nodes in your armpits to feel for any lumps or abnormalities. You will also be asked about family history and risk factors for breast cancer.
• Mammogram: A mammogram is often able to detect mucinous carcinoma. However, because it has well-defined edges and pushes against nearby healthy tissues, the mass typically looks like a benign (noncancerous) breast lump on a mammogram. (Other invasive breast cancers appear with irregular borders and calcium deposits, which appear as white specs on mammography.)
• Breast ultrasound: Ultrasound uses sound waves to obtain images of breast tissue and allows healthcare providers to see all sides of the breast. It is possible to see mucinous carcinomas on a breast ultrasound, but as with mammograms, they may be hard to distinguish from benign lumps.
• Breast MRI: A breast MRI can offer clearer images of the breast and check for other types of cancer.
• Breast biopsy: A breast biopsy involves making a small incision and taking samples from the suspicious area for examination under the microscope. Mucinous carcinoma appears as clusters of tumor cells floating in pools of mucin.

Treatment

Mucinous carcinoma should be treated to get rid of the cancer and to prevent it from returning (recurrence). Your treatment plan for mucinous carcinoma might include one or more therapies.

Surgery

With a lumpectomy, the surgeon will remove the part of the breast that contains the tumor and some of the healthy tissue around it.

Research shows low incidence of metastases to lymph nodes with mucinous carcinoma. Still, to be on the safe side, some surgeons may also opt to do a sentinel node biopsy (removal of one or two lymph nodes) to check the node or nodes closest to the tumor and see if the cancer has invaded other areas. If the mucinous tumor is 100 percent mucin, however, the cancer is unlikely to have spread.

Mastectomy may also be considered. This involves of the removal of the entire breast (or both breasts, if necessary) without removal of the underarm lymph nodes. As with lumpectomy, a sentinel node biopsy may be performed to check the lymph nodes for any signs the cancer has spread.

Radiation

Following a lumpectomy, your healthcare provider may suggest radiation therapy, which involves sending high-energy rays directly to the affected area of the breast to destroy any remaining cancer.

Cancer cells are more susceptible to radiation than healthy cells, which is the reason they are destroyed. While healthy cells may get damaged with radiation as well, they are able to repair themselves and recover, though you may experience some side effects.

Chemotherapy

Chemotherapy involves taking anti-cancer medications in pill form or intravenously (through a vein). These medicines travel through the bloodstream with a main goal to destroy any cancer cells that have separated from the initial tumor and entered the bloodstream to other parts of the body.

Hormone Therapy

Hormone therapy involves medications, such as tamoxifen, to block or lower the effects of estrogen. Since most mucinous carcinomas are estrogen- and/or progesterone positive, hormone therapy is likely to be an effective option for treating them. Moreover, hormone therapy lowers the risk for recurrence.

You should discuss all the risks and benefits of treatments with your healthcare provider. Both of you, as a team, are in the best position to determine what might be best for you.

Prognosis

Mucinous carcinoma of the breast is a medium- to- low-grade, slow-growing type of breast cancer. Since it is not aggressive, your prognosis is better than that of people with other invasive breast cancers.

According to a 2019 report in Radiology Case Reports, the five-year survival rate for mucinous carcinoma of the breast is 94 percent, compared to 82 percent for invasive ductal carcinoma. There is a more favorable long-term outlook as well.

A Word From Verywell

It is important for all women to do monthly breast self-exams, get annual OB-GYN examinations, and start having yearly mammograms from age 40 on, or earlier if breast cancer runs in your family. The earlier breast mucinous carcinoma is found, the better your chances of beating it will be. If you notice any lumps and changes to your breast during a self-exam, don’t put off telling your healthcare provider. Early detection is the key to the best possible outcomes.