Understanding Mucinous Adenocarcinoma (MAC)

Challenges in treating this form of colorectal cancer

Colorectal cancer is a term used to describe a wide range of cancerous malignancies affecting the colon and rectum. One form of colorectal cancer stands out as the primary form, affecting more than 125,000 Americans each year. It's called adenocarcinoma.

About 95% of all colorectal cancers in the U.S are adenocarcinomas. In addition to classic adenocarcinoma, two less common subtypes are:

MAC, the more frequently seen subtype, accounts for between 10% and 15% of all colorectal cancers.

Both MAC and signet-ring cell carcinoma are more aggressive than classic adenocarcinoma. While some studies have shown that the prognosis of mucinous adenocarcinoma colon cancer is poorer than non-mucinous adenocarcinoma colon cancer, other research has found that overall survival doesn't differ between the types.

Elderly patient with doctor
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Understanding Adenocarcinoma

Adenocarcinoma refers specifically to cancers that affect cells that are glandular in nature. "Adeno-" is the prefix for "gland" while "-carcinoma" is a term used to describe cancers that begin in the skin or in tissues that line the organs.

Adenocarcinomas develop because the colon is made up of a vast network of glands, which serve two key functions:

  • To absorb water from the feces back into the blood
  • To secrete mucus into the colon to lubricate feces as they are expelled from the body

If these cells are unable to produce ample mucus, the lining of the colon can become damaged by friction and scraping from unlubricated stools passing through. Over time, this can lead to damage on the genetic level, causing the cells to multiply abnormally without any means to prevent or temper replication. This factor triggers the formation of adenocarcinoma.

Definition of Mucinous Adenocarcinoma

MAC tumors in the colon originate from the same genetic cause as adenocarcinoma. However, there are important differences. Instead of producing less mucus, MAC cause the colon to produce far more mucus.

With MAC, your body forms tumors comprised of at least 50% mucin. Mucin is not mucus per se, but rather it is the glycoprotein component that makes up mucus along with other bodily fluids (such as saliva and breast milk). It is this mucinous component that many believe helps a tumor spread easily as it seeps beyond the walls of the tumor to the adjacent tissue.

Because of this ability to seep, MAC has long been considered a more aggressive form of adenocarcinoma.

Risk Factors

MAC has distinct characteristics that make it different from other types of colon cancer. Some of the factors that are closely associated with its development are similar to risk factors for other types of cancer. Some are different. Risk factors for MAC include:

How Mucinous Adenocarcinoma Is Diagnosed

MAC is diagnosed via a colonoscopy when polyps are biopsied or removed and later examined under a microscope. If a sample consists of at least 50% mucin, it's considered mucinous adenocarcinoma, which can require the entire tumor to be removed and studied.

The pathologist who studies the tumor will check the depth of invasion, looking at whether the tumor has spread to layers beyond the mucosa, and if so, how deep beyond it. The cancer is assigned a stage based on the TNM staging system, which grades the tumor (T), nodal (N), and metastatic (M) qualities.

  • T1 – Tumor is in the layer just below the mucosa on the inner surface of the colon
  • T2 – Tumor has spread two layers below the mucosa to the muscularis propria.
  • T3 – Tumor has invaded the muscular wall and the fat near the outer surface of the colon.
  • T4 – Tumor has spread out to the tissue on the outer surface of the colon or to surrounding organs.

Tumors are assigned a nodal stage to indicate if they have spread to the lymph nodes and a metastatic stage based on whether they have spread to distant areas beyond the colon.

​Sometimes early tests cannot provide an accurate diagnosis. If you have any early signs of colorectal cancer and you have a family history of the disease, it's important to take extra steps if early investigations are inconclusive. MAC is often easy to miss during a biopsy and may be more easily spotted using magnetic resonance imaging (MRI).

MAC is usually diagnosed in the more advanced stages of the disease. This is due, in part, to the fact that mucinous tumors have a far softer consistency than "standard" tumors and are often not detected until they're larger and more pronounced.

Even when detected early, the tumor's poorly defined shape and border make it hard for even experienced pathologists to properly stage.

Mucinous Adenocarcinoma Treatment

Researchers have long believed that the mucinous component helps a tumor spread quickly and makes it far less receptive to treatment than non-mucinous cancer of the colon. However, both of these beliefs have been challenged and are hotly debated among researchers, some of whom hypothesize that it is not the speed of development but rather the stage when the tumor is found that leads to poorer outcomes.

Comparing MAC and Adenocarcinoma

Determining if one type is more aggressive or harder to treat than another is difficult since MAC has an entirely different molecular "signature" than adenocarcinoma. While we don't yet know how this relates to disease progression—it may or may not—we do know that mucinous cancer tends to be less genetically stable (a state we refer to as microsatellite instability) than non-mucinous cancer.

Currently, mucinous colorectal adenocarcinoma is treated with the same approaches used for other types of colon cancer. These include:

However, chemotherapy may not be as effective for MAC as for other forms of colon cancer.

The excess production of mucin creates a barrier that may, in fact, prevent chemotherapy medications from effectively penetrating MAC cancer cells. In short, the chemo may not be able to get where it needs to go.

A Word From Verywell

MAC has distinct characteristics that make it harder to diagnose and make these tumors more likely to be found late. This may lead to shorter survival times. Thus, it is very important to work with your doctor if you suspect you might be at risk for MAC.

Even if an initial colonoscopy finds no cancer, do not hesitate to request further investigation if symptoms persist or worsen. Alternately, you can seek a second opinion from a colorectal specialist experienced in MAC and signet-ring cell carcinoma.

Frequently Asked Questions

  • Is mucinous adenocarcinoma aggressive?

    Mucinous adenocarcinoma (MAC) is usually considered an aggressive form of cancer. However, research increasingly shows that whether or not the tumors spread quickly depends on where the cancer originates and how early it's detected.

  • How fast does colon adenocarcinoma grow?

    In most cases, colon cancer, which is usually colon adenocarcinoma, grows slowly. This type of cancer begins as polyps in the colon and can take many years to develop into cancer. 

  • At what age are patients diagnosed with mucinous adenocarcinoma?

    The average age varies depending on where the tumors originate. In general, the average age of diagnosis is 66 to 68 years. With mucinous adenocarcinoma of the appendix, the average age drops to 58; for ovarian cancer, it’s 56.5; for uterine cancer, it’s 59.5.

  • Is adenocarcinoma colon cancer curable?

    Yes. Adenocarcinoma, the most common type of colon cancer, is often curable if it is found before it’s spread beyond the colon. Typically, surgery is used to treat colon cancer at this stage, which allows for a cure rate of 50%.

  • Can you survive mucinous adenocarcinoma?

    Yes. In most instances, the five-year survival rate for mucinous adenocarcinoma is good. For some types, the survival rate is better than for other types of cancer in the same area. This is not always the case, however.

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12 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading
  • National Cancer Institute: National Institutes of Health. "Cancer Stat Facts: Colon and Rectum Cancer." Bethesda, Maryland.

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