What Are Mucinous (Colloid) Cancers? Characteristics of Mucinous Tumors in the Breast, Colon, and Ovary

Tumors That Make a Jelly-Like Secretion: What Mucinous Cancers Mean in Each Organ

The word "mucinous," or its older name "colloid," appears often in pathology reports. It means the tumor produces a jelly-like secretion called mucin. But what it means depends very much on the organ: in the breast, mucinous carcinoma is usually a subtype with a favorable course; in the colon and rectum, it is a finding that can affect how well treatment works; and in the ovary, it first calls for checking whether the tumor has spread there from another organ. In this article, we look at mucinous tumors of the breast, colon and rectum, and ovary, the "jelly belly" condition that starts in the appendix, and mucinous cancers in other organs, drawing on current sources and large patient series (for example, a review of colorectal mucinous adenocarcinoma). We have covered the basic terms often seen in pathology reports, the Ki-67 value, and invasive micropapillary carcinoma, another special type of breast cancer, in separate articles.

Why does this matter?

Mucin is a slippery, jelly-like substance made by many glands in the body; it protects the digestive system and the airways. Some cancers produce this secretion in excess, and the tumor cells can be seen floating in pools of mucin. Pathologists call these tumors "mucinous carcinoma"; older sources and some reports also use the term "colloid carcinoma."

Although the mucinous pattern carries the same name, it means something different in each organ. That is why the word "mucinous" can be confusing for patients: the same word can be reassuring in the breast, yet in colon cancer it can be information that calls for a more careful treatment plan. Mucin can also make a tumor look different on imaging and can affect how some tests are interpreted (what is PET-CT).

What is mucinous (colloid) cancer?

Mucinous (colloid) cancer is a type of cancer in which the tumor cells produce large amounts of a jelly-like secretion (mucin) and float in pools of this mucin. It can occur in the breast, colon and rectum, ovary, appendix, pancreas, stomach and lung. What it means depends on the organ: in the breast it usually has a favorable course; in the colon and rectum it can affect response to treatment; and in the ovary, spread from the digestive system must be ruled out.

Mucinous tumors by organ: an overview

Rates are compiled from different series and vary by region and diagnostic criteria.

Mucinous carcinoma of the breast

Mucinous breast cancer is one of the "special types" of breast cancer. In the pure type, more than 90% of the invasive part of the tumor is mucinous; tumors with a smaller share of mucinous areas are called the "mixed" type and behave more like classic breast cancer (breast cancer stages and treatment).

• Who gets it: Usually at an older age; in US cancer registry data, the median age at diagnosis is 71.
• Biological features: Usually hormone receptor-positive and HER2-negative; slow-growing tumors.
• Spread: 86% of patients have disease limited to the breast at diagnosis; regional lymph node involvement is 12% and distant spread 2%.
• Imaging: Because it can look soft and smooth-edged, it can be mistaken for a benign lump on mammography and ultrasound; that is why a biopsy of suspicious lumps is important.

As with classic breast cancer, treatment is planned according to tumor size, lymph node status, hormone receptor status and HER2 status. Hormone therapy is the main treatment; because lymph node involvement is uncommon in pure mucinous tumors, chemotherapy is needed less often. If the lymph nodes are involved or the tumor is the mixed type, treatment is planned as for classic breast cancer (treatment of hormone-positive breast cancer, patients who will not need chemotherapy).

Mucinous adenocarcinoma of the colon and rectum

In the colon and rectum, a tumor is called "mucinous adenocarcinoma" if at least 50% of it is made up of extracellular mucin, and "adenocarcinoma with mucinous features" if the share is between 10% and 50%. How common it is varies by region: around 3% in Japan, and 10–15% in the US and Europe (colon and rectal cancer).

• Location: More common in the right colon, less common in the rectum.
• Molecular features: Strongly linked to microsatellite instability (MSI-high), BRAF mutation and CIMP (gene silencing). Because immunotherapy is an important option for MSI-high tumors, these tests matter especially in mucinous tumors (immunotherapy in dMMR colon cancer, encorafenib for BRAF mutation).
• Spread: Spread to the lining of the abdomen (the peritoneum) is more common.
• Course: In early-stage colon cancer, outcomes in most studies are similar to those of classic adenocarcinoma; in advanced disease and in the rectum, the course is less favorable.

Mucinous tumors of the ovary

Mucinous tumors of the ovary can be benign (cystadenoma), borderline, or malignant (carcinoma). When it comes to mucinous ovarian carcinoma, the first question is this: does the tumor really start in the ovary, or has it spread to the ovary from a digestive organ such as the appendix, colon, stomach or pancreas? In a study published in 2010, about 61% of tumors assessed as advanced-stage ovarian mucinous carcinoma turned out to have actually spread from other organs (ovarian cancer).

• Frequency and age: True ovarian mucinous carcinoma makes up 3–5% of epithelial ovarian cancers; it is the most common type of ovarian cancer in women under 40.
• Stage: 70–80% of patients are diagnosed at stage I–II; at an early stage, 5-year overall survival is above 90%.
• Growth pattern: The "expansile" (pushing) type has a better course than the "infiltrative" (spreading) type: recurrence rates are 6.9% vs 24.5%.
• Molecular features: KRAS mutation in about 66% and HER2 amplification in about 27%; unlike serous ovarian cancer, a link with BRCA and a benefit from PARP inhibitors are not clear.
• Treatment: Surgery is the main treatment. For the expansile type at stage IA, fertility-sparing surgery is an option; lymph node dissection is done on a risk-adapted basis. At an advanced stage, response to standard platinum chemotherapy is weaker; some centers prefer regimens used for digestive system cancers. Trastuzumab is being studied in HER2-positive tumors (ovarian cancer treatment, who benefits from PARP inhibitors).

The appendix and "jelly belly" (pseudomyxoma peritonei)

When the appendix ruptures, mucinous tumors of the appendix can cause mucin and tumor cells to spread inside the abdomen. This jelly-like material that slowly builds up in the abdomen is called pseudomyxoma peritonei ("jelly belly"). Patients may come in with a swollen abdomen, a change in weight or hernia-like symptoms; in women, it may first be noticed as an ovarian mass. That is why assessment of the appendix is also recommended for patients found to have a mucinous tumor in the ovary.

Pseudomyxoma peritonei is classified as low grade or high grade. The standard treatment is cytoreductive surgery (removal of all visible tumor) together with heated chemotherapy delivered inside the abdomen (HIPEC), done at experienced centers; in low-grade disease, this treatment can lead to long survival (appendix cancer and recurrence risk).

Mucinous cancers in other organs

• Pancreas: Colloid carcinoma of the pancreas usually develops from a mucinous cystic lesion (IPMN) and has a better course than classic pancreatic cancer (pancreatic cancer).
• Stomach: Mucinous and signet-ring cell types can occur; the signet-ring cell type behaves more aggressively (stomach cancer).
• Lung: Invasive mucinous adenocarcinoma can involve wide areas of the lung in a way that can mimic pneumonia; KRAS mutations are common (lung cancer).
• Other: Mucinous cancers can also rarely occur in the prostate, bladder and salivary glands.

Imaging and tumor markers

• MRI: Mucin looks distinctly bright on T2-weighted MRI images; this feature helps with diagnosis, especially in the rectum.
• PET-CT: Because mucinous tumors have few cells and a lot of mucin, uptake on PET can be low; this can make the tumor look smaller than it is, or not show up at all (misleading PET-CT results).
• Tumor markers: CEA and CA 19-9 can be raised in mucinous colon and appendix tumors, and CA 125 in ovarian tumors; however, none of them makes a diagnosis on its own (CEA test, CA 125 test).

Questions you may want to ask your doctor

• Is my tumor purely mucinous, or does it contain a mucinous component? What is the proportion?
• Are there any signet-ring cells? If so, what is their proportion?
• For colon or rectal cancer: Have MSI/MMR, RAS and BRAF tests been done?
• For an ovarian tumor: Has spread from the digestive system been ruled out? Have an endoscopy and an assessment of the appendix been done? Has HER2 been tested?
• Which imaging method will be used for follow-up? Have the limitations of PET-CT been taken into account?

For screening and early diagnosis of colon cancer, see our early diagnosis guide.

Frequently asked questions

What does mucinous (colloid) cancer mean?

It is a type of cancer in which the tumor cells produce a jelly-like secretion (mucin) and float in pools of this mucin. "Colloid cancer" is an older name. It can occur in the breast, colon and rectum, ovary, appendix, pancreas, stomach and lung; what it means depends on the organ.

Is mucinous breast cancer benign?

No, it is a malignant cancer; however, pure mucinous breast cancer usually grows slowly, is most often hormone receptor-positive and rarely spreads to the lymph nodes. In a large US series, 10-year breast cancer-specific survival was 89%, compared with 72% for classic breast cancer.

Is the mucinous type worse in colon cancer?

At an early stage, outcomes in most studies are similar to those of the classic type. However, mucinous tumors spread to the lining of the abdomen more often and, at an advanced stage, may respond less well to chemotherapy, and in the rectum to chemoradiotherapy. Because they are more likely to be MSI-high and BRAF-mutated, these tests are important when choosing treatment.

What happens if a mucinous tumor is found in the ovary?

First, doctors check whether the tumor really started in the ovary or has spread from the appendix, colon, stomach or pancreas. Most true ovarian mucinous carcinomas are caught at an early stage, and surgery gives good results; at an advanced stage, treatment is more difficult.

Glossary

Mucin

A slippery, jelly-like substance made by glands.

Mucinous carcinoma

A malignant tumor that produces abundant mucin outside the cells; its older name is colloid carcinoma.

Signet-ring cell carcinoma

A type of tumor, usually more aggressive, in which mucin collects inside the cell and pushes the nucleus to the side.

Pseudomyxoma peritonei

The "jelly belly" condition caused by a mucinous tumor of the appendix spreading inside the abdomen.

HIPEC

Heated chemotherapy delivered inside the abdomen during surgery.

MSI-high

A buildup of many mutations in the tumor because of a fault in the DNA repair system; linked to response to immunotherapy.

Borderline tumor

An ovarian tumor whose behavior lies between benign and malignant.

Expansile / infiltrative pattern

In mucinous ovarian carcinoma, whether the tumor grows by pushing against or by spreading into the surrounding tissue; it affects the outlook.