Mucinous Adenocarcinoma of the Large Intestine: Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC
April 1, 2026

Mucinous adenocarcinoma is a distinct subtype of colorectal cancer. It is called “mucinous” because at least 50% of the tumor is composed of a gel-like substance called mucin. Mucin is normally produced in small amounts by the cells lining the inside of the colon, but in mucinous adenocarcinoma, cancer cells produce it in large quantities. Under the microscope, the mucin forms pools that surround and separate tumor cell clusters, giving this type of cancer a distinctive appearance.

Mucinous adenocarcinoma accounts for approximately 10 to 20% of all colorectal cancers. Compared to the more common type of colorectal cancer — conventional adenocarcinoma — mucinous adenocarcinoma is more likely to be associated with changes in the mismatch repair system, the cellular machinery responsible for fixing errors in DNA. It is also often found on the right side of the colon and may present at a more advanced stage.

What are the symptoms of mucinous adenocarcinoma?

Many people with mucinous adenocarcinoma do not notice symptoms until the tumor is large or has spread. When symptoms do occur, they may include a change in bowel habits such as diarrhea or constipation, blood in the stool or rectal bleeding, abdominal pain or cramping, unexplained weight loss, or fatigue from anemia (low red blood cell count). In some cases, there are no symptoms, and the cancer is found during routine colorectal cancer screening, such as a colonoscopy.

What causes mucinous adenocarcinoma?

Like most colorectal cancers, mucinous adenocarcinoma develops when cells in the lining of the colon accumulate genetic changes that cause them to grow abnormally. In some people, these changes are inherited. The most important inherited condition associated with mucinous adenocarcinoma is Lynch syndrome, which is caused by an inherited mutation in one of the mismatch repair genes — most commonly MLH1, MSH2, MSH6, or PMS2. People with Lynch syndrome have a significantly higher lifetime risk of colorectal cancer as well as cancers of the uterus, stomach, ovaries, and urinary tract. Because Lynch syndrome is inherited, a diagnosis of mucinous adenocarcinoma may have implications for other family members.

In other cases, the changes that cause mucinous adenocarcinoma are acquired over a person’s lifetime and are not inherited. These sporadic cases are more common and may be influenced by lifestyle factors, long-standing bowel inflammation, or environmental exposures.

How is the diagnosis made?

The diagnosis is usually made after a tissue sample is removed from the colon during a colonoscopy — either as a biopsy or a polypectomy — and examined under the microscope by a pathologist. Mucinous adenocarcinoma is identified by its characteristic appearance: large pools of mucin surrounding clusters of cancer cells, sometimes with individual signet ring cells — tumor cells with mucin inside them that pushes the nucleus to one side, giving the cell a ring-like appearance.

Once the entire tumors are removed during surgery, the pathologist examines the entire specimen and provides additional information about how deeply the tumors have grown, whether the surgical margins are clear, whether cancer has spread to lymph nodes, and the results of molecular testing. If you received a biopsy diagnosis during a colonoscopy, our guide to understanding your colonoscopy biopsy report may also be helpful.

Tumour grade

Tumor grade describes how closely the cancer cells resemble normal colon cells. It is based on how much of the tumor forms recognizable gland-like structures under the microscope. Grade is reported on a scale from 1 (well differentiated) to 4 (undifferentiated):

• Grade 1 (well differentiated) — More than 95% of the tumor forms glands. Closely resembles normal colon tissue.
• Grade 2 (moderately differentiated) — 50–95% of the tumor forms glands—some resemblance to normal tissue.
• Grade 3 (poorly differentiated) — Less than 50% of the tumor forms glands. Looks quite abnormal.
• Grade 4 (undifferentiated) — No gland formation at all.

In practice, mucinous adenocarcinoma is most often grade 2 or grade 3. Higher-grade tumors tend to behave more aggressively and are more likely to spread. However, when a mismatch repair deficiency is present (see below), even poorly differentiated mucinous adenocarcinomas can have a more favorable outcome than expected from grade alone.

Level of invasion

Invasion refers to how deeply the tumor has grown into the layers of the colon wall. Mucinous adenocarcinoma, like all colorectal cancers, begins in the innermost layer (the mucosa) and can grow progressively deeper:

• Submucosa — supportive tissue just beneath the inner lining
• Muscularis propria — the thick muscle layer that moves stool through the colon
• Pericolorectal tissues (subserosa/pericolic fat) — the fatty tissue around the outer wall of the colon
• Serosa — the outermost surface layer of the colon (not present in all parts of the colon and rectum)
• Adjacent organs — in advanced cases, the tumor may grow directly into nearby structures

The deepest layer reached by the tumors is called the level of invasion and is used to assign the pathologic tumor stage (pT). Tumors that grow deeper into the wall are more likely to spread to lymph nodes or other parts of the body.

Tumor budding refers to small clusters of 1 to 4cancer cells found at the advancing edge of the tumor, breaking away from the main mass. The number of buds visible in the most active area under the microscope is used to assign a score: low, intermediate, or high. A high tumor budding score is associated with a greater risk of the cancer spreading to lymph nodes or recurring after treatment, and may influence decisions about additional therapy. Your report will state whether tumorudding was identified and whether the core was signed.

Lymphatic invasion

Lymphatic invasion means that cancer cells have entered the small lymphatic channels within and around the colon wall. Lymphatic channels carry fluid toward nearby lymph nodes, and cancer cells found inside them have a higher chance of reaching those nodes and spreading further. Your report will note whether lymphatic invasion is present or absent. Its presence is associated with a higher risk of lymph node involvement and may influence decisions about chemotherapy after surgery.

Vascular invasion

Vascular invasion means that cancer cells have entered blood vessels. Blood vessels provide a direct route for cancer cells to travel to distant organs, such as the liver or lungs. Your report may distinguish between invasion of vessels within the colon wall (intramural) and invasion of vessels in the surrounding fat outside the wall (extramural vascular invasion). Extramural vascular invasion is considered a particularly significant finding and is associated with a higher risk of distant spread and worse outcome. In some cases, the pathologist will use special stains to confirm this finding.

Perineural invasion

Perineural invasion means that cancer cells are growing along or around a nerve. Nerves run through the wall of the colon and into surrounding tissues, and tumor cells can use them as a pathway. Perineural invasion is more common in advanced tumors and is associated with a higher risk of recurrence after treatment. Your report will state whether perineural invasion was identified.

Immune response

The immune system often tries to fight cancer by sending immune cells — particularly lymphocytes — to the area around and within the tumor. A strong immune response is generally a favorable sign and is associated with better outcomes. In colorectal cancer, a prominent immune cell infiltrate within the tumor is sometimes called a tumor-infiltrating lymphocyte (TIL) response, and a cluster of immune cells at the outer edge of the tumor is called a Crohn-like reaction. Both patterns are associated with a better prognosis and are more commonly seen in tumors with mismatch repair deficiency.

In some cases, pathologists may use immunohistochemistry to characterize the immune cells present in the tissue. The degree of immune response is increasingly recognized as an important prognostic factor in colorectal cancer.

Surgical margins

Margins are the edges of the tissue removed during surgery. The pathologist examines the cut surfaces to determine whether cancer cells are present at the edge of the specimen.

• Negative margin — No cancer cells are present at the cut edge. This is the goal of surgery and indicates the tumor was fully removed in that area.
• Positive margin — Cancer cells are present at the cut edge, raising concern that some tumor cells may remain in the body.

In colon surgery, the pathologist examines the proximal and distal margins (the two cut ends of the bowel segment removed) as well as the mesocolic (mesenteric) margin — the outer soft-tissue surface along which the colon is attached to its blood supply. For rectal cancers, the circumferential resection margin (CRM) — the distance from the tumor to the closest non-mucosal surface — is particularly important and is often specifically commented on in the report.

Treatment effect

Some patients receive chemotherapy or radiation therapy before surgery (called neoadjuvant treatment) to shrink the tumor. After surgery, the pathologist evaluates how much of the tumor responded to treatment. This is called the treatment effect or tumor regression grade, and it is typically scored as follows:

• Score 0 — No viable cancer cells remain (complete response).
• Score 1 — Single cells or rare small groups of cancer cells remain (near-complete response).
• Score 2 — Residual cancer with evidence of regression, but more than just isolated cells (partial response).
• Score 3 — Extensive residual cancer with no significant evidence of regression (poor or no response).

A complete or near-complete response (scores 0–1) is associated with a better prognosis.

Tumour deposits

Tumor deposits are discrete nodules of cancer cells found in the fat surrounding the colon or rectum, outside the main tumor mass, and without the structure of a lymph node. They represent a form of local spread and are considered an adverse prognostic factor.

If tumor deposits are present but no lymph nodes contain cancer, the nodal stage is classified as pN1c. If positive lymph nodes are also present, the nodal stage is determined by the number of involved lymph nodes, but the presence and number of tumor deposits are still recorded.

Lymph nodes

Lymph nodes are small immune organs that filter fluid draining from the tissues of the colon. During colorectal cancer surgery, the lymph nodes in the mesentery (the fatty tissue attached to the colon) are removed along with the bowel segment and examined by the pathologist.

Your report will state the total number of lymph nodes examined and the number, if any, that contain cancer. Lymph node involvement is one of the most important factors in determining the pathologic nodal stage (pN) and in deciding whether chemotherapy is recommended after surgery.

When cancer is found in a lymph node, the report may also note the size of the largest tumor deposit and whether the cancer has broken through the lymph node’s outer wall into the surrounding fat — a finding called extranodal extension, which carries a worse prognosis.

Biomarker and molecular testing

Molecular testing is an increasingly important part of the workup for mucinous adenocarcinoma. The results can guide treatment decisions, identify patients who may benefit from immunotherapy, and determine whether the cancer is associated with an inherited syndrome.

Mismatch repair (MMR) protein testing

This is the most important molecular test performed for mucinous adenocarcinoma. Using immunohistochemistry, the pathologist tests for four mismatch repair proteins: MLH1, PMS2, MSH2, and MSH6. These proteins work in pairs to repair small errors in DNA. If one or more are missing, the result is reported as mismatch repair-deficient (MMR-deficient) — also known as microsatellite instability-high (MSI-high).

Mucinous adenocarcinoma is more likely than conventional colorectal adenocarcinoma to be MMR-deficient. This finding is clinically important for two reasons:

• MMR-deficient tumors may be eligible for immunotherapy (checkpoint inhibitor drugs such as pembrolizumab), which is particularly relevant in advanced or metastatic disease.
• MMR deficiency raises the possibility of Lynch syndrome, an inherited condition. Further testing is usually performed to distinguish sporadic MMR deficiency (caused by chemical silencing of the MLH1 gene, called MLH1 promoter hypermethylation) from hereditary mutations.

If mismatch repair proteins are all present, the result is called mismatch repair proficient (MMR-proficient), meaning the repair system appears to be functioning normally.

BRAF V600E and MLH1 promoter hypermethylation

When MMR testing shows loss of MLH1 and PMS2, additional tests are performed to determine the cause. A BRAF V600E mutation strongly suggests the MMR deficiency is sporadic (not inherited) and effectively rules out Lynch syndrome — if BRAF V600E is present, genetic testing for Lynch syndrome is generally not needed. If BRAF is negative, testing for MLH1 promoter hypermethylation or germline genetic testing may follow to investigate Lynch syndrome.

KRAS and NRAS

KRAS and NRAS are genes that, when mutated, keep cancer cell growth signals permanently switched on. Mutations in either gene predict resistance to a class of targeted drugs called anti-EGFR therapies (such as cetuximab and panitumumab), which are sometimes used in advanced colorectal cancer. Because of this, KRAS and NRAS testing are standard practices before these drugs are considered. Approximately half of all colorectal cancers carry a KRAS or NRAS mutation. Mucinous adenocarcinomas — particularly those with BRAF V600E — often lack KRAS/NRAS mutations, but testing is performed in all cases.

HER2

HER2 is a protein that promotes cell growth. In a subset of colorectal cancers — especially those without KRAS, NRAS, or BRAF mutations — the HER2 gene can be amplified, meaning extra copies are present and the protein is overproduced. HER2-positive colorectal cancers may respond to HER2-targeted therapies, and HER2 testing is increasingly performed in advanced disease, particularly when standard treatments are no longer effective.

PIK3CA and PTEN

PIK3CA and PTEN are genes involved in regulating cell growth and survival. PIK3CA mutations are found in approximately 10 to 20% of colorectal cancers and may reduce the benefit of anti-EGFR therapy. Some research suggests patients with PIK3CA mutations may benefit from regular aspirin use after surgery, though this is not yet established standard practice. Loss of PTEN function, as assessed by immunohistochemistry, may also predict reduced response to anti-EGFR drugs. These markers are more commonly assessed in advanced disease.

PD-L1

PD-L1 is a protein that some cancer cells produce to help them evade immune attack. Testing for PD-L1 expression may be performed when immunotherapy is being considered. Results are often reported as a Combined Positive Score (CPS), which reflects PD-L1 expression across both tumor and immune cells. In colorectal cancer, MMR deficiency is generally a stronger predictor of immunotherapy response than PD-L1 expression alone, but PD-L1 testing may provide additional information in certain clinical settings.

For detailed explanations of these and other biomarker tests used in colorectal cancer, visit our Biomarkers and Molecular Testing section.

Pathologic stage (pTNM)

The pathologic stage is determined after surgery and describes how far the cancer has spread. It is based on the internationally recognized TNM staging system, which considers the primary tumor (T), lymph node involvement (N), and distant metastasis (M). Metastasis is typically assessed by imaging rather than by the pathologist.

Tumor stage (pT)

• pT1 — Tumor has grown into the submucosa but not the muscle layer.
• pT2 — TuTumoras grown into the muscularis propria (muscle layer).
• pT3 — Tumor has grown through the muscle layer into the pericolorectal fat or tissues.
• pT4a — Tumor has reached the outermost surface of the colon (serosa/visceral peritoneum) or has perforated through it.
• pT4b — Tumor has grown directly into a nearby organ or structure.

Nodal stage (pN)

• pN0 — No cancer found in any lymph nodes examined.
• pN1a — Cancer found in 1 lymph node.
• pN1b — Cancer found in 2 to 3 lymph nodes.
• pN1c — TTumordeposit(s) in pericolorectal fat, but no cancer in lymph nodes.
• pN2a — Cancer found in 4 to 6 lymph nodes.
• pN2b — Cancer found in 7 or more lymph nodes.
• pNX — Lymph nodes were not assessed.

What is the prognosis for mucinous adenocarcinoma?

The prognosis for mucinous adenocarcinoma depends on many factors, including the pathologic stage, surgical margin status, and molecular test results. In general, mucinous adenocarcinoma tends to present at a slightly more advanced stage than conventional colorectal adenocarcinoma, which can affect outcomes. However, MMR-deficient mucinous adenocarcinomas tend to have a more favorable prognosis than MMR-proficient tumors of similar stage, likely because of the strong immune response these tumors generate.

The most important factors affecting outcome include lymph node involvement (the difference between node-negative and node-positive disease is significant), depth of invasion, margin status, and whether the cancer has spread to distant organs. Your treatment team will use all of the information in your pathology report, along with imaging and clinical findings, to estimate your prognosis and plan the most appropriate treatment.

Questions to ask your doctor

Your pathology report contains important information that will guide your care. The following questions may help guide your next conversation with your doctor or specialist.

• What stage is my cancer, and what does that mean for my treatment and outlook?
• How deeply did the tumor invade the colon wall?
• Was cancer found in any of my lymph nodes, and if so, how many?
• Were the surgical margins clear? Was the entire tumor removed?
• Was my tumor tested for mismatch repair deficiency (MMR) or microsatellite instability (MSI) — and what was the result?
• Do I need further testing to find out whether my cancer is related to Lynch syndrome?
• Should my family members be referred for genetic counseling or Lynch syndrome testing?
• Were any high-risk features found, such as extramural vascular invasion, perineural invasion, or a high tumor budding score?
• Was KRAS, NRAS, BRAF, or HER2 testing performed, and how do those results affect my treatment options?
• Will I need chemotherapy after surgery?
• Am I a candidate for immunotherapy based on my MMR or MSI results?
• What follow-up tests and appointments will I need, and how often?