by Ipshita Kak MD FRCPC
April 1, 2026
Poorly differentiated neuroendocrine carcinoma (NEC) is a rare and aggressive type of cancer that starts from neuroendocrine cells in the lining of the colon or rectum. Neuroendocrine cells are specialized cells found throughout the digestive tract that help regulate functions such as digestion by releasing hormones in response to signals from the nervous system.
There are two main subtypes of poorly differentiated neuroendocrine carcinoma, based on the size and appearance of the cancer cells under the microscope:
- Small cell neuroendocrine carcinoma — The cancer cells are small with very little surrounding cytoplasm, finely granular chromatin, inconspicuous nucleoli, and brisk cell division. This subtype is the same as small-cell carcinoma of the lung and is treated similarly.
- Large cell neuroendocrine carcinoma — The cancer cells are larger, with more visible cytoplasm and prominent nucleoli, yet retain features that identify them as neuroendocrine in origin. This subtype is also high-grade and treated aggressively.
Both subtypes are high-grade cancers that grow quickly and are likely to spread to other parts of the body. They are biologically and clinically distinct from well-differentiated neuroendocrine tumors of the colon and rectum, which grow much more slowly and have a better prognosis. Understanding the type of neuroendocrine tumor you have is important because the treatment and outlook differ significantly.
What are the symptoms?
Many people with poorly differentiated neuroendocrine carcinoma do not notice symptoms until the tumor has grown large or spread. When symptoms are present, they may include abdominal pain or cramping, changes in bowel habits such as diarrhea or constipation, rectal bleeding or blood in the stool, unexplained weight loss, and fatigue or weakness. Because these symptoms overlap with other conditions, a colonoscopy and biopsy are usually needed to confirm the diagnosis.
Unlike some well-differentiated neuroendocrine tumors, poorly differentiated neuroendocrine carcinomas do not typically produce excess hormones and rarely cause hormone-related symptoms such as flushing or diarrhea from carcinoid syndrome.
How is the diagnosis made?
The diagnosis is made by examining a tissue sample under the microscope. This sample is usually collected during a colonoscopy or at the time of surgery. A pathologist examines the tissue and identifies the characteristic features of poorly differentiated neuroendocrine carcinoma, which are confirmed by special laboratory tests called immunohistochemistry.
If you received your initial diagnosis during a colonoscopy, you may also find our guide to understanding your colonoscopy biopsy report helpful.
What does poorly differentiated neuroendocrine carcinoma look like under the microscope?
Under the microscope, poorly differentiated neuroendocrine carcinoma is composed of sheets or clusters of tightly packed, abnormal cells that look very different from normal neuroendocrine cells. The cells show signs of very rapid growth, with numerous mitotic figures (cells caught in the act of dividing) visible throughout. Areas of dead tissue, called necrosis, are common and distinguish poorly differentiated neuroendocrine carcinoma from lower-grade neuroendocrine tumors.
In small cell neuroendocrine carcinoma, the cells are small and oval with very little cytoplasm and often appear crushed or molded against each other. In large cell neuroendocrine carcinoma, the cells are larger and more irregular but still show neuroendocrine features such as granular cytoplasm and organoid growth patterns. The pathologist will specify which subtype is present in your report.
Immunohistochemistry
Immunohistochemistry is a laboratory test that uses antibodies to detect specific proteins inside tumour cells, helping to confirm the diagnosis and subtype. In poorly differentiated neuroendocrine carcinoma of the colon and rectum, the following results are typically seen:
- Synaptophysin — Usually positive. This is one of the most reliable markers of neuroendocrine differentiation.
- Chromogranin A — Often positive, though it can be weak or focal in poorly differentiated tumors.
- INSM1 — A sensitive nuclear marker of neuroendocrine differentiation. Often positive even when synaptophysin and chromogranin are weak.
- Ki-67 — Consistently very high (typically >55%, and often >80%), reflecting the rapid growth rate of these tumors. This is one of the key features used to confirm the diagnosis of poorly differentiated NEC rather than a high-grade well-differentiated NET.
- CDX2 and TTF1 — CDX2 is a marker of intestinal origin and may be positive, supporting a colorectal primary. TTF1, typically a lung and thyroid marker, can unexpectedly be positive in small cell neuroendocrine carcinoma arising outside the lung, which can occasionally be confusing. Your pathologist considers these results alongside clinical information to determine where the tumour originated.
The combination of positive neuroendocrine markers, a very high Ki-67 index, and the microscopic appearance together establishes the diagnosis.
Tumour grade
All poorly differentiated neuroendocrine carcinomas are, by definition, high-grade. This means the cancer cells grow and divide very rapidly, are highly abnormal in appearance, and are more likely to spread. The high number of mitotic figures confirms the grade, the presence of necrosis, and a Ki-67 labeling index typically greater than 55%.
It is worth noting that a small subset of well-differentiated neuroendocrine tumors can also have a high Ki-67 index (greater than 20%), and these are classified as grade 3 well-differentiated NETs. Distinguishing poorly differentiated NEC from grade 3 well-differentiated NET is important because they are treated differently, and your pathologist may comment on this distinction in the report.
Level of invasion
Invasion refers to how deeply the tumour has grown into the layers of the colon or rectal wall. Poorly differentiated neuroendocrine carcinoma begins from neuroendocrine cells in the innermost lining (the mucosa) and can extend into progressively deeper layers:
- Submucosa — the supportive tissue layer just beneath the mucosa
- Muscularis propria — the thick muscle layer that moves stool through the colon
- Pericolorectal tissues — the fatty tissue surrounding the outer wall of the colon
- Serosa — the outermost surface layer of the colon
- Adjacent organs — in advanced cases, the tumour may grow directly into nearby structures
The deepest layer the tumour has reached determines the tumour stage (pT). Because poorly differentiated neuroendocrine carcinoma is staged using the same system as colorectal adenocarcinoma (based on depth of invasion), this information is critically important for staging and treatment planning.
Perineural invasion
Perineural invasion means that tumour cells are growing along or around a nerve. When this occurs, tumour cells may use the nerve as a pathway to spread into surrounding tissues beyond the main tumour mass. Perineural invasion is associated with a higher risk of local recurrence after treatment. Your pathology report will state whether perineural invasion was identified.
Lymphovascular invasion
Lymphovascular invasion means that tumour cells have entered small blood vessels or lymphatic channels in the tissue around the tumour. Once inside these vessels, cancer cells can travel to nearby lymph nodes or distant organs such as the liver or lungs. Lymphovascular invasion is common in poorly differentiated neuroendocrine carcinoma, given its aggressive nature, and its presence is an important prognostic finding.
Surgical margins
A margin is the edge of the tissue removed during surgery. The pathologist examines the cut surfaces of the specimen to determine whether tumour cells are present at the edge.
- Negative margin — No tumour cells are present at the cut edge, suggesting the tumour was fully removed.
- Positive margin — Tumour cells are present at the cut edge, raising the concern that some cancer remains in the body. This may lead to a recommendation for additional treatment.
Treatment effect
Some patients receive chemotherapy — and occasionally radiation — before surgery (called neoadjuvant treatment) to try to shrink the tumour or control spread before an operation. After surgery, the pathologist evaluates the extent of viable tumour remaining and assesses the degree of treatment response. A complete or near-complete response, in which little or no viable cancer remains, is associated with a better outcome. Your report will describe the extent of residual tumour and any tissue changes that indicate a response to treatment.
Tumour deposits
Tumour deposits are small clusters of cancer cells found in the fat surrounding the colon or rectum, outside the main tumour mass, and separate from any lymph node. Their presence indicates local spread and is an adverse prognostic feature. If tumour deposits are present but no lymph nodes contain cancer, the nodal stage is classified as pN1c. If lymph node metastases are also present, the nodal stage is determined by the number of involved lymph nodes.
Lymph nodes
Lymph nodes are small immune organs located throughout the body. During surgery, the lymph nodes near the tumour are removed and examined by the pathologist. Your report will state the total number of lymph nodes examined and how many, if any, contain tumour cells.
Lymph node involvement is used to determine the nodal stage (pN) and is one of the most important factors in planning treatment. When cancer is found in a lymph node, the report may also note the size of the largest deposit and whether the cancer has broken through the lymph node’s outer wall — a finding called extranodal extension.
Biomarker and molecular testing
Molecular testing is increasingly used for poorly differentiated neuroendocrine carcinoma, particularly in advanced disease, to help confirm the diagnosis, guide treatment decisions, and distinguish NEC from other tumour types.
RB1 and TP53
Loss of RB1 protein and TP53 mutations are among the most common molecular alterations in poorly differentiated neuroendocrine carcinoma. These changes reflect the very abnormal and unstable nature of these cancer cells. Testing for RB1 by immunohistochemistry (loss of nuclear staining) and TP53 by immunohistochemistry (aberrant staining — either completely absent or diffusely strong) supports the diagnosis of poorly differentiated NEC. It helps distinguish it from high-grade well-differentiated NET, in which RB1 is usually intact. These findings may appear in your pathology report as part of the diagnostic workup.
KRAS, NRAS, and BRAF
Mutations in KRAS, NRAS, and BRAF are commonly found in conventional colorectal adenocarcinoma. Still, they are also present in a proportion of poorly differentiated neuroendocrine carcinomas, reflecting the fact that these tumors sometimes develop from or alongside a pre-existing adenocarcinoma. These results may be relevant if anti-EGFR or BRAF-targeted therapies are being considered. Poorly differentiated NEC mixed with adenocarcinoma components (sometimes called a mixed neuroendocrine-non-neuroendocrine neoplasm, or MiNEN) may show molecular features of both tumour types.
Mismatch repair (MMR) protein testing
Testing for mismatch repair proteins (MLH1, PMS2, MSH2, MSH6) is sometimes performed. Mismatch repair deficiency is uncommon in poorly differentiated neuroendocrine carcinoma, but when present, it may identify patients who could benefit from immunotherapy (checkpoint inhibitor drugs). If loss of one or more mismatch repair proteins is found, further testing to investigate Lynch syndrome may be recommended.
PD-L1
PD-L1 testing may be performed in advanced or metastatic disease when immunotherapy is being considered. The result helps identify patients who may be more likely to respond to checkpoint inhibitor treatment. However, immunotherapy is not yet a standard first-line option for most poorly differentiated neuroendocrine carcinomas.
For more information about molecular testing used in colorectal and neuroendocrine cancers, visit our Biomarkers and Molecular Testing section.
Pathologic stage (pTNM)
The pathologic stage describes how far the cancer has spread at the time of surgery. Poorly differentiated neuroendocrine carcinoma of the colon and rectum is staged using the same TNM system used for colorectal adenocarcinoma, based on depth of tumour invasion, lymph node involvement, and distant metastasis. This is different from the staging system used for well-differentiated neuroendocrine tumors, which is based on tumour size.
Tumour stage (pT)
- pT1 — Tumour has grown into the submucosa but not the muscle layer.
- pT2 — Tumour has grown into the muscularis propria (muscle layer).
- pT3 — Tumour has grown through the muscle layer into the pericolorectal fat or tissues.
- pT4a — Tumour has reached the outermost surface of the colon (serosa/visceral peritoneum).
- pT4b — Tumour 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 — Tumour deposit(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 poorly differentiated neuroendocrine carcinoma?
Poorly differentiated neuroendocrine carcinoma is one of the most aggressive forms of colorectal cancer. It grows rapidly, often spreads to lymph nodes and distant organs (particularly the liver) at an early stage, and has a higher risk of recurrence after treatment compared to other colorectal cancers. The prognosis depends on the extent of disease at diagnosis, particularly whether the cancer has spread to lymph nodes or distant organs, whether the tumour was completely removed with clear margins, and how well the cancer responds to chemotherapy.
Treatment typically involves a combination of surgery (when the disease is localized and resectable), systemic chemotherapy, and sometimes radiation therapy. The chemotherapy regimens used for poorly differentiated NEC are similar to those used for small-cell lung cancer, reflecting the biological similarities between these tumour types. Your doctor will discuss treatment options and likely outcomes with you based on the specific findings in your pathology report and the extent of disease on imaging.
Questions to ask your doctor
Your pathology report contains important information that will guide your care. The following questions may help you prepare for your next appointment.
- Is my tumour a small-cell or a large-cell neuroendocrine carcinoma, and does that affect my treatment?
- How is this different from a well-differentiated neuroendocrine tumour, and why does that distinction matter?
- What stage is my cancer, and has it spread to lymph nodes or other organs?
- Was the tumour completely removed with clear surgical margins?
- Were any molecular tests performed, such as RB1, TP53, or mismatch repair testing, and what did those results show?
- Is my tumour mismatch repair deficient, and could immunotherapy be an option for me?
- What chemotherapy regimen are you recommending, and why?
- Will I need radiation therapy?
- Are there any clinical trials available for my type of cancer?
- What follow-up testing and imaging will I need, and how often?
- What signs of recurrence should I watch for between appointments?
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