A spotlight on Colon Neuroendocrine Neoplasms

Disclaimer: This Spotlight is for general information only and should not be used as a substitute for personalised medical advice. Neuroendocrine neoplasms (NENs) are diverse, and individual cases vary. Always discuss your specific situation, test results, and treatment options with your own specialist team. Content reflects current evidence and classifications at the time of writing and may evolve as new data emerge.

Where the Colon Begins and Ends

The colon begins at the cecum The cecum is the first part of the colon, immediately after the terminal ileum. This marks the upper boundary for defining a colon NET.
The colon continues through the ascending, transverse, descending, and sigmoid segments These four anatomical sections make up the full length of the colon between the cecum and the rectosigmoid junction.
The colon ends at the rectosigmoid junction This is the point where the taeniae coli end and the bowel transitions into the smooth muscular tube of the rectum. Anything below this point is rectum, not colon.

Midgut/hindgut overlaps. However, this is often confusing when you consider the midgut–hindgut boundary frequently used used in NET which is an embryologic division, not an anatomical one. This boundary sits inside the transverse colon: the midgut ends at the proximal two‑thirds of the transverse colon, and the hindgut begins at the distal one‑third. This classification explains below why some colon segments behave differently, but it does not define where the colon itself starts or ends. Anatomically, the colon still begins at the cecum and ends at the rectosigmoid junction, while embryologically it spans both midgut and hindgut regions. This is why midgut vs hindgut is a biological nuance in colon NENs, whereas colon boundaries are anatomical.

Epidemiology

Some large epidemiology datasets group sites differently from clinical classification. A major 2025 US analysis grouped small intestine and cecum together, separate from colon and rectum.

This grouping reflects the midgut embryologic origin of the cecum and historical coding issues, not a change in WHO classification.
Clinically, the cecum remains part of the colon, and caecal NETs are classified as colon NETs. Overall, according to ENETS, right colon NET are uncommon, and ileal NET (small intestine) remain significantly more frequent despite both being midgut‑derived.
Colorectal NET figures are dominated by rectal NETs, so combined statistics can obscure the rarity and later stage of colon NET.

Some datasets group cecum with small intestine because both are midgut. Clinically, cecum is colon — and “colorectal NET” numbers are overwhelmingly rectal.

Colon NET Is a Heterogeneous Group

Colon NETs are not one disease. They vary because the colon spans a long anatomical and embryologic segment.

Location heterogeneity — because different colon segments arise from different embryologic regions, producing variation in tumour behaviour. ENETS highlights that there are no dedicated treatment data for metastatic right‑colon NET G1/G2, and carcinoid syndrome is rare compared with ileal NET (small intestine). Historically, because right‑colon NET arises in a midgut embryologic region, these tumours were included in “midgut NET” trials such as PROMID, NETTER‑1, and the midgut subgroup of CLARINET, although outcomes for this small subgroup were not reported separately. ENETS concludes that it is reasonable to treat right‑colon NET G1/G2 using the same algorithm as small‑intestinal (midgut) NET G1/G2, while NET G3 of the right colon is rare and managed according to general colorectal NET G3 principles. ENETS also confirms caecal NETs are the most indolent in Colon NETs.
Grade heterogeneity — because colon NETs originate from different neuroendocrine cell populations, leading to a range of proliferative activity.
Stage heterogeneity — because colon NETs are silent until late, and late‑stage disease produces the symptoms (pain, obstruction, bleeding) that finally lead to diagnosis. Non-functional states also add to the issue.
Behavioural heterogeneity — because tumour biology varies across segments, resulting in different growth rates and metastatic patterns. According to ENETS, Grade 3 tumours of the main part of the colon are often neuroendocrine carcinomas (NEC) more than NET G3, and can be mixed neuroendocrine non-neuroendocrine neoplasms (MiNEN) with an adenocarcinoma/adenoma component.

Stage at Diagnosis

SEER‑aligned GI‑NET analyses consistently show:

Colon NETs are more often regional or distant at diagnosis (vs Rectal NETs are mostly localized)
Combined “colorectal NET” staging figures may therefore appear misleading. Not helped by a lack of well differentiated Colon NET data. This will take time to come through.
Well differentiated Colon NETs has its own standalone staging definitions. Colon NEC uses same staging definitions as Colon Adenocarcinoma.

Functional Syndromes

Colon NETs are overwhelmingly non‑functional.

Carcinoid syndrome is rare
Hormone‑driven symptoms are uncommon
Symptoms usually arise from mass effect, not hormones

Genetics

Colon NETs have no strong inherited pattern and no routine germline testing requirement.

Inherited syndromes are very rare
Somatic genetics are quiet
No hallmark mutation
NEC/MiNEN may show TP53/RB1 loss which can help distinguish G3 well differentiated from poorly differentiated.

G3 Colon NET

G3 colon NET is uncommon, but it exists.

G3 NET of the colon is uncommon. ENETS notes that high‑grade colonic NEN are more often poorly differentiated NEC than well‑differentiated G3 NET, with many cases classified as MiNEN. No reliable percentage estimates exist for colon G3 NET or colon NEC.
Combined “colorectal G3 NET” figures possibly distort the picture.

Poorly Differentiated Colon NEC and MiNEN

Poorly differentiated colon NEC is a distinct entity from colon NET. It arises in the colon but behaves biologically like other high‑grade NECs, with rapid growth, early spread, and TP53/RB1 loss as hallmark features. These tumours are managed using NEC‑specific pathways rather than NET pathways. Colon MiNEN (mixed neuroendocrine–non‑neuroendocrine neoplasm) contains both a neuroendocrine component and a non‑neuroendocrine carcinoma component. Clinical behaviour is usually driven by the most aggressive component, which is often NEC. Both NEC and MiNEN are rare, high‑grade, and biologically separate from well‑differentiated colon NET. ENETS notes that tumours arising in the main part of the colon (i.e., beyond the cecum) are more often poorly differentiated neuroendocrine carcinomas (NEC) than well‑differentiated G3 NET. Many are mixed neuroendocrine–non‑neuroendocrine neoplasms (MiNEN), typically containing an adenocarcinoma or adenoma component. This reflects the fact that the biology of these tumours aligns more closely with colorectal carcinoma pathways than with well‑differentiated NET biology, which is why colon NEC uses adenocarcinoma TNM staging rather than the NET staging system.

Additional Key Point

Colon NET Is Not Rising in Early‑Onset Disease

According to medical news in the 2020s, early‑onset colorectal adenocarcinoma is rising. However, notwithstanding registry accuracy and meta data disadvantages to NENs, it does not appear to be reflected in Colon NET. That said the figures for G3 Colon NET/NEC may reflect better if only we had some. Also worth noting that Rectal NET is rising due to incidental detection and screening, not biology.

Resources

1. European Neuroendocrine Tumor Society (ENETS) 2023 guidance paper for colorectal neuroendocrine tumours

Ronny Allan’s Spotlight on NENs Series

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