A spotlight on Rectal Neuroendocrine Neoplasms

What are Rectal NENs

Rectal Neuroendocrine Neoplasms (NENs) (rNENs) account for approximately 1-2% of all primary cancers in the rectum. The other main cancer types found in the rectum are regular adenocarcinomas (the vast majority) and some other rare types including Gastrointestinal stromal tumors (GIST) and Lymphoma.

The vast majority of rNENs will be well differentiated, i.e. rectal Neuroendocrine Tumours (rNETs) and are mostly indolent tumours treated effectively with surgical procedures if less than 1cm in size. Above that size there are different scenarios.

Most will be found incidentally upon presentation of classic rectal symptoms or via colonoscopy screenings and faecal tests which lead to colonoscopy follow ups. They are typically diagnosed in older patients but the incidence in younger patients is on the increase, see author’s note below in epidemiology section.

Poorly differentiated rectal Neuroendocrine Carcinomas (rNEC) are much less common than well differentiated rNETs.

Mixed neuroendocrine-non-neuroendocrine neoplasms (MiNEN) are possible but ultra rare in that they are only found in case studies.

Anatomy

The rectum is the last several inches of the large intestine closest to the anus.

The location of the Rectum is shown below (this may be important for understanding radiology/pathology reports).

Epidemiology

Many sites combine colon and rectal NENs together for epidemiological data but rectal are more common than Colon NENs. Colorectal NECs are rare but Rectal NETs are said to account for up to 27% of all NETs and 20% of gastrointestinal NETs. Colon NETs, which are less frequent, account for only 9.6% of all NETs and 14.1% of gastrointestinal NETs,

The rectum appears to be the second most common place for NENs along the digestive tract and the third most common NEN behind lung and small intestine. But there is a global variation and the biggest NEN study, US SEER 2012, is now significantly out of date.

Poorly differentiated rectal Neuroendocrine Carcinomas (rNEC) are much less common than well differentiated rNETs. Only around 0.5% of all colorectal carcinomas are rNEC. Around 35% are Rectal located with a 40/60 split between small and large cell.

Author’s note: The incidence of rNETs may be understated given the output from this study which claims that rNETs were common, representing approximately 4% to 20% of all colorectal and 8% to 34% of all rectal cancer cases, depending on age group and calendar year. Click here.

Syndromes and Rectal NENs

So called ‘carcinoid syndrome‘ is rare in rNENs, mostly due to the low stage and curative surgery approach. But any manifestation of carcinoid syndrome would be similar to small intestine NETs, e.g. almost always in metastatic cases.

Hereditary connections

There are no known hereditary connections with Rectal NENs. However, Familial adenomatous polyposis (FAP) is said to account for 1% of all cancers in the colon or rectum (given the numbers above, highly unlikely in rNENs).

Guidelines

There are numerous clinical practice guidelines on management of rNETs where management of these tumours is stratified according to risk of locoregional and distant metastasis. Most rNETs are less than 1cm (80-90%) and when there is negative margin, they will be removed with curative intent with no further surveillance necessary according to most guidelines. Tumours less than 1cm but without negative margins and tumours between 1 and 2cm may need more complex handling. Tumours over 2cm may need larger resections and longer surveillance periods. Metastatic cases would be considered alongside other metastatic NENs. The best diagrams explaining this are NCCN referenced below.

You may find slight differences in consensus of the various regional guidelines regarding tumours that measure 1-2 cm given the number of factors involved. The ENETS paper on colorectal NETs provides useful technical detail on Rectal NETs. See further reading below.

Guidelines for rNEC and MiNEN will be similar to other NEC and MiNEN.

2025 New Statistics

Conclusions

Death attributable to rNETs did not occur among patients who underwent at least 10 years of follow-up after endoscopic resection.

A Spotlight on Neuroendocrine Neoplasms – a series from Ronny Allan

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