What are Appendiceal NENs
Appendiceal Neuroendocrine Neoplasms (NENs) account for approximately 60% of all primary cancers in the appendix. The other main cancer types found in the appendix are regular adenocarcinomas (including signet ring cell and goblet cell) and mucinous neoplasms such as Pseudomyxoma Peritonei (PMP) or Low-Grade Mucinous Neoplasms (LAMD).
Appendiceal neuroendocrine tumours (aNET) (i.e. well differentiated) account for 75% of all aNENs, are mostly indolent tumours treated effectively with simple appendectomy. In fact, most are incidentally found upon presentation of right lower quadrant (RLQ) pain in keeping with appendicitis. They are typically (but not exclusively) diagnosed in younger patients.
Poorly differentiated appendiceal Neuroendocrine Carcinomas (aNEC) resemble small-cell or large-cell neuroendocrine carcinomas of the lung, have aggressive behaviour, and usually present with metastatic disease at diagnosis.
Mixed neuroendocrine-non-neuroendocrine neoplasms (MiNEN) are possible, particularly with Goblet Cell Adenocarcinoma. T0 qualify as a MiNEN, there should be at least 30% cellular positivity in both cancers.
Anatomy
The appendix is a thin tube that is joined to the large intestine close to the junction (the cecum) of the large and small intestine. It sits in the lower right part of your abdomen. Scientists aren’t sure what the appendix does as a job, but the most common hypothesis is that it was once a store of good bacteria, and another is that is a remnant of human evolution.
The location of the Appendix is shown below, worth nothing the base of the appendix is at the joint of the cecum and the tip is the end furthest away from the cecum (this is important for understanding radiology/pathology reports).
Epidemiology
The appendix is the third most common place for NETs along the digestive tract and aNETs are the most common neoplasms of the appendix. aNETs are the fifth most common well differentiated NET behind lung, small intestine, rectum and stomach.
The 20-year prevalence of aNETs between 1993-2012, based on the US SEER database, was 2 per 100,000 persons. In the follow on study cover 20 years up to 2021, the summary stated “The appendix site specifically showed a notable increase in incidence over the past two decades.”
It follows on to say most aNETs are diagnosed at an early, localized stage, often incidentally during an appendectomy for appendicitis or other abdominal issues. In a study of 4,870 patients, 86.0% had T1 stage disease (least risk on the TNM scale). Appendiceal NETs generally have a favourable prognosis, especially when localised. The majority of cases (89.4%) were low-grade (Grade 1 (86.2%) or G2 (3.2%)) tumours, which exhibit slow growth. In guidelines, removal of small low grade tumours with R0 margins is considered curative.
Most commonly seen at the tip of the appendix (67% of adult patients, 73% of paediatric patients). Most appendiceal NETs are < 2 cm. 52 – 62% are < 1 cm, 28 – 30% are 1 – 2 cm. 8 – 19% are > 2 cm.
aNETs rarely cause Carcinoid Syndrome (see below).
All of the above figures may vary between countries. The biggest study, US SEER 2025 indicates diagnoses of Appendiceal NETs has increased by almost 13% over a period of 20 years.
Syndromes and Appendiceal NENs
So called ‘carcinoid syndrome‘ is rare in aNENs, 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
Most Multiple Endocrine Neoplasia (MEN) cases are related to foregut NETs but having a family history of multiple endocrine neoplasia type 1 (MEN1) syndrome can be a risk factor for appendiceal NET. For most, a family history of MEN1 will already be known.
Guidelines
There are numerous clinical practice guidelines on management of aNETs where management of these tumours is stratified according to risk of locoregional and distant metastasis. Most aNETs are less than 1cm and will be removed with curative intent and no further surveillance would be necessary. Tumours of 2cm plus would need to consider a right hemicolectomy.
However, there remains a lack of consensus regarding tumours that measure 1-2 cm. In these cases, some histopathological features such as size, tumour grade, presence of lymphovascular invasion, or mesoappendix infiltration must also be considered. Computed tomography or magnetic resonance imaging scans are recommended for evaluating the presence of additional disease, except in the case of tumours smaller than 1 cm without additional risk factors. Somatostatin receptor imaging should be considered in cases with suspected residual or distant disease. In general, controversy exists regarding the necessity of oncologic right hemicolectomy (RH) in aNET patients with histologic features suggestive of more aggressive disease.
This comparison of the main Appendiceal NET guidelines summaries the controversary. Consequently, staging definitions are different between Europe and North America.
Guidelines for aNEC and MiNEN will be similar to other NEC and MiNEN.
Staging of Appendiceal NETs
ENETS Guidelines for well differentiated Appendiceal NETs.
This is a very useful summary for those interested in the detail. Click here to read.
Guideline Controversy – Right Hemicolectomy for aNETs size 1-2cm
One of the issues faced by guideline writers is a lack of data assessing the risks of doing something or not doing something. With aNETs, and as mentioned above, there has been controversy over the requirement for a right hemicolectomy to ensure margins in case of aNETs with a tumour size between 1 and 2cm.
As I was writing this blog post, the following study came out of the annual European NET Society conference, a timely addition to this blog post.
Findings
282 patients with suspected appendiceal tumours were identified, of whom 278 with an appendiceal NET of 1–2 cm in size were included. 163 (59%) had an appendectomy and 115 (41%) had a right-sided hemicolectomy, 110 (40%) were men, 168 (60%) were women, and mean age at initial surgery was 36·0 years (SD 18·2). Median follow-up was 13·0 years (IQR 11·0–15·6). After centralised histopathological review, appendiceal NETs were classified as a possible or probable primary tumour in two (1%) of 278 patients with distant peritoneal metastases and in two (1%) 278 patients with distant metastases in the liver. All metastases were diagnosed synchronously with no tumour-related deaths during follow-up. Regional lymph node metastases were found in 22 (20%) of 112 patients with right-sided hemicolectomy with available data. On the basis of histopathological risk factors, we estimated that 12·8% (95% CI 6·5 –21·1) of patients undergoing appendectomy probably had residual regional lymph node metastases. Overall survival was similar between patients with appendectomy and right-sided hemicolectomy (adjusted hazard ratio 0·88 [95% CI 0·36–2·17]; p=0·71).
Interpretation
This study provides evidence that right-sided hemicolectomy is not indicated after complete resection of an appendiceal NET of 1–2 cm in size by appendectomy, that regional lymph node metastases of appendiceal NETs are clinically irrelevant, and that an additional postoperative exclusion of metastases and histopathological evaluation of risk factors is not supported by the presented results. These findings should inform consensus best practice guidelines for this patient cohort.
This 2024 study from USA indicates support for conservative approach (see discussion document 4 below). The question is “Do Appendiceal Neuroendocrine Tumors Metastasize Post Appendectomy or Right Hemicolectomy?”. Their conclusion was “Stage IV appendiceal NETs are exceptionally rare, and distant metastases are synchronous in nearly all cases. The risk of metastatic spread after resection of local appendiceal NETs is negligible. Patients with tumors <2 cm should not be managed with completion right hemicolectomy, and the role of this operation for larger tumors is questionable. Postoperative surveillance is unlikely to be of benefit.”
This 2025 study looks specifically at invasion criteria. Conclusions: Among patients with resected aNETs, Mesoappendiceal Invasion (MAI))/Subserosal Invasion (SI) alone in the absence of other established risk factors was associated with a low likelihood of nodal metastasis and equivalent long-term outcomes regardless of the extent of surgical resection. The presence of MAI/SI alone should not be an indication for RHC. Click here.
Ronny Allan note: Watching the commentary on twitter, this remains controversial amongst healthcare professionals. If you have a 1-2cm aNET, always ask your doctor to explain the benefits and risks of appendectomy vs right hemicolectomy.
To aid your discussion with the doctor, these 4 documents might help – the conclusions are useful for those to which this is relevant:
1. ENETS Guidelines (as linked above – click here
2. This study from Switzerland – click here
3. This study from France – click here
4. This study from USA (Moffit in Tampa) – click here
5. This invasion study – click here
Young Patient Population (Paediatric aNETs)
The paediatric patient in NETs is mainly in the locations covered by the Multiple Endocrine Neoplasia (MEN) hereditary syndromes. While there can be an overlap between MEN and appendiceal NETs, most are not hereditary cases. Other than that connection, the appendix appears to have a larger quota of paediatric patients than most other primary locations. Read more here.
Further Reading
- Surgical Management of Patients with Neuroendocrine Neoplasms of the Appendix: Appendectomy or More – PubMed (nih.gov)
- Pathology Outlines – Well differentiated appendix neuroendocrine tumor
- UKINETS Guidance Appendiceal NETs.
- Management of Appendix Neuroendocrine Neoplasms: Insights on the Current Guidelines
- Epidemiology of neuroendocrine tumors of the appendix in the USA: a population-based national study (2014-2019)
- NCCN Guidelines for Neuroendocrine Neoplasms (see page NET-3) – account login required.
- ENETS Guidelines for appendiceal NETs
- Not as rare as you think – Ronny Allan
- Pediatric appendiceal neuroendocrine neoplasms: SEER registry analysis
- Appendiceal Neuroendocrine Tumors: Prognostic Role of Mesoappendiceal Invasion and Implications for Recommending Right Hemicolectomy versus Simple Appendectomy
A Spotlight on Neuroendocrine Neoplasms – a series from Ronny Allan
Disclaimer
I am not a doctor or any form of medical professional, practitioner or counsellor. None of the information on my website, or linked to my website(s), or conveyed by me on any social media or presentation, should be interpreted as medical advice given or advised by me.
Neither should any post or comment made by a follower or member of my private group be assumed to be medical advice, even if that person is a healthcare professional. Some content may be generated by AI which can sometimes be misinterpreted. Please check any references attached.
Please also note that mention of a clinical service, trial/study or therapy does not constitute an endorsement of that service, trial/study or therapy by Ronny Allan, the information is provided for education and awareness purposes and/or related to Ronny Allan’s own patient experience. This element of the disclaimer includes any complementary medicine, non-prescription over the counter drugs and supplements such as vitamins and minerals.
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