Disclaimer:
The information in this Spotlight is provided for general information and education only. It is not intended to replace the advice of a qualified healthcare professional, nor to be used for diagnosis or treatment of any medical condition. Always discuss your individual situation, symptoms, test results, and treatment options with your own doctor or specialist team. Do not ignore or delay seeking professional medical advice because of something you have read here. While every effort is made to ensure accuracy and currency, no guarantee can be given that all information is complete, correct, or up to date, and clinical practice may vary between centres and countries. Any mention of drugs, doses, tests, or procedures is illustrative only and may not be appropriate for your specific circumstances.

 

1️⃣ What Do We Mean by “Ovaries”?

The primary job of the ovaries is egg production and hormone creation. As vital parts of the female reproductive system, they store and release eggs (ova) for potential fertilization, and they act as endocrine glands by secreting hormones like oestrogen and progesterone.   The ovaries are paired reproductive organs located in the pelvis. They contain:

• Surface epithelium
• Stromal cells
• Germ cells
• Teratomatous elements, which may include neuroendocrine‑capable tissue

Primary ovarian NENs arise from neuroendocrine elements within teratomas or, rarely, from native ovarian tissue. The ovary is also a recognised site for metastatic NETs, especially from the small intestine and appendix.

⭐ WHO / IARC Classification (currently Female Genital Tumours, 5th Ed.)

Ovary → Neuroendocrine Neoplasms

• Well‑differentiated neuroendocrine tumour (historically labelled “carcinoid”, but terminology is evolving and likely to be retired)
• Poorly differentiated neuroendocrine carcinoma – small‑cell type – large‑cell type
• Mixed neuroendocrine–non‑neuroendocrine neoplasm (MiNEN) – extremely rare
• Frequently associated with mature teratomas
• Must be distinguished from metastatic NETs (e.g., SI‑NET, appendiceal NET)

2️⃣ Epidemiology — NET vs NEC in the Ovary

Key takeaway:

Ovarian NENs are very rare. NETs are far more common than NECs, and their behaviours differ dramatically.

A. Well‑differentiated ovarian NETs

• Represent the majority of primary ovarian NENs
• Often arise within mature teratomas
• Usually stage I at diagnosis
• Typically indolent with excellent prognosis
• May produce serotonin → carcinoid‑type symptoms, but this is uncommon

B. Poorly differentiated ovarian NECs

• Exceptionally rare
• Highly aggressive, mirroring NECs in lung/GI
• Present at advanced stage
• Managed using platinum–etoposide regimens
• Prognosis is poor

C. Relative frequency

Ovarian neuroendocrine tumours are far more often metastatic than primary. Primary ovarian NETs are rare, usually unilateral, low‑grade, and often arise within a mature teratoma. Metastatic NETs — especially from small‑intestine NETs — are common, typically bilateral, and frequently occur with peritoneal carcinomatosis.

Pathology uses a small set of decisive immunohistochemical markers to determine origin:

• PAX8‑positive → supports primary ovarian origin
• CDX2‑positive → strongly supports small‑intestine or appendiceal origin
• TTF‑1‑positive → suggests lung origin

Ki‑67 helps distinguish tumour behaviour, but with an important nuance:

• Primary ovarian NETs are almost always low‑grade – High Ki‑67 with NET‑like morphology usually indicates a metastatic well‑differentiated NET G3 rather than a primary ovarian tumour
• Primary ovarian NECs are always poorly differentiated high‑grade (small cell or large cell)

In practice: e.g. A bilateral, multifocal, CDX2‑positive ovarian NET — especially with peritoneal disease — is overwhelmingly likely to be metastatic, not primary.

3️⃣ Ovarian Metastases From Extra‑Ovarian Primaries

Key takeaway:

Metastatic NETs to the ovary are more common than true primary ovarian NEC.

A. Small intestine NET (SI‑NET) metastases

• Ovaries are a known metastatic site
• Often bilateral
• May cause carcinoid syndrome without liver metastases (ovarian venous drainage bypasses the liver)
• Frequently discovered incidentally during pelvic surgery

B. Appendiceal NET metastases

• Less common than SI‑NET metastases but more common than primary Ovarian NET.
• Can mimic mucinous ovarian tumours
• May resemble Krukenberg‑type lesions

C. Distinguishing primary vs metastatic

Use:

• Laterality → metastases often bilateral
• Teratoma association → suggests primary
• Immunohistochemistry:
  • CDX2 (suggestive of GI origin)
  • PAX8 (suggestive of ovarian origin)
  • TTF‑1 (suggestive of lung origin)
• Clinical context → known GI NET, mesenteric mass
• Pattern of spread → peritoneal implants, nodal disease

4️⃣ Differentials — This area contains many possible abnormalities, often found incidentally via modern imaging. Unlikely to be NET related

A. Functional / Physiological

• Follicular cyst
• Corpus luteum cyst
• Haemorrhagic cyst

B. Endometriosis‑related

• Endometrioma

C. Inflammatory / Infective

• Tubo‑ovarian abscess

D. Non‑neoplastic cysts

• Paraovarian cyst
• Hydrosalpinx
• Peritoneal inclusion cysts

E. Vascular / Mechanical

• Ovarian torsion
• Ovarian vein thrombosis

F. Metabolic / Hormonal

• Polycystic ovary morphology

G. Pregnancy‑related

• Theca lutein cysts
• Luteomas of pregnancy

5️⃣ Summary — The Spotlight Message

1. Primary ovarian NETs are rare, usually well‑differentiated, often teratoma‑associated, and generally indolent.
2. Primary ovarian NECs are exceptionally rare and highly aggressive.
3. Metastatic NETs (especially SI‑NET and appendiceal NET) are more common than primary ovarian NEC.
4. Many ovarian abnormalities are non‑neoplastic, so a structured differential is essential.
5. The WHO/IARC classification places ovarian NENs within the Female Genital Tumours volume, and terminology is shifting away from “carcinoid”.

These recent peer‑reviewed publications provide high‑quality, clinically relevant insights into ovarian neuroendocrine neoplasms, ovarian metastases, and peritoneal disease in NETs.

1. Comprehensive review of ovarian neuroendocrine neoplasms
Modern overview of classification, pathology, imaging, and management of ovarian NENs.
https://pmc.ncbi.nlm.nih.gov/articles/PMC12733891/

2. Ovarian metastases from small‑intestine NETs
Key evidence that SI‑NET ovarian metastases are common, often bilateral, and frequently associated with peritoneal carcinomatosis.
https://pubmed.ncbi.nlm.nih.gov/32145918/

3. Radiologic–pathologic correlation of ovarian metastases from GI NETs
Detailed CT/MRI–pathology correlation showing how GI NET metastases present in the ovaries and how to distinguish them from primary ovarian tumours.
https://www.sciencedirect.com/science/article/pii/S0887217125000551

5. WHO classification updates for female genital tract NENs (paywall but index is available)
https://publications.iarc.who.int/Book-And-Report-Series/Who-Classification-Of-Tumours/Female-Genital-Tumours-2020

6. Pelvic neuroendocrine tumours: multimodality imaging review
Clinical Imaging (2023). Practical review of CT, MRI, and DOTATATE PET appearances of pelvic and ovarian NENs.
Imaging of Gynecologic Neuroendocrine Tumors: A Case-Based Pictorial Essay – ScienceDirect

7. Primary vs metastatic ovarian NETs: pathology guidance
Histopathology (2021). Clear immunohistochemical criteria (CDX2, PAX8, TTF‑1) for distinguishing primary ovarian NET from metastatic GI NET.
https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2021.770266/full

8. Management of peritoneal metastases in NETs
Cancers (2023). Open‑access review covering surgery, PRRT, systemic therapy, and the role of cytoreduction in NET peritoneal disease.
https://pmc.ncbi.nlm.nih.gov/articles/PMC12634710/