CUP — SCE Medical Oncology MCQ

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ModerateCancer of Unknown PrimaryCUPSCE Medical Oncology

A 65‑year‑old woman presents with multiple liver metastases and peritoneal carcinomatosis. Biopsy shows adenocarcinoma. Immunohistochemistry results are: CK7 positive, CK20 negative, ER/PR weakly positive, HER2 negative, TTF‑1 negative, and PAX8 positive. Serum CA‑125 is elevated. What is the most likely primary site?

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Correct answer: D — Ovary or primary peritoneal carcinoma

The CK7+/CK20− immunophenotype with PAX8 positivity is characteristic of Müllerian tract tumours such as ovarian or primary peritoneal carcinoma, whereas TTF‑1 negativity excludes pulmonary origin, and CK20 negativity makes colorectal or pancreatic primaries unlikely. Elevated CA‑125 in this clinical context (peritoneal disease in a post‑menopausal woman) further supports an ovarian/peritoneal source. ER/PR weak positivity is compatible but non‑specific. Other options (pancreas, lung, breast, colorectal) do not match the combined pattern of immunohistochemistry and tumour marker elevation.

Reference: NICE clinical guideline CG104 (2023) and CG122 (2026), relevant chapters on immunohistochemistry in CUP and CA‑125 elevation in ovarian or primary peritoneal cancer.