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Occult Breast Cancer — SCE Medical Oncology MCQ

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ModerateCancer of Unknown PrimaryOccult Breast CancerSCE Medical Oncology

A 45-year-old woman presents with metastatic adenocarcinoma in the left axillary lymph nodes. CT shows no other disease. Biopsy IHC: CK7+, CK20-, GATA3+, mammaglobin+, ER+, PR+, HER2-negative. PET-CT shows no primary. What is the most likely diagnosis and management?

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Correct answer: C — Occult primary breast cancer — treat as stage II-III ER+ breast cancer

Adenocarcinoma in axillary lymph nodes with GATA3+, mammaglobin+, ER+, PR+ immunophenotype strongly indicates an occult breast primary. This is classified as a favourable CUP subset and should be managed as stage II-III breast cancer (axillary lymph node dissection ± mastectomy or breast RT, followed by adjuvant systemic therapy guided by receptor status). MRI breast should be performed even if mammography/ultrasound is negative. Occult breast cancer presenting as axillary disease has similar outcomes to overt breast cancer with axillary involvement.

Reference: NICE NG36 Cancer of unknown primary; ESMO 2024 CUP Guidelines; ESMO 2024 Breast Cancer Guidelines