To distinguish a gastric primary from a breast primary in ascitic cytology, which IHC pattern supports gastric origin?

• A. ER+, PR+, MUC2-
• B. MUC5+, GATA3-
• C. CK5/6-, GCDFP-15+
• D. CLDN18.2-, NapsinA+

Answer: (B)

Gastric tumors often produce gastric-type mucins and lack breast-lineage markers. MUC5 is a gastric mucin, so its presence suggests gastric differentiation. GATA3 is a transcription factor commonly expressed in breast carcinomas. If tumor cells in ascites are MUC5-positive and GATA3-negative, this pattern supports a gastric origin and argues against a breast primary.

In contrast, a pattern showing breast-associated markers (such as ER/PR positivity or GCDFP-15 expression) would point toward a breast origin, and a marker like Napsin A would suggest a lung primary. The combination of MUC5 positivity with GATA3 negativity is the most consistent with gastric origin in this setting.