To distinguish gastric primary from breast origin in ascites cytology, which panel supports gastric origin?

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

Answer: (A)

When trying to tell apart a gastric primary from breast cancer in ascites cells, you rely on tissue-specific immunoprofiles. MUC5 is a mucin produced by gastric epithelium, so positivity for MUC5 supports a gastric origin. GATA3 is a transcription factor commonly expressed in breast carcinomas, so a negative result (GATA3-) argues against breast origin. The combination of gastric mucin positivity with absence of a Breast marker makes gastric origin the best explanation for the ascites cytology. Other panels point toward breast (ER/PR positivity or GCDFP-15) or toward non-gastric sources (NapsinA for lung), or would not support gastric origin if CLDN18.2 is negative.