Pulmonary immunohistochemical markers may be positive in gastric adenocarcinomas associated with autoimmune metaplastic atrophic gastritis

Background Autoimmune metaplastic atrophic gastritis (AMAG) causes pulmonary trans‐differentiation of the gastric mucosa and is known to increase the risk of developing gastric adenocarcinoma. AMAG‐associated gastric adenocarcinomas were examined for immunoreactivity for TTF‐1 and Napsin A. Design Eighteen AMAG‐associated gastric adenocarcinomas and 36 non‐AMAG‐associated gastric adenocarcinomas were stained for TTF‐1 (clones SP141 and 8G7G3/1) and Napsin A (clone 1P64) by immunohistochemistry. AMAG stage and staining patterns were characterized. Results The AMAG group was older (mean age 74 vs. 65 years) and was not significantly different with regard to sex or tumour differentiation. Zero (0%) AMAG cases were classified as early phase, four (22%) as florid phase and 14 (78%) as end stage. AMAG‐associated adenocarcinomas showed more frequent TTF‐1 immunoreactivity compared to control adenocarcinomas in both clone SP141 (39% vs. 11%; P = 0.04) and 8G7G3/1 (22% vs. 6%; P = 0.17). AMAG‐associated adenocarcinomas showed more frequent Napsin A immunoreactivity compared to control adenocarcinomas (28% vs. 0%; P < 0.01). Immunoreactivity in AMAG‐associated adenocarcinomas was patchy, ranging from 1% to 20% of tumour cells staining positive (1+ to 3+ intensity). In the AMAG cases with background gastric mucosa (n = 14), the background showed TTF‐1 positive foci in seven (50%) cases (both clones) versus one (3%; clone SP141) to two (6%; clone 8G7G3/1) of 33 controls ( P < 0.01) and Napsin A positive foci in five (36%) cases versus zero (0%) controls ( P < 0.01). Staining in background mucosa was also patchy, involving 1%–10% of gastric glands without intestinal metaplasia. Conclusion TTF‐1 and Napsin A immunoreactivity was present in the background gastric mucosa almost exclusively in the AMAG‐associated cases. This immunoreactivity is also present in AMAG‐associated adenocarcinomas more commonly than non‐AMAG‐associated adenocarcinomas. This knowledge may aid pathologists in avoiding the pitfall of diagnosing metastatic lung cancer, as the expression is patchy and may also be seen in atrophic background mucosa.