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

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.