Gastric cancer is rare in the US (though quite common in Far East Asia). It’s very hard to diagnose before it’s too late, which is when worrisome signs like weight loss appear. At the beginning, the earliest symptom is simply vague upper abdominal pain. In Japan and maybe other Asian countries, they do regular screening tests on everyone, the way we do colonoscopies for colon cancer here. But gastric cancer is way too rare in the U.S. for that.
Gastroenterologists diagnose stomach cancer by performing endoscopy; only a rare type can be seen on CT scan. But we obviously don’t refer everyone with upper abdominal pain, since many patients get better with treatment for more common conditions. There are several types of antacid medications that work for all the stomach diseases (ulcer, gastritis, dyspepsia, GERD); only the strongest one (proton pump inhibitors, aka PPIs) will also relieve cancer pain (and thus misleading us). So if we need or choose that one, we only prescribe it for 2 months. If pain goes away but then returns, we refer for endoscopy.