I suspect it would be just as easy for doctors in the US to miss stomach cancer in a teen girl.
“The average age of people when they are diagnosed is 69. About 6 of every 10 people diagnosed with stomach cancer each year are 65 or older.”
“A major cause of stomach cancer is the bacteria Helicobacter pylori (H. pylori), which is also a major cause of stomach ulcer.”
Those below the age of 30 often show far fewer symptoms that suggest stomach cancer. One thing that could have helped the diagnosis is if stomach cancer was common in her family, if she smoked, ate a lot of smoked fish and other smoked meat. Beyond that, physical palpitation of the stomach can indicate hardness or unusual lumps.
So the bottom line is that missing that she had stomach cancer would be easy to do.
It’s not that a doctor wous say, “oh that’s stomach cancer.” It’s that a doctor would pursue determining what the matter was.