A hiatal hernia is a herniation of abdominal contents through the opening of the esophagus as it connects with the stomach. Most commonly the stomach herniates into the chest. This can vary greatly in size, to the the point where the entire stomach resides in the chest. A hiatal hernia increases in prevalence with age. Patients can be asymptomatic or at times have chest pain, burning, reflux and pain in the middle of the stomach.
Often a patient with symptoms that are thought to arise from an abnormality in the chest will get a chest X-ray. A hiatal hernia will often be visible on a chest X-ray. It will classically appear as a mass with an air fluid level behind the heart. That is because most commonly the stomach herniates into a hiatal hernia. The stomach often has an air fluid level. That means that the stomach has both air and fluid, where fluid settles at the bottom and forms a sharp interface with the air.
Unfortunately, sometimes there is no air fluid level and a hiatal hernia will look like any other mass, including cancer. Even a mass that has an air fluid level can represent other things like an abscess or walled off infected collection. This appearance can also be seen when the esophagus is removed for cancer and the stomach is pulled up, called a gastric pull up procedure. In this case, the surgical history will be known however.
I usually recommend another study to confirm a hiatal hernia when I see a mass behind the heart on a chest X-ray, even if it has an air fluid level. Unless the patient is known to have a hiatal hernia from another test, I will recommend a CT just to make sure. CT is definitive and will show the classic appearance of the stomach protruding into the chest. A barium swallow or esophogram is another good test to confirm.