Complex sentence: Most of the people diagnosed with esophageal cancer have late-stage disease, because people usually do not have significant symptoms until half of the inside of the esophagus, called the lumen, is obstructed, by which point the tumor is fairly large.
Split into simpler sentences: Most of the people diagnosed with esophageal cancer have late-stage disease. This is because people usually do not have significant symptoms until half of the inside of the esophagus, called the lumen, is obstructed, by which point the tumor is fairly large.