You may have followed, or seen a report of, the guy who used a pregnancy test on himself and it turned out positive. A friend posted it to reddit as a funny rage comic, and a commenter warned that it might be a sign of cancer. The guy saw a doctor, a small tumor was found, and his life was saved, as this very rare form of cancer is usually not detected early. The pregnancy test reacted to human chorionic gonadotropin, or HCG.
Yes, HCG in men can be a sign of a rare (and dangerous) form of testicular cancer — choriocarcinoma. This is a cancer made up of syncytiotroblastic cells, said Katherine McGlynn, a senior investigator with the National Cancer Institutes. The tumor secretes HCG because that's what syncytiotroblasts do. They secrete HCG. And they don't particularly care whether they're secreting it into a man or a woman.
But how do they get into a guy, to begin with? That's where things get really weird. The truth is that nobody is entirely certain, McGlynn told me. But there are a couple of theories. One possibility is that these syncytiotroblasts that turn cancerous were leftovers — remnants of the time when that guy was just a ball of 70-100 cells. One way or another, they persisted in his body and then started to grow out of control.