Knee
If you happen to have a tumor in your knee, you might be in for one of the weirdest surgeries I know of. It's something that you'd most likely see in a body horror movie.
They'd basically amputate the leg (including the knee) and chop off the cancer infected knee portion. You'd then take what you're left with, rotate it 180 degrees and reattach it. The foot is now your new knee joint. Your can thus attach a prosthetic and almost have a fully functional leg.