Leonard Betts - S4.E12

After the body of a decapitated EMT disappears from the morgue and an identical man starts work at the same hospital, Mulder believes that the man has the ability to regrow parts of his body, including his head. The agents soon discover that Betts' body is riddled with cancer and he is able to live in that condition but needs to feed on cancerous tumors to keep up his strength.