The
Cancer Man would be more accurately named C.J. Spender. A close, personal friend of
Fox Mulder's father, and co-conspirator in dealings with extra-terrestrials.
He is not named because he
may have cancer due to his ever-present Morley
cigarettes, but rather, was implicated somehow when
Agent Scully discovered she had an
inexplicable implant beneath the skin at the
nape of her neck.
As soon as the implant was removed, she developed cancer, but when it was replaced, at Spender's suggestion, her
illness disappeared.
...hence "the Cancer Man".