I remember seeing this movie in the early 1960's. As I recall, an astronaut was launched into space, the spaceship was lost and he was presumed dead. But he had actually gotten back to earth and had become a "radiation monster".
His head was oversized, spherical in shape and had pock-marks that looked like someone has taken an ice-cream scoop to the head. He killed people just by being near them since he was sending out radiation all the time.
That's about all I recall - aside from getting the bejeebers scared out of me. I think I was about six at the time!