Maggie Koerth-Baker, on "How space radiation hurts astronauts". I did not know about this part:
Cucinotta calls this pre-flight calibration. Scientists take a blood sample from an astronaut before the launch. While the astronaut is in space, the scientists divide that blood sample up and expose it to various levels of gamma rays the kind of damaging radiation we're used to dealing with on Earth. Then, when the astronaut comes back, they compare those gamma ray-affected samples to what has actually happened to the astronaut while in space. "You see about a two-to-three fold difference across the population of astronauts," Cucinotta told me.
The sample size of astronauts is small enough that I was surprised to see significant effects for one condition: cataracts. The article notes that the Mercury and Gemini astronauts had less spaceflight time than Mir and Skylab cosmonauts and astronauts, which is obvious, but I wonder how they control for the extensive flight time of astronauts who were former test pilots and the consequent history of radiation exposure before going to space.