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.