This study is very bad news: http://www.nature.com/articles/srep29901
Of course the sample size was very small, but they've been able to replicate vascular degeneration in mice by exposing them to radiation.
With the Apollo mission, time spent outside of the earth's magnetosphere was typically 8 days or so, a trip to mars might take 8 months, plus the same to get back. There's no magnetosphere on Mars, and no protection on the surface from the very sparse atmosphere. Shielding a spacecraft is a problem, then the problem of shielding once you got there. You'd actually have to be quite deep underground to be safe from spallation / showers.