Ramsar, Iran has similar levels of background radiation as being on the surface of Mars. And people live completely healthy long lives there. Chronic low-level radiation isn't nearly as bad as we once thought. It's acute high-level doses or consuming radioactive substances that you really need to worry about. Mars really won't be bad at all with some easily implemented mitigation measures.
It's not that different between the two; Mars lacks a magnetosphere, so the planet itself is left to block about half of what'd hit you in interplanetary space.
On that, if you orient the ship so that the rear is pointed directly at the sun (trivial to do once you're coasting) you'll have a hundred feet of liquid fuel and other solid material which will block the primary source of radiation (the sun).
I was always led to believe that the primary source of radiation we need to worry about for space travel was Cosmic Radiation [1]. The shielding requirements for CR relative to solar radiation requires much more material, to protect from rays from every angle.
[1] https://www.nasa.gov/missions/analog-field-testing/why-space...
There are different types of radiation and body's response to them are different. Elevated levels of simple gamma rays (and maybe also protons) are reasonably tolerated - the body has mechanisms to recover from that.
However, a steady dose of relativistic heavy nuclei is entirely different matter. Gun wound vs bulldozed over. Cells have mechanisms for repairing local damage, even in the cell nucleus. However, a fact heavy nucleus wrecks so much that the repair mechanisms have no chance - a cell that got hit simply dies (or, at least, is never going to be able to divide again). The damage is cumulative, does not get repaired.
In game parlance: Gammas reduces your HP (that can come back over time, if the hits do not occur too fast). Fast heavy nucleus reduces your max UP. You are doomed, given enough time.
