No one has ever done a "years long" expedition to ISS, and the radiation flux in transit to Mars, in particular GCR dose, is much higher than experienced on the space station.
I’m well aware, just mistyped. The total equivalent radiation dose on a fast transit to Mars is less than some ISS expeditions.
Note that the magnetic field only deflects lower energy galactic cosmic rays which have a lower gyro radius than the real whoppers. The magnetic field is less important to overall radiation shielding than the earth’s atmosphere.
Total GCR dose is 3-5x in transit to Mars compared to what you get on ISS; on the Martian surface it's from 1.5-2x the ISS dose. (see https://www.swsc-journal.org/articles/swsc/pdf/2020/01/swsc2...).
On a long-stay Mars mission, that adds up to 12-18 times the accumulated GCR exposure compared to a six-month ISS increment.
Ah, you’re being tricky. You’re ignoring the substantial trapped radiation dose on ISS (it is, after all, the total dose that causes the defects). That, combined with GCR, gives an equivalent dose the same at Curiosity altitude as on ISS.
In fact, look at Table 2. It shows that at ISS, the dose from the SAA is about the same as the GCR dose, so by ignoring trapped radiation, you’re manipulating the result by a factor of 2.