AFAIK, under EASA, airlines have to keep a record of a crew members exposure to radiation. Many do this via the CARI programme integrated into crew rosters. Is it worth asking your admin department? As far as measures to minimise go, I assume it is nearly impossible to reroute to lower latitudes in your airline, leaving flying at lower altitudes as the only method. Personally, I don’t go above FL350 if I can help it above 60N. If that means sticking on an extra tonne or two, so be it.