Put things in perspective: how much radiation will you get when you have an x-ray? Or an MRI? Or a CT scan? In all probability, the average house has much, much more radiation in it from appliances. All of those are infinitely more powerful than occasional exposure to a thorium-tainted lens. No big deal. Old Pentax Takumars had it. Whatever it was purported to do has long since faded into history, but such lenses are sought out by collectors for their curio value, if not actually used for photography. Speaking of which, it's probably time to get more lens work done than worrying about radioactivity.