Quote Originally Posted by Rich Ullsmith View Post
Is it possible that you are getting the effect of solarizing? 300w is pretty bright.
That's also the only thing I could think of. One method of producing positives from positives is to overexpose to the point where the silver halide grains "do a 180". That's oversimplified for such a complex reaction, but I believe that's kinda how it works. There is a discussion on this topic somewhere...

If you've found a way to get a direct positive from mammography film, that might be a very interesting technique to exploit!