I would tend to agree with Steve. Who was your doctor and what did they say? I assume it would have been Chang and he usually would not be ambiguous about interpreting the results with a patient from my experience. Otherwise, I think it's important to keep two things in mind .
First, all AN's react to the radiation differently based on how often the cells replicate and the necrosis shows up as the damaged cells die when they can't reproduce.
Second, MRI's are a snap shot in time. The cycle is generally from white to darker and spotted to white again ( scarred over). See point 1 for how fast or slow that happens. There is nothing magical about 6 months. Some people see the AN in the dark necrosis stage, others miss it entirely and see it already starting to scar over, some don't see anything significant at that point. As long as it didn't grow, I would wait to see what the 1 year MRI showed before becoming concerned. Certainly by that point evidence of necrosis should be notable from my understanding
Mark