radiation necrosis or tumor?

I had gamma knife a year ago for a brain met and had been doing well. My last scan in Feb was great. I just had a routine scan this week and have an area that lights up in the same area as the tumor that was radiated. Luckily, I have no new lesions. The rad onc says she thinks this is something called necrosis or dying tissue from the radiation but she can't be sure it isn't new tumor and that the radiation didn't work. So they are going to watch it and rescan in 6 weeks.

This of course makes me nervous! Has anyone heard of this? She says that they can't retreat an area once treated and if it turns out to be tumor, I would need surgery to remove it.

Help, I am freaking out!

Thanks all,
Rosie