My husband had his first post-rad MRI after 4 weeks, then it was every 8 weeks thereafter. We see the Onc every month for blood work, exam, etc. and he gets an MRI every two months.
The effects of the radiation and chemo are cumulative (as I understand it) so they want to give it time to do it's full job. Our onc said even the 4 week post-rad MRI was really just a new baseline, it was the one two months after that where we'd truly start being able to measure results.
Sending healing thoughts and prayers to your Mom.
Nancy
**husband dx 8/07, GBM, surgery, rad/temodar, currently on 5/23 temodar