scottbeammeup,
I had a similar experience. After radiation I had a -5.6 on dxa in the femur. I wasn't able to verify cause but had edema in the sacrum. I speculate that the dxa measured an area of edema. So edema weakens the bone, but can resolve more quickly than genuine bone loss. This is complete speculation on my part. I haven't had dxa reading that low since, -3.3 was the lowest.
Incorrect positioning for a dxa scan can skew the readings.
You might repeat the dxa in a few months. I would but I wouldn't delay a bone anabolic.
With this idea, I should add that radiation can damage bone in many ways--through the marrow, destruction of blood vessels within the bone, by increasing the number of acid producing cells in the matrix.
I'd be curious if a reading at a different facility would give you a better reading. If in a few months resolution of edema would change the low reading.
I do suspect that you are better off than it seems. Still, I wouldn't risk fracture by waiting.
And especially with all this information in consideration, I would suggest that you think about Forteo or Tymlos rather than an antiresorptive bisphosphonate.
Bless your choice.