Just a correction of a misconception.
The xray treatment you get from IMRT does not make you radioactive. There is no radiation level that needs to decrease.

Instead what is happening is that your tissues are inflamed and damaged after the radiation. Since the tissue is rebuilding/healing it is metabolically more active and it would therefore show up on a PET scan. This is one reason why you need to wait before you can run a meaningful PET scan after rad treatment (also surgery). The length of time probably varies from person to person (and extent of damage). I have heard numbers like > 6 weeks after radiation being thrown around, which might still be early. In any case your RO should know how soon a PET scan can be done (insurance is an issue too).

M