I hadn't heard of this before, so I resorted to my old friend Google: but like you, have been able to find very little about it.
I'm pretty sure it's not a standard therapy, so are you being offered it as part of some kind of trial?
Some trials have used radiotherapy as a precursor to stem cell treatment, but radiotherapy as a treatment in its own right seems even rarer. You haven't mentioned stem cells, so I assume they're not proposing that?
I also assume it's the NHS who's offering, is it? If it is any other organisation (e.g. private clinic) I'd be extremely wary of false promises and controversial and unproven treatments.
Sorry this doesn't help much.
Haven't you been given any info to consider?