I think it would be more rare in head and neck than say if you radiated the leg bones or hips where most of the body's marrow resides - but anytime you radiate bones and thus the marrow - you can kill off the cells inside. And, when they are trying to regenerate - there is a greater chance of mutation in one which can cause the whole chain reaction and lead to leukemia. That is what I understand to be the way it works. Please correct me if someone else knows the answer to be different.

That being said - if case requires radiation - I don't think the doctors would forgo it b/c of this very minimal risk. If your case was questionable whether radiation would help or not - then it might be a factor in the decision process.

I believe there is a similar slight risk for chemotherapy as Chemo generally kills off living cells and most do greatly affect the bone marrow - but if you require that - then the benefit far outweighs the minimal risk.