Yes, radiation is routinely done in patients with transplant and on immunosuppressive therapy. Efficacy of RT may be reduced in this setting.

If one considers how many immunosuppressants cancer patients get today (Rituxan, Humira, prednisone, even Seroquel and Risperdal) for conditions often unrelated to the need for RT, it's an increasingly uncommon patient who isn't immunocompromised; the local control numbers and toxicities don't appea...

I have treated patients on immunosuppression for renal and other transplants without apparent problems. However, I do not know of any studies in this area.