I too have an agressive form of cancer, so I can totally relate to how you are feeling right now. Cancer certainly does SUCK big time!
Are you having any other treatments other than the radiation? The radiation itself had a tendency to make me feel exhausted and tired although it did not effect my white cells as much as the chemo did. How many more radiation treatments are you scheduled for?
Have you had your PET scan yet? Hang in there one day at a time. You can and will get through this!