Hi ken, so sorry about your diagnosis. I too had stage 4 tumour in my tongue but I had a total glossectomy done to remove it. I had the same amount of radiation. I found I couldn't eat from the end of week 5. I didn't use fentanyl though, I was on pregablin (from the gabapentin family) and rinsed my mouth out with warm water to help ease the pain. It wasn't too bad, just the ulcers were painful