The opera singer underwent the treatment programme at the Christie Hospital in Manchester following emergency surgery last October.
He had had an operation in 2006 to remove the tumour, but was told it had redeveloped.
He wrote on his blog that the course was "no walk in the park", saying: "I'm not out of the woods yet. I dearly wanted to keep the side-effects to a minimum and I've been keeping myself fit and healthy as I can, but I'm afraid they've caught up with me, and I don't mind admitting I've not been myself."
He added that he had experienced "moments under that machine where I just wanted to stop the bloody thing and walk away", but the support of friends and family had helped him keep going.
The 41-year-old will now undergo regular scans to establish whether the treatment has succeeded in eradicating the tumour.
His manager Giles Baxendale said the radiotherapy did not appear to have damaged Watson's voice or eyesight.