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McKellen, 73, played Gandalf in the hit "Lord of the Rings" movie trilogy, and reprises the role in three prequels based on J.R.R. Tolkien's novel "The Hobbit".
The first of those, "The Hobbit: An Unexpected Journey", recently had its world premiere in New Zealand, where it was shot under the directorship of Peter Jackson.
"I've had prostate cancer for six or seven years," McKellen told the Daily Mirror tabloid. "When you have got it you monitor it and you have to be careful it doesn't spread. But if it is contained in the prostate it's no big deal."
His representatives in London were not immediately available to comment on the interview.
"Many, many men die from it but it's one of the cancers that is totally treatable," added McKellen, one of Britain's most respected actors who is also well known in Hollywood for appearances in the X-Men franchise.
"I am examined regularly and it's just contained, it's not spreading. I've not had any treatment."
He admitted he feared the worst when he heard he had the disease.
"You do gulp when you hear the news. It's like when you go for an HIV test, you go 'arghhh is this the end of the road?'
"I have heard of people dying from prostate cancer, and they are the unlucky ones, the people who didn't know they had got it and it went on the rampage. But at my age if it is diagnosed its not life threatening."