The Lord of the Rings icon feared the worst when a biopsy discovered cancerous cells, but the 73-year-old Brit has not undergone treatment for the disease and instead has regular tests to ensure it hasn't spread.
He tells Britain's Daily Mirror, ""You do gulp when you hear the news. It's like when you go for an HIV test, you go, 'Argh, is this the end of the road?' They say you have cancer of the prostate and then that you can have it zapped.
""You can have it snipped but you are not a candidate for that. You are 'waitful watching'. I've had it for six or seven years, 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.
""Many, many men die from it but it's one of the cancers that is totally treatable so I have 'waitful watching'. I am examined regularly and it's just contained, it's not spreading. I've not had any treatment.
""You are told what the situation is: you can have an operation but there is no point me having an operation because there is no need for it. They are concerned about the cancer spreading outside the prostate. If it doesn't you are fine. How do you know if it is spreading? You keep being tested.""
And despite cloudy vision and a hearing problem, Sir Ian insists he still feels ""absolutely fine"" - thanks to regular exercise.
He adds, ""I just got hearing aids, I am going to have a cataract removed from my eyes, I am having an implant in my mouth, I am dealing with the prostate... (But) I am absolutely fine, thank you very much!
""I am lucky, I don't have aches and pains. I do Pilates regularly which is a series of stretching exercises and I recommend it to anyone of my age because the temptation is not to exercise when you get older. Well, you should. I always walk up the escalator on the Tube (subway) and I live in a house with a lot of stairs. That's good exercise but you need more.
""The wonderful thing about modern medicine is that so many of these complaints that used to signify old age and decline can be coped with.""