"The Hobbit" actor, 73, is "examined regularly" and his cancer is "contained," according to the site.
This suggests that McKellen is taking an observational approach to his cancer called "active surveillance," said Dr. Durado Brooks, the director of prostate and colorectal cancers at the American Cancer Society.
"Prostate cancer is often a very slow-growing disease," Brooks said. "If you take a slow-growing disease in an older man, there is a likelihood that that man is going to end up dying from some other problem."
Brooks said many men with early-stage prostate cancer can live 10, 12 or even 15 years without ever experiencing symptoms. Because men are often in their mid-60s and older when they choose to forgo treatment in favor of observational approaches, there is minimal data on how the disease progresses over 20 years and more.
When a man has slow-growing prostate cancer that's contained within the organ, he and his doctor can choose between two observational approaches: "watchful waiting" or "active surveillance," Brooks said, adding that the latter is more popular.
Watchful waiting means the patient will wait for urinary or sexual complications - which sometimes never happen - before seeking treatment. Active surveillance means the patient undergoes annual or semiannual prostate biopsies and routine blood tests for high levels of prostate-specific antigen (PSA), a marker of the disease.
Between 30 and 40 percent of men in the United States with prostate cancer are potential candidates for these observational approaches, but fewer than 10 percent of them decide against surgery and radiation in favor of waiting for symptoms to appear, Brooks said.
"There is some cultural component to it," Brooks said, citing a Swedish study that showed 40 percent of Swedish men with prostate cancer chose observational approaches. "Swedish men are willing to consider this while in the United States, at this point, it hasn't taken off."
The website post contradicts what McKellen's agent told ABC News Tuesday, which was that the actor does not have cancer. But it also clarifies an article in the U.K. tabloid The Sun, which was titled "Sir Ian McKellen: I'm fighting prostate cancer."