This is being reported in NY Post, Daily Mail and Newsweek which are not believable. UK's "The Independent" may be somewhat more reliable. He has an indication of some medical condition. A leukemia "diagnosis" from a random unnamed oligarch? Not believing this. I call bullshit.

"insider" is a more believable news source. That doesn't make the Putin cancer story factual, but it does make me believe some Russian oligarch might have said it. [businessinsider.com]

Here's the link from New Lines, the source quoted by Business Insider. It goes into great detail, but it still doesn't mean Putin's dying or even very sick.
[newlinesmag.com]