Too bad it actually didn't happen. The entire image is photoshopped by 4chan. Justin Bieber never tweeted that, those people never responded to him and kayne never tweeted it out either...
Type in "Bieber cancer" on Google news and a ton of articles come up about it. It definitely happened, but it's questionable as to whether people actually shaved their heads for it or those are just old photos of girls with shaved heads.