Here's a thought -- the risk/concern is that the placebo arm is performing far better than normal due to doctor/clinic skill blah blah blah.
79 sites -- it's inevitable in my experience no matter how great/experienced/skilled the pool of experts, the law of large numbers inevitably has some incompetent baboons in the mix. There's going to be a few doctors or sites at least that simply suck no matter how hard you try and those sites/doctors are going to have patients with low PFS or even treatment failures. It's just inevitable. 79 sites aren't going to be perfect.
Those few incompetent sites will keep it under 18 months (worst case for investors) IMO.