OS using ECA is NOT confounded.
Hypothetically this could have happened....
NWBO to FDA " hey there buddy, know body knew about this at the time, but we and others discovered psuedo-progression. It shows immune system is attacking the cancer, but some placebo patients crossed over to save their life.
FDA: oh, can you just start a new trial ?
NWBO: Nah, we don't have the money, we're just a clinical stage company.
FDA: How about using control patients from other external trials... the patient data should be almost identical??
NWBO: Wow, that would be great.