Proof will definitely be necessary as acceptance for approval by regulators would not be based on just taking NWBO' word for it however, as I posted the necessary proof by way of the work reviewing MRI scans has been conducted by the company and will IMO be sufficient to prove the PFS endpoint was successful.
If not working or futile or dangerous, FDA would take action immediately. Case in point which happened recently was Vaping which was favorite to youth. FDA took action to create awareness how dangerous they are.
I believe FDA allow it to mature long enough without those dreaded k-m curve which I happen to hate so much since it can mislead patients.
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