The challenge has to do with the limits of imaging technology up to the present day. Tumor resection decisions rely upon judgement calls based on imperfect scans.You can't see the tumor and the effected area at a granular level. Immunotherapy can appear to worsen the tumor, but after a period of time under observation, it turns out that the tumor, which was thought to have gotten bigger, in reality was just a positive inflammatory response. Subsequent scans after time to resolve shows the tumor getting smaller or even disappearing. But during that first interim scan, the timing on how they want to proceed is critical. If it's real progression, it doesn't make sense to create more vaccine, or if the confidence is high that it's peudo, then batches need to be prepared and administered at the appropriate intervals. I'm not sure, if I have it exactly right, but the bottom line is that imaging tech still has a ways to go, and as long as that's the case, PFS is not a reliable surrogate to determine treatment effectiveness.
PfS is merely a check at a given time point to guage which direction the treatment is trending. Positive translates to improved OS. As opposed to negative OS means you're dead or dying.
I don't understand why so much is being made over PFS, when it's only a marker of a trend, but useless beyond that.
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