What is known for certain is that brain volume shrinks as AD progresses. That is a fact.
The reason that there hasn't been a correlation with reduced rate of brain volume decline with clinical meaningful result is that there has never been a drug that slows brain volume loss in AD or old age. So there hasn't been anything to correlate with before this result.
We do know that all of the people in the trial have been diagnosed with AD. So the comparison is between a control group that is known to have AD, which is known to cause brain volume loss, and the treatment group which also has AD. We are not talking about the brain loss rate of just old age, although these people are also aging and whatever brain volume loss due to old age is happening to both groups in addition to the AD related loss.
The issue will be this is a post hoc finding and not previously specified in the trial. As such it won't be considered a valid correlation if one is found. For the FDA to consider any correlation that is found, if any, it would have to be specified in a new trial as part of the stated outcomes.
Having said all that, it seems reasonable that if the reduction in volume loss takes the rate back to that of just old age, the agencies will give that some weight.
After all, that has never been seen before.
I'm with you, hopefully the brain volume data will correlate with reduced cognitive decline.
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