There has been more than one instance with immune/biological therapies ( I think one was in PCa) where early progression doesn't necessarily mean benefit, even overall survival (OS), won't accrue later. In the study above, calling "progression" before the measured immune response peaks seems premature. Seems telling they follow with
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however, 300-day overall survival was 92%
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I suspect once progression (according to RECIST criteria) was called, the therapy was discontinued. The issue of continue benefit of Herceptin past progression seems likely related.
There are even instances with non-traditional therapies where tumors have internal necrosis with fluid build up that can actually increase the radiologic (CT) size while it is dying. I'm not clear on whether PET can fully escape that issue.