A good question. My answer is: Well - no, I don't think they are deciding dose escalation based on scans of tumour size. The scans are only every 6 weeks or so, and direct effects on tumour size take longer than that - at least to begin with - due to pseudo progression. The CR in Bile Duct cancer appeared to get worse in initial scans - judged by tumour size. I think it was scan 3 before tumour size reduced.
They have made it pretty clear in their most recent conference presentation that they are measuring a range of immunological data - something like 20 or more immune cell indicators - and that they can demonstrate a correlation between those immunological changes, and tumour response. ie. "Immunological changes in responders are characterized by higher frequencies of T cells, natural killer cells, and monocyte subsets post-treatment." Source: https://static1.squarespace.com/static/5b63d41b3e2d09b1f56bf483/t/65a9aff332d3180e08db85c7/1705619461250/749+ASCO-GI+MAST-Final+%281%29.pdf
So I assume that they are measuring the immune cell indicators - and using them as a proxy for likely tumour response and efficacy of the treatment, which would take much longer to develop.
(20min delay)