I'm kind of fascinated that this sort of thing is still considered a publishable result. It's been known for at least twenty years that intra-observer variability is a significant factor in such things.

I'm not knocking the work you've done, but pointing out that when I was working in radiotherapy in the early 90's this kind of stuff (not on bidimensional product specifically, but whatever tumor volume measures were being used at the time) was being done. I wasn't involved in any of it directly, but recall colleagues who were, more-or-less ranting on about the degree to which chance predominated.

One issue I was more involved in was that every modality (CT, MR, fluoro...) gave significantly different tumour volumes and no one knew how to reconcile them in a clinically useful way.

Given that every measure we've ever devised seems subject to similar issues, there's an argument to be made that the very concept of "tumour size" is problematic. It seems to make sense, but the edges are too fuzzy to be very diagnostically useful in a depressingly large number of cases.