Ohm,

let me answer your post to me on IHUB here ( I have one post a day there )

You said...

" That the cancer has already spread to metastasis there must already be a heavy burden of CTCs "

That may be true or not . They can accept for a study any patients even with minimal metastasis ,
They just need to see any metastasis..
but sometimes patients after developing minimal of metastasis not progressing for some time , weeks or even longer , or even becoming little better.

I guess our Immune system try to fight this cancer , and we may see at this time minimal number of CTC...

We don't know what medical situation is this patient , we only know that she has some metastasis...