When, if ever, would you initiate chemotherapy for recurrent colorectal cancer based on a positive ctDNA assay alone, such as Signatera, in the absence of tumor on imaging or physical exam?

No. There are many ongoing trials to look at this question and stay tuned for a soon in press JCO review that will reference many of the studies but currently, no data to treat based on a positive test. I will be more mindful of surveillance i.e., a 3-month scan, for example.

I also agree that I would not initiate therapy. The use of ctDNA for surveillance has posed a problem for me as to what I would do with the information and whether I am causing trauma to the patient without improvement in survival, just lead time. However, the possible use of PET to find disease and...

Agree with Dr. @Dr. First Last. Couple of additional thoughts:

1. In addition to the surveillance with cross sectional imaging, switching out the modality can often identify what a CT scan missed (PET/CT, MRI [liver or pelvis], or a diagnostic lap - in the appropriate clinical context). Often have stu...