One idea: if you have a biomarker you can track via CTCs or ctDNA, then you could watch the rate at which that marker's availability increases. The PSA doublin

One idea: if you have a biomarker you can track via CTCs or ctDNA, then you could watch the rate at which that marker's availability increases. The PSA doubling time for indolent prostate cancers can often be measured in years, whereas CA 19-9 doubling time for pancreatic cancer is more often measured in days or months.