How useful are circulating tumor cells as a tumor marker?

For certain cancers: Circulating tumor cell count is done and quite useful to see the response of treatment given for certan type of metastatic cancers only -i, e, metastatic breast cancers, prostate, and colon cancer. Not indicated / used for other type of cancers and not used for earlier stage of cancer.

Not good: In my opinion, that test has not been perfected yet. There still are a lot of false positives. I believe that a good clinical exam, a compliant patient, and standard lab (if needed) is all that is needed. Now, is someone has known metastatic disease, then ct scans are important if there is measurable disease. Hope this helps.