[gdpawel]

It would be important to develop a method of in vivo labelling of tumor cells in the circulation and to monitor their trafficking and homing to other sites. If these cells are viable and therefore able to disseminate, I think the most robust test to this end is to document their ability to metastasize. It's perhaps useful as an adjunct to traditional methods for following tumor response, such as x-rays, blood tests, CTs, MRIs, history, physical exam, etc.