Disclosed herein are methods for identifying cancer cells and monitoring anti-cancer therapy in the body of a mammal by systemically delivering Sindbis viral vectors. The vector can specifically target and identify tumor cells in mice growing subcutaneously, intraperitoneally, intrapancreatically, or in the lungs. These findings demonstrate the remarkable specificity of the Sindbis vector system that is relatively safe and can specifically target tumor cells throughout the body via the bloodstream.