Body-mountable devices are provided to detect the presence or status of a tumor in a body by detecting probes associated with circulating cells of the tumor that travel to subsurface vasculature of the body. The probe enters a tumor and associates with cells of the tumor before the cells metastasize. A wearable body-mountable device can be worn for a protracted period of time to detect the probe associated with circulating tumor cells in the vasculature at low concentrations and/or at low rates. A body-mounted device could detect the presence of such released, tumor-cell-associated probes to determine a presence or status of a tumor in the body.