A method includes implanting an implantable biosensor within a subject where the implantable biosensor has an array of light sources and an array of light detectors, activating the array of light sources to direct light signals at a targeted tissue site in the subject, capturing, with the light detectors, the light signals reflected off the targeted site, calculating a roundtrip propagation time for each of the light signals and comparing the roundtrip propagation time for each of the light signals against previous calculated respective roundtrip propagation times to determine an occurrence of a change in the targeted tissue site.