Focal laser ablation offers a minimally invasive method of treating solid organ tumors via hyperthermia. Real-time monitoring of the induced tissue damage is critical for clinical success, and is typically accomplished using thermal measurements and Arrhenius models. In this manuscript, the utility of interstitial fluence probes in assessing coagulation directly in real-time was assessed through a Monte Carlo simulation and an experimental study in tissue mimicking prostate phantoms. In the simulation results, fluence increases greater than 100% were observed inside the coagulation zone, as coagulation effectively acts as a ‘light trap’. Moreover, the passing of the coagulation boundary at any given point was shown to correspond with an inflection in fluence with a mean absolute difference of 0.1mm and 0.4mm observed for the simulation and phantom respectively. These results suggest that interstitial fluence probes may be capable of providing real-time feedback during focal laser ablation.