Changes in relative light fluence measured during laser heating: implications for optical monitoring and modelling of interstitial laser photocoagulation

Dynamic changes in internal light fluence were measured during interstitial laser heating of tissue phantoms and ex vivo bovine liver. In albumen phan toms, the results demonstrate an unexpected rise in optical power transmitt ed approximate to1 cm. away from the source during laser exposure at low po wer (0.5-1 W), and a decrease at higher powers (1.5-2.5 W) due to coagulati on and possibly charring. Similar trends were observed in liver tissue, wit h a rise in interstitial fluence observed during 0.5 W exposure and a drop in interstitial fluence seen at higher powers (1-1.5 W) due to tissue coagu lation. At 1.5 W irradiation an additional, later decrease was also seen wh ich was most likely due to tissue charring Independent spectrophotometric s tudies in Naphthol Green dye indicate the rise in fluence observed in the h eated albumen phantoms may have been primarily due to light exposure causin g photobleaching of the absorbing chromophore, and not due to heat effects. Experiments in liver tissue demonstrated that the observed rise in fluence is dependent on the starting temperature of the tissue. Correlating change s in light fluence with key clinical endpoints/events such as the onset of tissue coagulation or charring may be useful for on-line monitoring and con trol of laser thermal therapy via interstitial fluence sensors.