Hyperthermia from interstitial laser irradiation in normal rat brain

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Abstract
This study examined both the effect of variations in optical fiber tip and in light wavelength on laser-induced hyperthermia in rat brain. Normal rat brains were exposed to argon laser light (454–514.5 nm) delivered through an intracerebral end-emitting (bare-tipped) or a diffusion-emitting (sapphire-tipped) optical fiber probe. Interstitial thermister probes recorded temperatures after thermal equilibration at varying distances from the emitting source. The end-emitting fiber produced significantly (P < 0.05) higher elevations in tissue temperature than the diffusion-emitting fiber at the same laser power output. This is due to the smaller surface area (1.2 mm2 versus 7.8 mm2) of the end-emitting fiber, which results in a greater rate of energy delivery to tissue adjacent to the fiber tip. Changes in intracerebral temperature measurements were also recorded at similar distances from a diffusion-emitting fiber at a continuous total laser power output of 150 mW for light wavelengths of 454–514.5 nm, 700 nm, and 750 nm and at a total laser output of 1.1 W for 1,060 nm. Variations in brain tissue temperature with distance from the laser emission source were compared for each laser group with the tissue temperature profile generated by a radiofrequency (wavelength 600–625 m) interstitial probe. Similar temperature changes were found for all visible wavelengths near the probe, suggesting that the thermal response of brain adjacent to an interstitial laser fiber is primarily dependent upon the rate of energy delivery and not upon wavelength. The thermal profile versus distance from the light source depends mostly upon the level of temperature rise near the interstitial laser fiber tip and not the wavelength of laser light used. These results have important implications in interstitial applications of laser for hyperthermia and photochemotherapy.