Nd:YAG surgical laser effects in canine prostate tissue: temperature and damage distribution.

An in vitro model was used to predict short-term, laser-induced, thermal damage in canine prostate tissue. Canine prostate tissue samples were equipped with thermocouple probes to measure tissue temperature at 3, 6, 9 and 12 mm depths. The tissue surface was irradiated with a Nd:YAG laser in contact or non-contact mode for up to 20 s, using powers from 5 to 20 W. Prediction of thermal damage using Arrhenius theory was discussed and compared to the in vitro damage threshold, determined by histological evaluation. The threshold temperature for acute thermal tissue damage was 69 +/- 6 degrees C (means +/- SD), irrespective of exposure time. Contact mode laser application caused vaporization of tissue, leaving a crater underneath the fiber tip. The mean extent of tissue damage underneath the vaporization crater floor was 0.9 +/- 0.6 mm after 5, 10 or 20 s of contact mode laser irradiation at 10 W, whereas 20 W non-contact exposure up to 20 s causes up to 4.7 +/- 0.2 mm coagulation necrosis. It was concluded that short-term acute thermal tissue damage can be comprehensively described by a single threshold temperature.

Endovenous laser ablation: an experimental study on the mechanism of action.

OBJECTIVES: The aim of this experimental study was to investigate the mechanism of action of endovenous laser ablation (EVLA) using an 810-nm diode laser. METHODS: We compared intermittent and continuous delivery of laser energy and studied the absorption of laser light by blood, intravascular temperatures in ex vivo human vein segments using an intravascular thermography catheter and heat dissipation in a model tissue using the Schlieren technique. RESULTS: Laser light is absorbed by blood and converted to heat leading to coagulation, vaporization and carbonization, and forming an isolating layer at the fibre tip. Laser energy is then absorbed into the isolating layer forming black patches that burned on the laser fibre. Intravascular temperature increased rapidly above carbonization temperatures (300 degrees C) after the fibre tip reached the thermocouple, stayed at this temperature for a few seconds and decreased gradually to around 30 degrees C, 10 s after the fibre tip passed the thermocouple. Schlieren techniques revealed that heat spread from the laser was locally distributed and closely around the laser fibre tip while heat dissipation is minimal and comparable for both exposures. Compared with intermittent exposure, continuous exposure results in more carbonization, higher mean maximum intravascular temperature (128 +/- 7 vs. 75 +/- 4 degrees C), and long-lasting temperature of 100 degrees C (1.2 +/- 0.4 vs. 0.1 +/- 0.1 s). CONCLUSION: In this experimental study, application of endovenous laser shows to be dominated by carbonization at the fibre tip. Although intraluminal laser-induced heat was heterogeneously distributed, with laser tip temperatures up to 1200 degrees C, heat dissipation was minimal. Continuous exposure of laser light appears to be better suited in EVLA than intermittent.

Transscleral laser thermotherapy of hamster Greene melanoma: inducing tumour necrosis without scleral damage.

The feasibility of using transscleral thermotherapy (TSTT) to induce necrosis of choroidal melanoma without causing damage to the sclera was investigated. Fifty-two subcutaneously implanted hamster melanomas covered by human donor sclera were irradiated for 1 min with an 810 nm laser using a 3 mm spot diameter, with and without cooling of the scleral surface. Immediately after irradiation the temperature of the scleral surface was measured with an infrared camera. Irradiation at 2000 mW, without cooling of the sclera, resulted in tumour necrosis to a mean depth of 4.4 mm and a mean diameter of 5.5 mm, without causing damage to the scleral collagen; the surface temperature of the sclera was 44.5 degrees C, and the estimated temperature at the sclera-tumour interface was 60.1 degrees C. There was a sharp demarcation between the viable and the necrotic part of the tumour. TSTT at 2500 mW, without cooling of the sclera, caused maximal tumour necrosis to a mean depth of 5.2 mm and a mean diameter of 5.9 mm; the scleral layers adjacent to the tumour had an estimated temperature of 67.6 degrees C and showed signs of damage in 14% of the experiments. Cooling of the sclera resulted in less thermal damage to the sclera but also less tumour necrosis. Results indicate that TSTT has potential in the treatment of choroidal melanoma.

Temperature dependence of thermal damage to the sclera: exploring the heat tolerance of the sclera for transscleral thermotherapy.

Thermal damage to the human sclera in relation to temperature and duration of exposure was studied in order to determine the heat tolerance of the sclera with respect to transscleral thermotherapy of choroidal melanoma. Samples of human sclera were submerged in saline for 10 sec to 10 min at temperatures of 37-100 degrees C. The effects of heat on the shape, weight and size of the samples were studied. Thermal damage of scleral collagen was examined by polarized light microscopy (LM) and electron microscopy (EM). The sclera was embedded in epoxy resin and stained with toluidine blue for LM and with uranyl acetate and lead citrate for EM. Thermal damage of scleral collagen on polarized LM was graded on a five point scale. Scleral damage was visible on macroscopic examination and on LM and EM in sclera heated at 65 degrees C for 20 sec or longer, at 70 degrees C for 10 sec or longer, and at higher temperatures. A sigmoidal function was used to define the relation between temperature and changes in diameter, thickness, and weight of scleral samples. Using fitted functions, the threshold temperature for thermal damage was estimated to be 59-61 degrees C when samples were heated for 10 min, 62-63 degrees C when heated for 1 min, and 66-67 degrees C when heated for 10 sec; the threshold exposure time at 60 degrees C was estimated to be 7-12 min. These results indicate a temperature of 60 degrees C for 1 min is well tolerated by human donor sclera; information of in vivo studies is required to validate whether this setting can be used in transscleral thermotherapy (TSTT) for choroidal melanoma.

Uniform irradiation of irregularly shaped cavities for photodynamic therapy.

It is difficult to achieve a uniform light distribution in irregularly shaped cavities. We have conducted a study on the use of hollow 'integrating' moulds for more uniform light delivery of photodynamic therapy in irregularly shaped cavities such as the oral cavity. Simple geometries such as a cubical box, a sphere, a cylinder and a 'bottle-neck' geometry have been investigated experimentally and the results have been compared with computed light distributions obtained using the 'radiosity method'. A high reflection coefficient of the mould and the best uniform direct irradiance possible on the inside of the mould were found to be important determinants for achieving a uniform light distribution.