Dear Editor,

Drs. Miura, Nishiwaki and co-authors have contributed valuable experimental data on transpupillary thermotherapy (TTT) for choroidal neovascularization (CNV) in a rat model [1]. In their scholarly discussion section, they speculate that the variability in power settings they encountered in heating experimental CNV may be due to a “variation of heat conduction in experimental CNV” [1]. There are more probable explanations for that variability. As reported previously in the authors’ reference 7: “light absorption in pigment clumps from prior focal photocoagulation can cause local hot spots in large TTT treatment fields” [2]. Additionally, local choroidal blood flow [2] may have been altered by vascular remodelling that occurred in the 14 days between the intense focal laser photocoagulation that the authors used to produce CNV and their subsequent liposomal-monitored TTT at the site.

Chorioretinal temperature rise from a lengthy 60 sec TTT exposure is affected (1) by pigmentation at the treatment site, which determines how effectively laser radiant energy is converted locally into thermal energy, and to a lesser extent; (2) by choroidal blood flow [3] which transfers thermal energy by heat convection away from the exposure site. It is unlikely that local heat conduction is altered significantly by the initial photocoagulation or subsequent tissue remodelling because heat conduction in most normal biological tissues is essentially the same as that of water [4-6].

References

[1]. Miura S, Nishiwaki H, Ieki Y et al. Chorioretinal temperature monitoring during transpupillary thermotherapy for choroidal neovascularisation. Br J Ophthalmol 2005;89:475-9.

[2]. Mainster MA, Reichel E. Transpupillary thermotherapy for age- related macular degeneration: long-pulse photocoagulation, apoptosis, and heat shock proteins. Ophthalmic Surg Lasers 2000;31:359-73.

[3]. Welch AJ, Wissler EH, Priebe LA. Significance of blood flow in calculations of temperature in laser irradiated tissue. IEEE Trans Biomed Eng 1980;BME-27:164-6.

[4]. Mainster MA, White TJ, Tips JH, Wilson PW. Retinal-temperature increases produced by intense light sources. J Opt Soc Am 1970;60:264-70.

[5]. Welch AJ, van Gemert MJC. Optical-thermal response of laser- irradiated tissue. New York: Plenum Press; 1995.

[6]. Mainster MA. Decreasing retinal photocoagulation damage: principles and techniques. Semin Ophthalmol 1999;14:200-209.