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TTT: local light absorption and heat convection versus heat conduction
  1. M A Mainster1,
  2. D H Silney2
  1. 1Department of Ophthalmology, University of Kansas Medical School, 3901 Rainbow Boulevard, Kansas City, KS 66160-7379, USA
  2. 2Laser/Optical Radiation Program, US Army Environmental Hygiene Agency, Aberdeen Proving Ground, MD 21010, USA
  1. Correspondence to: Professor Martin A Mainster Department of Ophthalmology, M53009, University of Kansas Medical School, 3901 Rainbow Boulevard, Kansas City, KS 66160-7379, USA; mmainsterkumc.edu

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Miura and co-authors have contributed valuable experimental data on transpupillary thermotherapy (TTT) for choroidal neovascularisation (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 flow2 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 seconds 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 (2) to a lesser extent 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

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