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TTT for occult CNV: check the power!
  1. A J Mueller,
  2. M Ulbig,
  3. A Kampik
  1. Eye Clinic of the University, Mathildenstrasse 8, 80336 Munich, Germany
  1. Correspondence to: Dr Arthur J Mueller; amueller{at}ak-i.med.uni-muenchen.de

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Transpupillary thermotherapy (TTT) was originally introduced for the treatment of small choroidal melanoma.1 Although the precise mechanism of action for this treatment is unknown yet, the reported data appear to be beneficial compared to the natural course of the disease.

Currently, the following treatment parameters are recommended to treat occult choroidal neovascularisation (CNV) with TTT: one exposure, 60 second exposure time, 800 mW power for the 3 mm spot, 530 mW for the 2 mm spot, and 320 mW for the 1.2 mm spot. With these parameters applied via the Goldmann fundus lens the power per field is calculated to 247 mW/mm.2 This usually results in no visible change of the treated area at the end of exposure. Thus, if the applied power would be inadvertently lower than 800 mW for the 3 mm spot (or the power equivalent for other spot sizes) the treating physician would not notice he has to compensate for the lower power by adjusting the instrument’s power level.

We have used a diode laser (Iridex Corporation, Mountain View, CA, USA) with the recommended fibreoptic adapter since 1997 for the treatment of choroidal melanomas and haemangiomas without any apparent problems.3,4 Following the report by Reichel and coworkers, we started treating patients with occult CNV according to the recommended treatment parameters described above.

However, we did not observe satisfactory functional results in the majority of the first consecutive 28 cases treated and in most cases no effect at all could be demonstrated in the fluorescein angiogram or OCT.

Lacking any possible explanation other than that the treatment method didn’t work, we finally asked a technician to check the laser system. Immediately he noticed barely visible defects at the adapter of the fibreoptic resulting in a measurable reduction of power output from the 800 mW on the display to 560 mW at the end of the fibreoptic. Apparently the fibreoptic had been damaged by its frequent use in treating patients with choroidal tumours. However, this remained unnoticed by the oncologist because in treating choroidal melanomas the end point of an exposure is a “visible” grey to white lesion. If this is not achieved with the initially chosen power level, the physician usually increases the power in a stepwise manner until this end point is reached.

In summary, we highly recommend to all physicians who are treating occult CNV with subthreshold TTT to check their aiming spot regularly, especially if the system has been in use for a longer time. Additionally, the output power of the entire application system (laser and fibreoptic) should probably be rechecked at regular intervals depending on the frequency of the instrument’s use. This might prevent future reports of the (“false negative”) inefficiency of this possible beneficial treatment method.

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Footnotes

  • None of the authors has a proprietary interest in any of the products or procedures mentioned in this manuscript.

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