Affiliations: [a] Institute for Biomedical Engineering, University and Swiss Federal Institute of Technology, Zürich, Switzerland | [b] Postfach, Bern, Switzerland
Correspondence:
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Corresponding author: P. Niederer, Institute for Biomedical Engineering, University and Swiss Federal Institute of Technology, 8092 Zürich, Switzerland. E-mail:[email protected]
Abstract: Photothermal treatment of tumors of the retina and choroid such as
retinoblastomas, malignant melanomas, benign tumors as well as of vascular
malformations can be performed by using laser radiation. A number of basic
physical laws have to be taken into account in this procedure. Of particular
importance thereby are: Arrhenius' law to approximate the kinetics of
protein denaturation and photocoagulation, furthermore the electromagnetic
radiation field, the distribution of both radiant and thermal energy induced
in tumors and vascular structures, the influence of the wavelength and laser
pulse duration (exposure time), as well as of the optical properties of the
tissue. Strict confinement of the extent of the photothermal damage is
critical since such pathological entities are frequently located close to
the macula or optic nerve head. The conditions for tumor destruction are best fulfilled when using radiation
in the near-infrared range of the electromagnetic spectrum such as that
emitted from the diode (810 nm) and the Nd: YAG (1064 nm) laser, because of
the good optical penetration properties of these radiations in tissue. Short
wavelength sources of radiation, such as the argon ion (488, 514 nm) or the
freqeuency-doubled Nd: YAG (532 nm) laser are less well suited for the
irradiation of large vascular structures due to their poor penetration
depths. However, for vascular formations with a small thickness (1 mm or
less), short wavelength sources appear to be the most appropriate choice.
Optical coupling of radiant energy to the eye by means of indirect
ophthalmoscopic systems or positive contact lenses is furthermore of
importance. Strong positive lenses may lead to severe constrictions of the
laser beam within the anterior segment, that leads to high irradiance
increasing the probability for structures to be damaged; with negative
contact lenses, such as the -64 D Goldmann type lens, this danger is
largely absent.
