Transpupillar laser phototherapy for retinal and choroidal tumors: A rational approach

Background: The physical laws are considered that need to be taken into acc ount for optimal photothermal treatment of solid and vascular tumors, as we ll as other vascular anomalies of various etiology of the retina and choroi d. Optimal irradiation therapy should take into account the distribution of both radiant and thermal energy in tumors, such as retinoblastomas, malign ant melanomas and vascular malformations. Strict confinement of the extent of photothermal damage is critical since such pathological entities are fre quently located close to the macula or optic nerve head. Methods: A formal treatment of the optical quantities related to these requirements is presen ted. In this analysis we emphasize the following topics: Arrhenius' law, th e kinetics of protein denaturation, the electromagnetic radiation field, wa velength, laser pulse duration (exposure time), optical properties of tissu e, photocoagulation and thermotherapy. Results: Generally, the conditions a re best fulfilled when using radiation in the near-infrared range of the el ectromagnetic spectrum, such as that emitted from the diode (810 nm) and Nd :YAG (1064 nm) laser, because of the good optical penetration properties of this radiation in tissue. The xenon are lamp was a very effective and part icularly appropriate energy source for such purposes, and its withdrawal fr om the world market may have been untimely. Short wavelength sources of rad iation, such as the argon ion (488, 514 nm) or the frequency-doubled Nd:YAG (532 nm) laser, are unsuitable for the irradiation of large vascular struc tures, as they have poor penetration depths. However, for vascular formatio ns with a short path length (1 mm or less), short wavelength sources appear to be the most appropriate choicer Optical coupling of radiant energy to t he eye by means of indirect ophthalmoscopic systems or contact lenses to th e eye is crucial. Strong positive lenses may lead to severe constriction of the laser beam which leads to high irradiance within the anterior segment, increasing the chances of it being damaged; with negative contact lenses, such as the -64 D Goldmann type, this danger is reduced. Conclusions: Photo thermotherapy is not without risk unless the temperature field can be well adapted to the tumorous structure, as temperature elevations outside a smal l therapeutic range that affect vital structures are considered to be a ris k factor.