Transpupillary laser phototherapy of tumors and vascular anomalies of retina and choroid: theoretical approach and clinical implications

Background: Small retinal and choroidal tumors situated near the optic nerve or macula, such as retinoblastomas and malignant melanomas, as well as various other anomalies, in particular vascular malformations, may successfully be treated by photocoagulation. Model assumptions geared towards maximizing efficiency and minimizing undesirable side effects are forwarded, and the most important parameters subserving photothermal destruction, such as radiation field and thermal energy, analyzed. The influence exerted by physical traits of various tissues involved are also considered. Methods: The model approximations presented are based on classical radiation and absorption laws, as well as on the scattering properties of the various tissues implicated, these being considered as a function of wavelength and their relevance to the photodestructive task at hand. Particular attention is paid to the rate processes and reaction kinetics of irradiated proteins. Conclusions: Radiation sources emitting in the near-infrared range of the electromagnetic spectrum, such as the diode (810 nm) and the cw Nd:YAG (1064 nm) lasers, are optimal for the treatment of tumors and large, voluminous entities (such as Hippel-Lindau angiomas), owing to the good tissue penetration properties of their light. Those emitting in the shorter wavelength range, such as the argon ion (488 and 514 nm) and first harmonic -- mode Nd:YAG (532 nm) lasers, are not suitable for such tasks, but they are ideal for the destruction of fine sanguinous structures, within which their light is strongly absorbed but through which it penetrates poorly. For the treatment of large anomalous structures, a combinded short/long wavelength -- strategy should also be considered as a viable alternative. Such a "wavelength mixture" is emitted by the xenon high pressure lamp of the once renowned Meyer-Schwickerath light coagulator. The precision and safety of photothermal destruction methods depend, in the first approximation, upon the details of the pulse energy deposition (wavelength, pulse hight and duration, pulse dynamics, mode composition, focussing), the choice of which is determined by the optical and thermal constants of the irradiated tissues. Higher pulse energies will, of course, be more effective in destroying neoplastic cells, but the risk of producing undesired collateral heat damage will also increase concomitantly. Until we can ascertain the physical properties of the pathological tissues treated with certainty, we will hardly be able to achieve more than an approximation of an ideal treatment strategy. But with increasing developments in in vivo-diagnostic techniques, we expect that this goal will be attainable in the not too distant future.