On the physical rationale of laser induced hyperthermia

Abstract

The rationale of hyperthermic treatment of tumours is based on the phenomenon that several tumours have a reduced tolerance for exposure to temperatures in the region 42-47 °C for 20-30 min. Hyperthermia may also be used as an adjunct to other modalities such as chemotherapy, conventional radiation therapy or photodynamic therapy. Methods for introducing hyperthermia include electromagnetic radiation in the radiofrequency and microwave region as well as ultrasonic waves. The use of lasers emitting within the visible and near infrared part of the spectrum has the potential to initiate a local elevation of the temperature. The extension of the heated region will depend on optical wavelength, tissue composition and blood perfusion. Typical linear dimensions of the 42-47 °C temperature region will range from 2-3 mm and up to about 10 mm. Superficial tumours may be heated by direct irradiation whereas deeper lying tumours or lesions of large extent may require light delivered by one or several inserted optical fibres. Laser-induced hyperthermia may be of particular relevance in the treatment of retinal or choroidal tumours. Visible and near infrared radiation can be transmitted through the cornea, the lens and the vitreous with negligible loss. The absorption in the tumour is, however, significantly larger and the main part of the optical power will be absorbed within 0.5-5 mm into the neoplastic tissue. This paper emphasizes a discussion of the basic principles. © 1990 Baillière Tindall.