EMFs and Alzheimer's.

Red light (c. 630 nm) is almost universally used in tumour phototherapy as it is the most penetrating of the porphyrin excitation wavebands. However, measurements of tumour attenuation of light of different wavelengths and of the excitation spectrum of haematoporphyrin derivative in vitro suggested that green light might be more efficient than red in destroying thin tumours. Experimentally, we confirmed this for tumours up to-1.2mm thick, a depth exceeding that of most carcinomas-in-situ. The superiority of green light over red in terms of the illumination time required to produce equivalent depths of necrosis may extend to greater depths (3-4 mm) if the former is produced by an argon laser and the latter by an argon-pumped dye laser. The relation between depth of necrosis zn and light dose D is shown to be zn=a x 'n(D/lA) where 0A iS the attenuation coefficient for light at wavelength A and 0A the threshold light dose for producing necrosis at that wavelength. This logarithmic relationship suggests that it may be difficult to eradicate large tumours merely by increasing the light dose, and indicates the need for other approaches. Tumour phototherapy depends on the ability of certain photosensitizing agents to localise in tumours, so that exposure of the tumour to sufficiently intense light of appropriate wavelength causes its rapid necrosis. The photosensitiser most widely used at present is "haematoporphyrin deriva-tive" (HpD), a complex mixture of porphyrins (Bonnett et al., 1980, 1981; Berenbaum et al., 1982). HpD is excited at a number of discrete wavebands, most intensely by violet light at 400-410nm (the Soret band), and with much less (and decreasing) intensity by bands at 500-505nm, 535-540nm, 565-575 nm and 620-635 nm. Only the red waveband (620-635nm) is used in tumour photo-therapy as it has the greatest tissue penetration, an essential requirement in treating most human tumours, where light of adequate intensity may be needed through 1 cm or more of tissue. However, the use of red light has disadvantages. First, generation of sufficiently intense red light requires (at present) use of a dye laser pumped by an ion laser, a costly and complicated arrangement. Second, the red waveband is the least efficient exciter of the porphyrin molecule. Many human carcinomas do not demand use of highly penetrating light. For example, carcinoma-in-situ is usually well under 1 mm thick in most sites, and many early invasive carcinomas are only a few mm thick. The question therefore …