Читать книгу Intracranial Gliomas Part III - Innovative Treatment Modalities - Группа авторов - Страница 16

Light absorption is dependent on the various properties of the target tissue, but water content has the most significant impact. Its absorption sharply decreases from around 200–500 nm wavelength and steadily increases thereafter, but below 800 nm it is still very low in comparison with other chromophores. The absorption of oxyhemoglobin has its largest peak at 405–420 nm and 2 secondary peaks at 538 and 578 nm, and sharply decreases after 600 nm. Light absorption by melanin gradually decreases from 200 to 800 nm. In general, absorption spectra of the normal tissues of the human body make them least affected in a wavelength range between 650 and 700 nm. No significant light absorption by biological tissues is observed in the visible wavelength range above 600 nm with the exception of water [2].

Basically, the photo-irradiation wavelength for PDT is defined at the upper level of the optimal light absorption of each photosensitizer, since it is well known that the longer the wavelength is, the more deeply the light penetrates into tissues. The upper limits for clinically applicable photosensitizers Photofrin^®, Alaglio^® (5-ALA), Foscan^®, and Laserphyrin^® are 630, 630, 652, and 664 nm, respectively. Usually photo-irradiation is attained with a strong laser beam of a single wavelength. The light source for PDT can be either coherent or incoherent, and can operate in continuous or pulsed mode. Light fractionation may produce beneficial results [7]. Recently, a new concept of metronomic PDT was introduced, in which low-dose photosensitizer and low-dose photo-irradiation are delivered continuously during extended periods of time [21].

PDT of brain tumor can be performed in 2 ways. First, it can entail surface illumination of the lesion and/or walls of resection cavity, which can be done either directly during surgery or after implantation of the balloon diffuser after mass removal. Second, an optical fiber may be stereotactically inserted directly into the neoplasm for interstitial therapy [2, 10].