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The use of the photosensitizer Photofrin' (dihematoporphyrin ether, DHE)' in the photodynarnic treatment of human disease is becoming well known and widely used. While a number of photosensitizers may be used for photodynamic therapy (PDT), the porphyrins, in particular Photofrin', have received the most attention. This is specifically true for human trials.25 Clinical studies in photodynamic therapy (PDT) have utilized lasers to take advantage of coupling efficiencies to optical fibers allowing light to be delivered to many areas of the body. This is particularly true in endoscopic PDT. Both interstitial and superficial delivery techniques can be applied using one of a variety of delivery fibers available. A fiber with an optically flat end with a lens to produce a spot with a homogeneous intensity is used for superficial applications. Diffusers of various lengths, at the tip of a fiber, produce a cylindrical isotropic pattern and are suited for either intraluminal or interstitial illuminations, Previous studies have measured space irradiance produced by cylindrical fibers to determine the extent to which conditions of use would affect light distribution.6 Further, those studies attempted to assess the impact of variations in light distribution on the therapeutic effect. The therapeutic effect, however, was based upon the use of the cylindrical fiber as an interstitial implant and conclusions, therefore, were made in that context. The manner in which energy is distributed in air by fiberoptics can be described in terms of those surfaces through which the energy flux (energy per unit area per unit time) is everywhere constant. Since the therapeutic light dose has units of energy per unit area, these surfaces are properly called 'isodose-rate' surfaces. A field of treatment that conforms to an isodose-rate surface receives an incidental uniform light dose in a time interval (sec) given by the numerical ratio of the light dose (Joules/cni2) to the dose rate (watts/cm2). The isodose-rate surfaces in the air of the microlens and the sphere diffuser are simple surfaces, namely planes perpendicular to the light beam and spheres centered on the light source, respectively. The isodose-rate surfaces of the cylinder diffuser do not have a simple cycometry. In the following, these surfaces are presented for a line source which is a good approximation of the cylinder diffuser.
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