An illuming device for use in medically illuminating tissue of interest within a patient generally includes a light diffusing optical fiber and an at least partially optically opaque sheath that is mounted over the fiber for selective, sliding movement along the length thereof. By sliding the sheath relative to the optical fiber after it has been inserted into a patient to illuminate a particular area of tissue, the length, size, and/or shape of the light that is exposed to the tissue can be selectively controlled. Such in situ variation of the properties of the exposed light permit use of a single instrument and a single procedure regardless of variation in the size and shape of the diseased tissue being medically treated.
