A system and method for interstitial photodynamic light therapy (I-PDT) of a tissue. A plurality of light-transmitting catheters (LTCs) are provided and placed in the tissue according to a pre-determined treatment plan, wherein an LTC includes a first treatment fiber disposed therethrough, and an LTC includes a dosimetry fiber disposed therethrough. A dose light is provided to the tissue by way of the first treatment fiber according to the pre-determined treatment plan. Light received at the dosimetry fiber is measured using a spectrometer in operable communication with the dosimetry fiber. One or more properties of a photosensitizer in the tissue are determined. The treatment plan is modified based on the properties of the photosensitizer, and an updated dose light is provided to the tissue by way of the first treatment fiber according to the modified treatment plan.
