Disclosed is a device for interstitial laser therapy. The device comprises an optical waveguide extending about a central longitudinal axis and having an optical output end； an optical diffuser optically coupled to, optically associate with, or positioned about the optical output end, wherein the optical diffuser comprises a housing having an open end for receiving the optical output end and a first longitudinal portion of the optical waveguide； and a temperature sensor interposed, positioned or located between the central longitudinal axis and an exterior surface of the housing, and preferably within the longitudinal extent of the first longitudinal portion of the optical waveguide. The optical diffuser can be provided with one or more holes, one or more slits, one or more openings, and/or one or more vents. The device can also include a second temperature sensor. Also disclosed is a system for interstitial laser therapy.
