Techniques are generally described for a treatment balloon, and methods and systems effective to detect a position of a beam. The system may include a treatment balloon effective to receive the beam. The treatment balloon may include a shaft and a balloon portion with an open end in communication with the shaft. The balloon portion may be effective to be at least partially filled with a fluid. The treatment balloon may be effective to generate a light wave when the treatment beam is incident on the treatment balloon. A waveguide in the treatment balloon may be effective to receive the light wave. A light detector in optical communication with the waveguide may be effective to detect the light wave. A processor in communication with the light detector may be effective to detect the position of the beam.