A method and system for controlling and adjusting light in interstitial photodynamic light therapy (IPDT) in a subject is disclosed. More particularly, a method for controlling the light in interstitial tumor photodynamic light therapy is described using a calculation method for determination of status of tissue during the PDT treatment. The status is used in a feedback loop to control the continued PDT treatment. Methods are disclosed that constitute pre-treatment and realtime dosimetry modules for IPDT on the whole prostate glandular tissue. The method includes reconstruction of the target geometry, optimization of source fiber positions within this geometry, monitoring of the light attenuation during the treatment procedure and updating individual fiber irradiation times to take into account any variation in tissue light transmission. A control device that is arranged to restrict delivery of therapeutic light treatment at least temporary in dependence of at least one attribute of one of photodynamic treatment parameters. In comparison to no treatment feedback, a significant undertreatment of the patient as well as damage to healthy organs at risk are avoided.
