Interstitial photodynamic therapy (IPDT), involving light delivery through thin optical fibres inserted into the tumour mass could eliminate the limitations of PDT and allow treatment of solid, thicker and deeper-lying tumours. An instrument for interstitial photodynamic therapy has been developed in which six to eighteen bare-end, optical fibers are used to deliver the therapeutic light to the tumor mass. The same fibers can also be used to measure relevant dosimetry parameters during the treatment session. Throughout the therapy session, the instrument repeatedly interrupts the treatment mode and switches to monitoring mode, during which the light fluencerate, the photosensitiser fluorescence signal and the tissue oxygen saturation level are measured. The results are used as input for new dosimetry calculations, resulting in an updated treatment plan. This should lead to a more efficacious and safe treatment with IPDT.

The requirement for the tissue destruction is therefore the simultaneous presence of photosensitizer, tissue oxygen and light of a specific wavelength. Neither the photosensitizer nor the light exerts any therapeutic effect until it is combined in presence of tissue oxygen. Accurate PDT dosimetry, leading to a complete treatment of the tumour, while sparing adjacent normal or sensitive tissues, should allow for accurate prediction of the treatment-induced tissue damage based on the light and drug doses delivered and knowledge of variations in tissue oxygenation status during the treatment session.