PILOT-STUDY ON LIGHT DOSIMETRY FOR ENDOBRONCHIAL PHOTODYNAMIC THERAPY

Endobronchial photodynamic therapy (EB-PDT) using photofrin as the pho tosensitizer is currently being evaluated as a new treatment modality for inoperable endobronchial tumors. One of the current problems with EB-PDT is the lack of adequate light dosimetry, which hampers proper i nterpretation of treatment results. In this study exploratory light do simetry experiments were performed in plastic bronchus models using ei ther a microlens-tipped fiber (suitable for illumination of small supe rficial tumors) or a cylindrical diffuser fiber (suitable for intralum inal illumination or interstitial illumination of partially obstructin g tumors). It is shown that the light fluence prescriptions of current clinical protocols yield a different fluence in tissue for each illum ination modality. Depending on the actual placement of the cylindrical diffuser within the lumen, the light fluence at 5 mm depth in the hom ogeneous tissue model may vary by a factor of 3. The results were conf irmed by in vivo experiments in the trachea of a pig. There is a possi bility of enhanced tissue response by accidental hyperthermia induced during EB-PDT. The temperature rise was therefore estimated in vivo us ing a rat tumor model to mimic clinical EB-PDT. Temperature rises of a t least 5-degrees-C and 10-degrees-C can be expected for intraluminal and intratumoral illumination, respectively, at 3.5 +/- 1 mm depth in tissue and 400 mW/cm diffuser output. Light fluence and its distributi on in the bronchus strongly depend on the geometry and the optical pro perties of the tissue as well as on the technique of illumination. As a result of inadequate dosimetry, significant variations in treatment response between patients may be expected.