DAMAGE THRESHOLD OF NORMAL RAT-BRAIN IN PHOTODYNAMIC THERAPY

Normal brain tissue response to photodynamic therapy (PDT) must be qua ntified in order to implement PDT as a treatment of brain neoplasm. We therefore calculated the threshold for PDT-induced tissue necrosis in normal brain using Photofrin (porfimer sodium, Quadralogic Technologi es Inc., Vancouver, BC) as the photosensitizer. The absolute light flu ence-rate distribution for superficial irradiation and effective atten uation depth were measured in vivo using an invasive optical probe, Ph otosensitizer uptake in cerebral cortex was measured with chemical ext raction and fluorometric analysis. Photodynamic therapy-induced lesion depths at various drug dose levels were measured as a biological end point. The PDT threshold for normal brain necrosis was calculated as i n the magnitude of 10(16) photons/cm(3). Thus normal rat brain is extr emely vulnerable to PDT damage, This suggests that extra precautions m ust be exercised when PDT is used in brain.