Dosimetric concepts for PDT

The introduction of drug consumption in the model of dose as the product of light dose together with the existence of thresholds for tissue necrosis results in a profound alteration in the perception of PDT dosimetry. Light doses had been limited by normal tissue toxicities which were the result of levels of drug. The consequence of that thought process was the dose of light (when limited to that which spares normal tissue) falls off sharply with depth [4,5]. Based upon the theories presented the consideration of consumption of drug can increase the effective photodynamic depth greater that twofold. The maximum injected drug dose which allows normal tissue to fully recover from an unlimited light dose appears from studies conducted in human patients with various cutaneous malignancies, to be 1 mg/kg. There is however, no reason a priori, to assume that for other tissues or specific applications (i.e., obstructing tumors vs. superficial disease) this is the optimum value. The gap between the current clinical practice of employing PDT using the simplest parameters of drug dose administered, light dose applied and the timeframe between them and a more concise description of specific dosimetry factors is presumably wide. However, in practice, based upon empirical data the drug and light dosage prescribed is sufficient to produce the desired clinical effect particularly in the approved indications. The challenge of developing methods and instrumentation in order to achieve a more effective dosing scheme which will allow for a more personalized dose modification based upon various factors to produce clinically relevant tissue response endpoints is yet to be accomplished. While promising for the future of PDT the concept of dosimetry ultimately needs to be determined and evaluated via clinical trials.