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The basic flow diagram for PDT is shown in the figure.

LIGHT + DRUG --> ACTIVATED DRUG --> REACTIVE OXIDIZING SPECIES ---> OXIDATION ---> CELL DEATH

The amount of LIGHT and the amount of photosensitizing DRUG are the two key parameters for an effective PDT dose.

The activated drug transfers its activated energy to molecular oxygen to yield reactive oxidizing species (ROS) such as singlet oxygen that oxidize sites within the cells of a tissue. Accumulation of oxidative injury causes either (1) membrane damage to yield cell death by necrosis, or (2) oxidative injury that elicits apoptosis or programmed cell death. Hence OXYGEN is the third important factor in PDT. People are working on methods for routinely measuring tissue oxygen levels at the time of treatment.

A practical "Photodynamic Dose" was proposed by Patterson, Wilson, and Graff based on the number of photons absorbed by photosensitizing drug per gram of tissue or [ph/g]. This number equals the number of activated drug molecules per gram tissue. A typical value for the threshold "Photodynamic Dose" is 10¹⁸ [ph/g]. Since the drug accumulation in the tissue and the light penetration into the tissue are measurable at the time of treatment, this practical "Photodynamic Dose" is a currently feasible measurement, despite its neglect of the later steps of activating oxygen and achieving lethal oxidative damage.

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