Existing dosimetric quantities do not fully account for the dynamic interactions between the key components of photodynamic therapy (PDT) or the varying PDT oxygen consumption rates for different fluence rates. Using a macroscopic model, reacted singlet oxygen ( [ O 2 1 ] rx ) was calculated and evaluated for its effectiveness as a dosimetric metric for PDT outcome. Mice bearing radiation-induced fibrosarcoma tumors were treated with benzoporphyrin derivative monoacid ring A (BPD) at a drug-light interval of 3 h with various in-air fluences (30 to 350 ?? J / cm 2 ) and in-air fluence rates (50 to 150 ?? mW / cm 2 ). Explicit measurements of BPD concentration and tissue optical properties were performed and used to calculate [ O 2 1 ] rx , photobleaching ratio, and PDT dose. For four mice, in situ measurements of O 2