Abstract
Photodynamic therapy (PDT) is a well-established treatment modality for cancer and other malignant diseases;
however, quantities such as light fluence, photosensitizer photobleaching rate, and PDT dose do not fully account
for all of the dynamic interactions between the key components involved. In particular, fluence rate (Φ) effects are
not accounted for, which has a large effect on the oxygen consumption rate. In this preclinical study, reacted singlet
oxygen [1O2]rx was investigated as a dosimetric quantity for PDT outcome. The ability of [1O2]rx to predict the
long-term local tumor control rate (LCR) for BPD-mediated PDT was examined. Mice bearing radioactivelyinduced
fibrosarcoma (RIF) tumors were treated with different in-air fluences (250, 300, and 350 J/cm2) and in-air ϕ
(75, 100, and150 mW/cm2) with a BPD dose of 1 mg/kg and a drug-light interval of 3 hours. Treatment was
delivered with a collimated laser beam of 1 cm diameter at 690 nm. Explicit dosimetry of initial tissue oxygen
concentration, tissue optical properties, and BPD concentration was used to calculate [1O2]rx. Φ was calculated for
the treatment volume based on Monte-Carlo simulations and measured tissue optical properties. Kaplan-Meier
analyses for LCR were done for an endpoint of tumor volume ≤ 100 mm3 using four dose metrics: light fluence,
photosensitizer photobleaching rate, PDT dose, and [1O2]rx. PDT dose was defined as the product of the timeintegral
of photosensitizer concentration and Φ at a 3 mm tumor depth. Preliminary studies show that [1O2]rx better
correlates with LCR and is an effective dosimetric quantity that can predict treatment outcome.
