Irradiance, Photofrin(®) Dose and Initial Tumor Volume are Key Predictors of Response to Interstitial Photodynamic Therapy of Locally Advanced Cancers in Translational Models

The objective of the present study was to develop a predictive model for Photofrin(®)‐mediated interstitial photodynamic therapy (I‐PDT) of locally advanced tumors. Our finite element method was used to simulate 630‐nm intratumoral irradiance and fluence for C3H mice and New Zealand White rabbits bearing large squamous cell carcinomas. Animals were treated with light only or I‐PDT using the same light settings. I‐PDT was administered with Photofrin(®) at 5.0 or 6.6 mg kg(−1), 24 h drug‐light interval. The simulated threshold fluence was fixed at 45 J cm(−2) while the simulated threshold irradiance varied, intratumorally. No cures were obtained in the mice treated with a threshold irradiance of 5.4 mW cm(−2). However, 20–90% of the mice were cured when the threshold irradiances were ≥8.6 mW cm(−2). In the rabbits treated with I‐PDT, 13 of the 14 VX2 tumors showed either local control or were cured when threshold irradiances were ≥15.3 mW cm(−2) and fluence was 45 J cm(−2). No tumor growth delay was observed in VX2 treated with light only (n = 3). In the mouse studies, there was a high probability (92.7%) of predicting cure when the initial tumor volume was below the median (493.9 mm(3)) and I‐PDT was administered with a threshold intratumoral irradiance ≥8.6 mW cm(−2).