In situ light dosimetry during whole bladder wall photodynamic therapy: clinical results and experimental verification.

Photodynamic therapy (PDT) using Photofrin IIR (PII) as a photosensitizer is currently being evaluated as a new treatment modality for superficial bladder cancer. An optimum therapeutic ratio requires uniform illumination of the whole bladder wall and accurate light dosimetry. The first clinical light dosimetry results (16 patients) are reported, obtained using a system which allows in situ measurement and control of the light fluence at the bladder wall. The true light fluence at the bladder wall (i.e. non-scattered incident light plus scattered light) appeared to be on the average a factor beta = 4.8 +/- 1.2 (mean +/- SD) larger than the non-scattered incident light fluence. The latter is often, but incorrectly, used in reporting light fluence. The factor beta varied between patients with extremes of 2.5 and 7.1. Because such large variations were unexpected, but may have significant clinical consequences, experiments in plastic bladder models were performed to study separately the various factors (e.g. bladder shape, air bubble) affecting the dosimetry in clinical treatments. The results imply that the clinical variations are most likely to be the result of variations in optical properties of the bladder wall mucosa, probably due to the disease and prior treatments. If light dosage is based on non-scattered light only (without in situ light dosimetry, according to a current clinical protocol) the present results indicate variations in the true (total) light fluence between patients by a factor of at least 2. At the least this may cause unnecessary discomfort to a number of patients.