Integrating sphere effect in whole bladder wall photodynamic therapy: I. 532 nm versus 630 nm optical irradiation

The optical absorption, scattering and anisotropy coefficients of piglet bladder, with and without Photofrin, and of diseased human bladder were determined in vitro with a double integrating sphere set-up in the wavelength range 450-800 nm. Monte Carlo simulations were performed in a spherical geometry, representing the bladder, using the optical properties at 532 nm and 630 nm determined in vitro. The calculated fluence rates support the fluence rates that were measured at the bladder wall of a piglet during an in vivo whole bladder wall (WBW) irradiation at 532 nm and 630 nm. Fluence rates calculated and measured in vivo at 630 nm are in agreement with those measured previously in clinical photodynamic therapy (PDT) at 630 nm. WBW-PDT with red light (630 nm) will be technically more advantageous than with green light (532 nm) because of a stronger integrating sphere effect, which reduces the variations of the fluence rate at the bladder wall when the isotropic light source is moved away from the centre of the bladder. Since the optical properties show considerable variations from bladder to bladder, and since as a result the light fluence rate at the bladder wall can vary by a factor of 3 to 4 for the same non-scattered light fluence rate, we conclude that in situ light dosimetry during clinical WBW-PDT is a necessity