INTEGRATING SPHERE EFFECT IN WHOLE-BLADDER-WALL PHOTODYNAMIC THERAPY .3. FLUENCE MULTIPLICATION, OPTICAL PENETRATION AND LIGHT-DISTRIBUTIONWITH AN ECCENTRIC SOURCE FOR HUMAN BLADDER OPTICAL-PROPERTIES

Whole-bladder-wall (WBW) photodynamic therapy (PDT) is performed using approximate to 630 nm light emitted by an isotropic light source cent red in the bladder cavity. The phenomenon of an increased fluence rate in this spherical geometry, due to light scattering, is denoted as th e integrating sphere effect. The fluence rate and the optical penetrat ion depth depend on a single tissue optical parameter, namely the redu ced albedo. The optical properties of (diseased) human bladder tissue, i.e. absorption coefficient, scattering coefficient, anisotropy facto r and refractive index, were determined in vitro in the wavelength ran ge of 450-800 nm. The integrating sphere effect and optical penetratio n depth were calculated with diffusion theory and compared to Monte Ca rlo (MC) computer simulations using approximate to 630 nm optical prop erties. With increasing albedo, the integrating sphere effect calculat ed with diffusion approximation is increasingly larger than that found with MC simulations. Calculated and simulated optical penetration dep ths are in reasonable agreement. The smaller the integrating sphere ef fect for a given tissue absorption, the larger the optical penetration depth into the bladder wall, as the effective attenuation coefficient decreases. Optical penetration depths up to approximate to 7.5 mm (de finition dependent) can be responsible for unintended tissue damage be yond the bladder tissue. MC simulations were also performed with an ec centric light source and the uniformity of the light distribution at t he bladder wall was assessed. The simulations show that even for a sma ll eccentricity, the extremes in deviation from the mean fluence rate are large. All these results indicate that WBW PDT should be performed with some kind of in situ light dosimetry.