Determination of in vivo light fluence distribution in a heterogeneous prostate during photodynamic therapy.

Light fluence delivered to the tumor volume is an important dosimetry quantity in photodynamic therapy (PDT). The in vivo measurements in four patients showed that light fluence rates varied significantly in a prostate during PDT. The maximum and the mean fluence rates in a quadrant varied from 74 to 777 mW cm(-2) and from 45 to 385 mW cm(-2), respectively, among 13 quadrants of four patients' prostates. To determine three-dimensional (3D) light fluence rate distribution in a heterogeneous prostate, a kernel model was developed. The accuracy of the model was examined with a finite-element-method (FEM) model calculation, a phantom measurement, and the in vivo measurements. The kernel model calculations showed good agreements with the FEM model calculation and the measurements. The maximum and the mean deviations of the kernel model calculation from the in vivo measurements in the four patients were 23% and 4%, respectively. The kernel model, which is based on an analytic expression of a point source in a spherically symmetrical heterogeneity, has the advantage of fast calculation and is suitable for real-time PDT treatment planning.