Photodynamic therapy (PDT) is a cancer treatment that has been used experimentally for various types of malignancies including head and neck, bronchial, gastric, skin, lung, bladder and esophageal carcinoma. PDT with Photofrin(r) has recently received FDA approval in the US for treatment of esophageal cancer.
PDT involves the interaction of a photosensitizing dye and red light, neither of which have any effect alone. Twenty-four hours after intravenous administration, the dye is cleared from normal tissue and localized in tumor tissue. Red laser light (chosen for optimal penetration) is applied to the tumor site. The red light is absorbed by the dye and raises the dye to an excited singlet state. The excited dye transfers energy to molecular oxygen in the tumor, resulting in highly reactive singlet oxygen that oxidizes biological targets such as cell membranes and organelles. The dye has a finite excited lifetime so the oxygen must be available near the excited dye molecule. The singlet oxygen also has a short lifetime and must be created near the desired target. PDT requires dual dosimetry for the drug dose and the light dose to ensure effective treatment of the entire tumor. It has been shown that a threshold light absorption is necessary to cause irreversible tumor necrosis. The light dose applied to the front surface must be sufficient to allow a threshold absorption at the deepest part of the tumor. Light penetration through the skin depends on tissue pigmentation, dye absorption and light scattering.
A material is optically characterized at the wavelengths of interest by microscopic absorption and scattering coefficients,k and s, and the anisotropy, g, or average cosine of the scattering angle. The light flux in a highly scattering medium can be calculated with the photon diffusion equation which is derived from radiative transfer theory. The distribution of light throughout the material as well as the diffuse reflectance and transmittance, R and T, of a material can be determined from k, s and g with mathematical tissue optics theories or with a Monte Carlo computer simulation. In order to determine the microscopic coefficients, the "inverse method" is employed: R and T are measured for an optically thin sample and k and s(1-g) are mathematically fitted to the measurements with optical theory.
The propagation of light through tissue has been successfully modeled with Monte Carlo
simulation. Photon "bundles" are injected into the tissue and allowed to move
throughout the specified dimensions with scattering and absorbing events occuring until
the bundle is attenuated. Light that emerges from the front or rear surface is counted as
reflection or transmission. Absorbed light is counted in a two or three-dimensional array
to allow for flux density output. Monte Carlo
Scattering Program V1.0 is the program I wrote that allows one to see light transport
in three dimensions through a user defined tissue sample.
This page originally written by Dr. Linda Jones from the College of Charleston's Physics department.