Previous dosimetric studies during PDT of superficial cavitary lesions such as Barret’s esophagus, trachea and nasopharyngeal malignancies demonstrated significant intra- and inter-patient variations in fluence rate build up as a result of tissue surface re-emitted and reflected photons. The total fluence rate at the cavity surface consists of a direct incident term, i.e. photons directly from the source entering the tissue and a scatter term and will therefore affect the response to PDT as a result of the number of photons available for photosensitizer absorption. The build-up factor depends on the optical properties and geometry. Recently a Foscan™ mediated PDT study of tumors in the paranasal sinuses after partial maxillectomy and/or endoscopic sinus surgery was initiated. These geometries are complex in shape, and have spatially varying optical properties, therefor pre-planning and in-vivo dosimetry to ensure a steady and effective light dose delivered to the tumor while securing the vital structures is required.
For this purpose, two 3D mesh based light transport models were developed. An empirical model that utilizes a linear function that determines the build-up factor as function of the source to surface distance, and an analytical model based on Lambert’s cosine law and an average diffuse reflectance. Both models were evaluated by means of 3D printed tissue optical phantoms, and a porcine tissue model. The models are within ± 20% accurate and have the potential to determine the optimal source location (OSL) along with the output power settings. EM navigation can be used to guide the source to the OSL.