Ex vivo light dosimetry and Monte Carlo simulations for endobronchial photodynamic therapy

Abstract

The light distribution during photodynamic therapy of the bronchial tree has been estimated by measuring the fluence rate in ex vivo experiments on dissected pig bronchi. The trachea was illuminated (630 nm) with a cylindrical diffuser and the fluence rate was measured with a fibre optic isotropic probe. The experiment with the diffuser on the central axis was also simulated with Monte Carlo techniques using the optical properties that were determined with a double-integrating-sphere set-up. The results from ex vivo experiments and the Monte Carlo simulations were found to agree within the error of measurement (15%), indicating that the Monte Carlo technique can be used to estimate the light distribution for varying geometries and optical properties. The results showed that the light fluence rate in the mucosa of the tracheal tract may increase by a factor of six compared to the fluence rate in air (in the absence of tissue). This is due to the scattering properties of the tissue and the multiple reflections within the cavity. Further ex vivo experiments showed that the positioning of the diffuser is critical for the fluence rate in the lesion to be treated. When the position of the diffuser was changed from the central axis to near the lesion, the fluence rate in the mucosa increased significantly by several orders of magnitude as compared to the initial (central) illumination. The inter- and intraspecimen variations in this increase were large (+/- 35%) because of variations in optical and geometrical properties and light source positioning, respectively. These variations might cause under- or overdosage resulting in either insufficient tumour necrosis or excessive normal tissue damage.

Short- and long-term normal tissue damage with photodynamic therapy in pig trachea: a fluence-response pilot study comparing Photofrin and mTHPC

“…Similar variations in fluence rate caused by variations in optical properties (with red light of 630 nm) are found in the bladder (D'Hallewin et al, 1992) and in the skin (Hudson et al, 1994). In addition, the positioning of the diffuser in the lumen (Marijnissen et al, 1993;Murrer et al, 1995) and the output characteristics of the linear diffuser used (Murrer et al, 1996) have great influence on the fluence rate distribution in the tissue.…”

“…With the aid of a Monte Carlo model for the geometry of a linear diffuser in a trachea (Murrer et al, 1995) we estimated an overlap of less than 5% of the maximum fluence rate between separate fields when a diffuser of 1.5 cm was used with an inter-lesion spacing of 5 cm. This is possible in the pig's trachea which is generally longer than the 15 cm (3 × 5 cm) needed for the experiment.…”

“…When the probe is pressed against the mucosa, the reading of the probe calibrated in air has to be multiplied by a factor 1.07 (± 2%) (Murrer et al, 1995, Marijnissen and. It is important that the isotropic probe is kept free of mucus and blood to ensure correct calibration.…”

In situ light dosimetry during photodynamic therapy of Barrett's esophagus with 5‐aminolevulinic acid

“…[cm] of the diffuser corresponding to the Barretts epithelium length [14]. In this approach, the cylindrical diffuser is assumed to behave as a radially emitting line source with the output homogeneously distributed over the cylinder surface.…”

Multipurpose Monte Carlo simulator for photon transport in turbid media

“…This work deals with photon transport simulation in a complex setup in which multiple light sources illuminates the sample and several detectors are placed around the object. The goal is to develope a flexible code capable of simulating diagnostic [2,3] and therapeutic [4] particular emphasis on optical tomography, either diffuse (DFOT) [5,6], or projective [7]. Fluorescence optical tomography (FOT), is a particular case of DFOT, where the aim is to retrieve the localization of fluorochrome-labeled agents.…”