Light dosimetry for multiple cylindrical diffusing sources for use in photodynamic therapy

Abstract

Since prostatic carcinoma is usually multifocal within the prostate, effective photodynamic therapy (PDT) of prostatic carcinoma is expected to require the photochemical destruction of the entire organ. Accurate light dosimetry will be essential to avoid damage to proximal sensitive tissue such as the rectum. The prostate will be illuminated using interstitial cylindrical fibreoptic light sources and, because of the limited transparency of prostate tissue, these sources will be mounted in a parallel array analogous to the source array used in brachytherapy. Both source spacing and the light delivered to each source will control light dosimetry from a parallel array of fibreoptic sources implanted into tissue. Clinical PDT will require dose planning in order to determine the position and illumination of each source prior to treatment, but unfortunately few methods of predicting light fluence from cylindrical interstitial sources currently exist. In this paper, a novel light fluence model is used to predict tissue transillumination resulting from cylindrical interstitial sources. The cylindrical source is modelled as a finite array of infinitesimal small sources using Christian Huygens' famous single-slit diffraction model. We show that this source model when combined with a robust derivation of fluence in a spherical geometry using diffusion theory, accurately predicts fluence levels from a single cylindrical source in a variety of media. This method is found to retain its accuracy near the sources. With a simple extension, this fluence model is used to predict the light fluence levels from an array of three sources and the predicted fluence is found to compare favourably with experimental data.

Fractionated versus Standard Continuous Light Delivery in Interstitial Photodynamic Therapy of Dunning Prostate Carcinomas

“…Potentially, this change in blood flow and oxygenation can be monitored by sequential illumination and detection in the multifiber array and the duration of on/off sequences adjusted. Furthermore, real-time photosensitizer fluorescence detection in tumor is possible, thereby customizing light dosimetry, as much as possible, to shape PDT effect (35).…”

Magnetic resonance temperature imaging validation of a bioheat transfer model for laser‐induced thermal therapy

“…The resulting photon source is referred to as the conformal discretisation approximation (CDA) in this manuscript. The CDA model is similar to fully analytic attempts to capture the cylindrical geometry of the interstitial fibre-optic [13] but exploits the underlying finite element discretisation of the computational domain to evenly distribute the laser power amongst the axial length and finite diameter in the active tip region. An analogous discretisation approach was implemented on structured grids [12] and results in multiple individual optical diffusion approximation (ODA) along the centre line of the applicator.…”

“…An applied volumetric power density, $P^{\ast }\left[\frac{W}{m^3}\right]$, provides a photon flux throughout the active region of the interstitial laser fibre, Ω tip . Huygens superposition principle [13] is used to treat each position in the domain, x̂ ∈ Ω tip , as an isotropic differential point source of irradiance, d E ,…”

“…For these purposes, a fundamental model that couples the Pennes bioheat equation [9] to the diffusion theory of light transport in tissue [10, 11] is rigorously derived and critically evaluated against MR-derived thermal imaging data in a canine brain model. Similar to previous investigators, the presented model attempts to capture the geometrical effects of the cylindrical applicator [12, 13] and effects of the actively cooled applicator on the maximum temperature distribution [14]. The current study utilises isotropic constitutive parameters to evaluate the computer model of the laser–tissue interaction against real-time MR temperature imaging.…”

Feasibility of interstitial diffuse optical tomography using cylindrical diffusing fibers for prostate PDT

“…Most existing prostate DOT techniques were performed using point-like light sources, which may not be the same as the treatment light source (Li et al , 2007; Jiang et al , 2008; Xu et al , 2008; Wang and Zhu, 2009). Cylindrical diffusing fibers (CDFs) are often used for interstitial prostate PDT (Profio and Doiron, 1987; Sandell and Zhu, 2011), it provides more uniform light fluence distribution when there are highly absorbable media, such as blood, around the light source (Du et al , 2006; Dickey et al , 2004; Weersink et al , 2005; Zhu et al , 2005b). In this paper, we propose an interstitial CDF-DOT method to reconstruct optical properties for prostate PDT using the existing treatment CDFs as light sources.…”