A light dosimetry system is developed for prostate PDT, which integrates four main components: a light fluence rate
calculation engine, an optimization tool for treatment planning, a light delivery system, and an in vivo light fluence rate
measurement system. Three-dimensional light fluence rate distribution in a prostate is calculated using a kernel
algorithm, which takes into account of heterogeneous optical properties. A Cimmino optimization algorithm is used to
optimize the parameters of the cylindrical diffusing fibers (CDFs) to generate uniform PDT dose (or light fluence rate
under uniform drug distribution) to cover the heterogeneous prostate. The light delivery system is composed of a 12-
channel beamsplitter and the intensities of each channel (i.e., source) are controlled individually by programmable
motorized attenuators. Our tests show that the light fluence rate calculation is fast and the accuracy is close to that of a
finite-element method model, and the approach that uses the treatment CDFs to determine optical properties, improves
the accuracy of light fluence rate prediction. The light delivery system allows real time control of the light source
intensities for both PDT dosimetry and PDT light delivery. Integrating the fast light fluence rate calculation, optimization,
instant source intensity adjustment, and in vivo light fluence rate measurement, the dosimetry system is suitable for
prostate PDT.
Endoscopic and interstitial diffuse optical tomography have been studied in clinical investigations for imaging prostate
tissues, yet, there is no comprehensive comparison of how these two imaging geometries affect the quality of the
reconstruction images. In this study, the effect of imaging geometry is investigated by comparing the cross-section of the
Jacobian sensitivity matrix and reconstructed images for three-dimensional mathematical phantoms. Next, the effect of
source-detector configurations and number of measurements in both geometries is evaluated using singular value
analysis. The amount of information contained for each source-detector configuration and different number of
measurements are compared. Further, the effect of different measurements strategies for 3D endoscopic and interstitial
tomography is examined. The pros and cons of using the in-plane measurements and off-plane measurements are
discussed. Results showed that the reconstruction in the interstitial geometry outperforms the endoscopic geometry when
deeper anomalies are present. Eight sources 8 detectors and 6 sources 12 detectors are sufficient for 2D reconstruction
with endoscopic and interstitial geometry respectively. For a 3D problem, the quantitative accuracy in the interstitial
geometry is significantly improved using off-plane measurements but only slightly in the endoscopic geometry.
An improved interstitial diffuse optical tomography (iDOT) system has been developed to characterize the optical
properties of prostate gland during the photodynamic therapy (PDT). Multiple cylindrical light diffusers with different
lengths (instead of point sources used in an earlier version) and isotropic detectors are introduced interstitially in the
prostate gland in-vivo. During the data acquisition, linear sources and detectors are stepping into prostate sequentially
controlled by a motorized system. A computerized multi-channel attenuator system is developed to automatically control
the power strength of each linear source and on times to speed up data acquisition. Three dimensional optical properties
are obtained by solving the inverse problem of steady-state diffusion equation based on an adjoint model with Moore-
Penrose scheme. The convergence, accuracy and the speed of the algorithms are tested in mathematical phantoms and in
prostate simulating phantoms with known optical properties. For comparison, the optical properties of tissue simulating
phantoms are also reconstructed using iDOT with multiple isotropic point sources. Data acquisition time in iDOT using
linear sources is at least 10 times faster than using the point sources with the total data acquisition time to be less than 1
minutes. Reconstruction results showed both algorithms can successfully recover the optical properties. Reconstruction
using linear sources/detectors acquisition mode is 20 times faster than the point sources/detectors method (30 minutes vs.
4 hours on a 3.4 GHz Pentium PC with 4 GB memory). We have demonstrated that linear-source/detector acquisition
mode out-performs the point-source mode, and is more practical to be implemented in the clinical settings.
Photodynamic therapy (PDT) dose, D, is defined as the absorbed dose by the photosensitizer during photodynamic therapy. It is proportional to the product of photosensitizer concentration and the light fluence. This quantity can be directly characterized during PDT and is considered to be predictive of photodynamic efficacy under ample oxygen supply. For type-II photodynamic interaction, the cell killing is caused by the reaction of cellular receptors with singlet oxygen. The production of singlet oxygen can be expressed as &eegr;D, where &eegr; is the singlet oxygen quantum yield d is a constant under ample oxygen supply. For most PDT, it is desirable to also take into account the effect of tissue oxygenation. We have modeled the coupled kinetics equation of the concentrations of the singlet oxygen, the photosensitizers in ground and triplet states, the oxygen, and tissue receptor along with the diffusion equation governing the light transport in turbid medium. We have shown that it is possible to express eta as a function of local oxygen concentration during PDT and this expression is a good approximation to predict the production of singlet oxygen during PDT. Theoretical estimation of the correlation between the tissue oxygen concentration and hemoglobin concentration, oxygen saturation, and blood flow is presented.
The parameters which limit supply of photosensitizer to the cancer cells in a solid tumor were systematically analyzed
using microvascular transport modeling and histology data from frozen sections. In particular the vascular permeability
transport coefficient and the effective interstitial diffusion coefficient were quantified for verteporfin-for-injection
delivery of benzoporphyrin derivative (BPD). Orthotopic tumors had a higher permeability and diffusion coefficients (Pd= 0.036 &mgr;m/s and D = 1.6 &mgr;m2/s, respectively) as compared to subcutaneously grown tumors (Pd = 0.025 &mgr;m/s and D = 0.9 &mgr;m2/s, respectively), likely due to the fact that the vessel patterns are more homogeneous orthotopically. In general,
large inter-subject and intra-tumor variability exist in the verteporfin concentration, in the range of 25% in plasma
concentration and in the range of 20% for tissue concentrations, predominantly due to these micro-regional variations in
transport. However, the average individual uptake of photosensitizer in tumor tissue was only correlated to the total
vascular area within the tumor (R2 = 64.1%, p < 0.001). The data is consistent with a view that micro-regional variation
in the vascular permeability, interstitial diffusion rate, all contributes the spatial heterogeneity observed in verteporfin
uptake, but that average supply to the tissue is limited by the total area of perfused blood vessels. This study presents a
method to systematically analyze microheterogeneity as well as possible methods to increase delivery and homogeneity
of photosensitizer within tumor tissue.
Explicit dosimetry of photodynamic therapy requires detailed knowledge of the light, drug, and oxygenation
distributions within the target tissue. We present a method for the optical detection and three-dimensional reconstruction
of hemoglobin concentration and oxygenation and sensitizer concentration within the human prostate. Spectrally
resolved diffuse transmission measurements were made using a small isotropic fiber-based white light source and an
isotropic detector inserted into the prostate via parallel closed transparent catheters. The spectra were modeled using the
diffusion approximation appropriate for infinite media. The optical absorption of the prostate was assumed to be a linear
combination of the absorption spectra of oxy- and deoxyhemoglobin and MLu, and the scattering was assumed to be of
the form A(&lgr;/&lgr;0)-b. The separation of absorption and scattering coefficients was accomplished based on the spectral
shape of the diffuse transmission, rather than the spatial variation in intensity. By making multiple measurements at
various source-detector separations, we investigate the signal-to-noise sensitivity of our algorithm. In addition, the
redundancy in our source-detector position matrix creates several positions in which the tissue parameters can be
reconstructed from multiple independent measurements, allowing an assessment of the repeatability of the algorithm.
We find significant heterogeneity in the reconstructed optical properties; however the recovery of spectrally consistent
absorption and scattering spectra is improved compared to wavelength-wise reconstruction algorithms.
We have developed an efficient Levenberg-Marquardt iterative algorithm utilizing a three-dimensional field
measurements coupled to a two-dimensional optical property reconstruction scheme. This technique takes advantage of
accurate estimation of light distribution in 3D forward calculation and reduced problem size and less computation time
in 2D inversion. Important advances in terms of improving algorithm efficiency and accuracy include use of an iterative
general minimum residual method (GMRES) for computing the field solutions, application of the dual mesh scheme and
adjoint method for Jacobian construction, and implementation of normalization scheme to reduce the absorption-scattering
cross talk. The synthetic measurement data were calculated for a cubic phantom containing a single
absorption anomaly and a single scattering anomaly. The model had a background of &mgr;a=0.03mm-1 and &mgr;s=1.4mm-1.
The absorption and scattering anomalies have the &mgr;a = 0.06 mm-1 and &mgr;s' = 2.0 mm-1. Five sources and 72 detectors are
used per slice. A typical human prostate is composed of 6 slices. The reconstruction images successfully recover the
both anomalies with good localization. Experiment data from tissue simulated phantom are also presented. The clinical
DOT imaging was performed before photodynamic therapy based on the protocol. The preliminary results showed the
reconstructed prostate &mgr;a varied between 0.025 and 0.07 mm-1 and &mgr;s' ranged from 1.1 to 2 mm-1. These results show
that this new 2D-3D hybrid algorithm consistently outperform the 2D-2D or 3D-3D counterparts.
Effective Photodynamic therapy (PDT) treatment depends on the amount of active photosensitizer and the delivered light in the targeting tissue. For the same PDT treatment protocol, variation in photosensitizer uptake between animals induces variation in the treatment response between animals. This variation can be
compensated via control of delivered light dose through photodynamic dose escalation based on online dosimetry of photosensitizer in the animal. The subcutaneous MAT-LyLu Dunning prostate tumor model was used in this study. Photosensitizer BPD-MA uptake was quantified by multiple fluorescence micro-probe measurements at 3 hours after verteporfin administration. PDT irradiation was carried out after photosensitizer uptake measurement with a total light dose of 75 J/cm2 and a light dose rate of 50 mW/cm2. Therapeutic response of PDT treatments was evaluated by the tumor regrowth assay. Verteporfin uptake varied considerably among tumors (inter-tumor
variation 56% standard deviation) and within a tumor (largest intra-tumor variation 64%). An inverse correlation was found between mean photosensitizer intensity and PDT treatment effectiveness (R2 = 37.3%, p < 0.005). In order to compensate individual PDT treatments, photodynamic doses were calculated on an individual animal basis, by matching the light delivered to provide an equal photosensitizer dose multiplied by light dose. This was completed for the lower-quartile, mean and upper-quartile of the photosensitizer distribution. The coefficient of variance in the surviving fraction decreased from 24.9% in non-compensated PDT (NC-PDT) treatments to
16.0%, 14.0% and 15.9% in groups compensated to the lower-quartile (CL-PDT), the median (CM-PDT) and the upper-quartile (CU-PDT), respectively. In terms of treatment efficacy, the CL-PDT group was significantly less effective compared with NC-PDT, CM-PDT and CU-PDT treatments (p < 0.005). No significant difference in effectiveness was observed between NC-PDT, CM-PDT and CU-PDT. The results indicate that by measuring the mean photosensitizer concentration prior to light treatment, and then adjusting the light dose appropriately,
a more uniform treatment can be applied to different animals thereby reducing the inter-individual variation in the treatment outcome.
This paper summarizes ten approaches to quantifying fluorescence in tissues, and contrasts their strengths and weaknesses, relative to what their common applications are, and should be. The major issues involved in this analysis are to compare the accuracy of the method and the ability to quantify the active (i.e. non-aggregated) fraction of fluorophore in the tissue. In addition, issues of the depth of penetration and the availability of the method come into play when clinical applications are required. In general, tissue extraction and liquification methods are the 'gold standard' in this field, yet these are plagued by large variance in the values, raising questions about their ability to report on the true active fraction of drug in the tissue. Confocal and fiber optic microsampling methods allow direct quantification of the active fluorescence in vivo and are able to quantify the heterogeneity in the tissue. Yet both of these methods sample the most superficial layers of a tissue, unless invasive injection of the probe is done. Macroscopic sampling of the tissue is therefore the preferred choice for clinical use, yet there is truly no optimum method which can sample the drug concentration to arbitrary accuracy. Empirical bulk tissue sampling methods are the most commonly used, yet without model-based interpretation of the values it is generally not possible to be quantitative. Even relative uptake values can be distorted by the shape of the tissue, and so raster scanning or model-based assessment of the fluorescent yield is preferable, if available. Extending this concept further, tomographic methods can be implemented to quantify fluorescence, and can even be coupled into existing clinical imaging systems, but development and optimization of these methods will be required in the coming years. These are outlined, and case examples illustrated in this paper.
The effect of sampling region size and tissue heterogeneity is examined using fluorescence histogram assessment in a rat prostate tumor model with benzoporphyrin derivative fluorophore. Spatial heterogeneity in the fluorescence signal occurs on both macroscopic and microscopic scales. The periphery of the tumor is more fluorescent than the center. Fluorescence is also highest nearest the blood vessels immediately after injection, but over time this fluorescence becomes uniform through the tumor tissue. Using microscopy analysis, the fluorescence intensity histogram distributions follow a normal distribution, yet as the sampling area is increased from the micron scale to the millimeter scale, the variance of the distribution decreases. The mean fluorescence intensity is accurately measured with a millimeter size scale, but this cannot provide accurate measurements of the microscopic variance of drug in tissue. Fiber probe measurements taken in vivo are used to confirm that the variance observed is smaller than would be expected with microscopic sampling, but that the average fluorescence can be measured with fibers. Sampling tissue with fibers smaller than the intercapillary spacing could provide a way to estimate the spatial variance more accurately. In summary, sampling fiber size affects the fluorescence intensities detected and use of multiple region microscopic sampling could provide better information about the distribution of values that occur.
Evaluating the light distribution in the esophagus is central to understanding how to optimize treatment of Barrett's esophagus with PDT. The light distribution of a cylindrical light diffuser in the esophagus was investigated by Monte Carlo simulations and experiments in esophageal simulating phantoms. Simulation of light transport in the esophagus requires knowledge of several biological parameters including radius, height, mucosal wall thickness and optical parameters such as reduced scattering coefficient, absorption coefficient, scattering anisotropy and refractive index. Results of Monte Carlo simulations were tested by measuring the light fluence rate in an esophagus phantom (absorption coefficient = 0.27 mm-1, reduced scattering coefficient = 1.9 mm-1, g = 0.9, n= 1.37) irradiated at 633nm with a 5cm long cylindrical light diffuser. We evaluated the light distribution within the esophagus for the on-axis and off-axis treatment fiber conditions both experimentally and mathematically. Results showed the reflected light from the walls of the esophagus increased the incident light dose at all points being treated by a factor of 1.6, compared to the dose given off by the fiber itself. Also, when the fiber was moved from one side of the esophagus to the other, it increased the light dose to the proximal area by a factor of 2, and decreased the light dose to the distal region by a factor of 2 in the phantom study, and in Monte Carlo simulation, factor of 18 and factor of 3 respectively.
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