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

Interstitial PDT using diffuser fiber-investigation in phantom and in vivo models

Photodynamic therapy (PDT) has been used for local treatment of several types of tumors. Light penetration of biological tissue is one limiting factor in PDT, decreasing the success rates of the treatment of invasive and solid tumors. In those cases, a possible solution is to use interstitial PDT, in which both diffuser optical fibers are inserted into the tumor. The uniformity of the diffuser emission plays a crucial role in planning the delivery of the appropriate light fluence and in ensuring treatment success. In this study, we characterized a diffuser optical fiber concerning its homogeneity. We showed that the diffuser emission can be inhomogeneous and that the necrosis generated by interstitial PDT using such a diffuser for illumination is asymmetrical in volume as a result. This observation has relevant consequences in achieving success in PDT and phototherapies in general, as the delivered light fluence depends on adequate previous knowledge of the irradiation profile.

Determination of the radiance of cylindrical light diffusers: design of a one-axis charge-coupled device camera-based goniometer setup

A one-axis charge-coupled device camera-based goniometer setup was developed to measure the three-dimensional radiance profile (longitudinal, azimuthal, and polar) of cylindrical light diffusers in air and water. An algorithm was programmed to project the two-dimensional camera data onto the diffuser coordinates. The optical system was designed to achieve a spatial resolution on the diffuser surface in the submillimeter range. The detection threshold of the detector was well below the values of measured radiance. The radiance profiles of an exemplary cylindrical diffuser measured in air showed local deviations in radiance below 10% for wavelengths at 635 and 671 nm. At 808 nm, deviations in radiance became larger, up to 45%, most probable due to the manufacturing process of the diffuser. Radiance profiles measured in water were less Lambertian than in air due to the refractive index matching privileging the radial decoupling of photons from the optical fiber.

Optical dosimetry probes to validate Monte Carlo and empirical-method-based NIR dose planning in the brain

A three-dimensional photon dosimetry in tissues is critical in designing optical therapeutic protocols to trigger light-activated drug release. The objective of this study is to investigate the feasibility of a Monte Carlo-based optical therapy planning software by developing dosimetry tools to characterize and cross-validate the local photon fluence in brain tissue, as part of a long-term strategy to quantify the effects of photoactivated drug release in brain tumors. An existing GPU-based 3D Monte Carlo (MC) code was modified to simulate near-infrared photon transport with differing laser beam profiles within phantoms of skull bone (B), white matter (WM), and gray matter (GM). A novel titanium-based optical dosimetry probe with isotropic acceptance was used to validate the local photon fluence, and an empirical model of photon transport was developed to significantly decrease execution time for clinical application. Comparisons between the MC and the dosimetry probe measurements were on an average 11.27%, 13.25%, and 11.81% along the illumination beam axis, and 9.4%, 12.06%, 8.91% perpendicular to the beam axis for WM, GM, and B phantoms, respectively. For a heterogeneous head phantom, the measured % errors were 17.71% and 18.04% along and perpendicular to beam axis. The empirical algorithm was validated by probe measurements and matched the MC results (R²>0.99), with average % error of 10.1%, 45.2%, and 22.1% relative to probe measurements, and 22.6%, 35.8%, and 21.9% relative to the MC, for WM, GM, and B phantoms, respectively. The simulation time for the empirical model was 6 s versus 8 h for the GPU-based Monte Carlo for a head phantom simulation. These tools provide the capability to develop and optimize treatment plans for optimal release of pharmaceuticals in the treatment of cancer. Future work will test and validate these novel delivery and release mechanisms in vivo.

New Monte Carlo model of cylindrical diffusing fibers illustrates axially heterogeneous fluorescence detection: simulation and experimental validation

We present a new Monte Carlo model of cylindrical diffusing fibers that is implemented with a graphics processing unit. Unlike previously published models that approximate the diffuser as a linear array of point sources, this model is based on the construction of these fibers. This allows for accurate determination of fluence distributions and modeling of fluorescence generation and collection. We demonstrate that our model generates fluence profiles similar to a linear array of point sources, but reveals axially heterogeneous fluorescence detection. With axially homogeneous excitation fluence, approximately 90% of detected fluorescence is collected by the proximal third of the diffuser for μ(s)'∕μ(a) = 8 in the tissue and 70 to 88% is collected in this region for μ(s)'∕μ(a) = 80. Increased fluorescence detection by the distal end of the diffuser relative to the center section is also demonstrated. Validation of these results was performed by creating phantoms consisting of layered fluorescent regions. Diffusers were inserted into these layered phantoms and fluorescence spectra were collected. Fits to these spectra show quantitative agreement between simulated fluorescence collection sensitivities and experimental results. These results will be applicable to the use of diffusers as detectors for dosimetry in interstitial photodynamic therapy.

Performance of a dedicated light delivery and dosimetry device for photodynamic therapy of nasopharyngeal carcinoma: phantom and volunteer experiments

The objective of this study was to develop a light delivery and measurement device for photodynamic therapy (PDT) in the nasopharyngeal cavity, which achieves a homogeneous and reproducible fluence rate distribution to a target area and provides proper shielding of predefined risk areas.

MATERIALS AND METHODS

A flexible silicone applicator was developed, incorporating light delivery and dosimetry fibers. The applicator can be inserted through the mouth and fixed in the nasopharyngeal cavity. Tissue optical phantoms were prepared on the basis of optical properties measured in vivo using diffuse reflectance spectroscopy (DRS). The fluence rate over the length of the applicator surface was measured in air, in tissue optical phantoms and in five healthy volunteers.

RESULTS

The fluence rate distribution over the applicator surface in air and tissue optical phantom was found to be more homogeneous (SD/mean 3.8% and 18.3%, respectively) than the fluence rate distribution in five volunteers (SD/mean ranging from 19% up to 52%). The maximum observed fluence rate build-up in the nasopharynx varied between subjects and ranged from a factor of 4.1-6.9. Shielding of the risk area such as the soft palate and tongue was effective.

CONCLUSIONS

In air and in tissue optical phantoms the fluence rate distribution of the device was highly homogeneous. The observed inter-subject and intra-subject variations in fluence rate in healthy volunteers originated from differences in optical properties and nasopharyngeal geometry. Light delivery based on a single tissue surface measurement will not be adequate. In situ dosimetric measurements are required to determine the light fluence delivered to a geometrically complex site such as the nasopharynx. These observations should be taken in consideration when developing light applicators for PDT of the nasopharynx and other non-uniform surfaces.

Innovative therapies: photodynamic therapy

Photodynamic diagnosis could be a useful tool for improving the diagnostic yield of tumor biopsy, especially for mesothelioma tumors that are sclerotic and particularly hypocellular. For PDD, the use of low doses of a sensitizing drug, such as 5-ALA, must be investigated further. The initial results of 5-ALA-mediated PDD are promising. The role, if any, for PDT in the treatment of mesothelioma has yet to be established. The number of centers exploring this technology is limited because the procedure is labor intensive and requires not only specialized equipment but also physician support. The number of patients treated in the different trials is small, and no definitive conclusions can be drawn. Further complicating the interpretation of published results is the number of variables (i.e., type of sensitizer, light dose, drug dose, drug light interval, methods of light measurement, technique of light delivery, surgical debulking techniques), which differ between studies. Most reports are phase I and II studies. The final outcome of these studies with respect to survival is of limited value. The only phase III study, which was performed with an earlier generation photosensitizer, reported no advantage to the use of PDT in combination with surgery and immunochemotherapy. To date, the most that can be said is that intraoperative PDT can be performed safely in experienced centers and that there are some encouraging results, especially in patients with stages I and II MPM, particularly with the newer generation photosensitizers. One attractive aspect of this adjuvant treatment is that PDT, as opposed to some of the other adjuvant treatments combined with surgery, may offer the option of effecting adequate tumor debulking with a pulmonary-sparing procedure.

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

BACKGROUND AND OBJECTIVES

Previous studies with PhotoDynamic Therapy (PDT) in bladder and bronchi have shown that due to scattering and reflection, the actually delivered fluence rate on the surface in a hollow organ can be significantly higher than expected. In this pilot study, we investigated the differences between the primary calculated and the actual measured fluence rate during PDT of Barrett's Esophagus (BE) using 23 independent clinical measurements in 15 patients.

STUDY DESIGN/MATERIALS AND METHODS

A KTP-dye module laser at 630 nm was used as light source. Light delivery was performed using a cylindrical light diffuser inserted in the center of an inflatable transparent balloon with a length corresponding to the length of the Barrett's epithelium. The total light output power of the cylindrical diffuser was calibrated using an integrating sphere to deliver a primary fluence rate of 100 mW cm(-2). Two fiber-optic pseudo sphere isotropic detectors were placed on the balloon and were used to measure fluence rate at the surface of the esophageal wall during PDT.

RESULTS AND CONCLUSIONS

The actual fluence rate measured was 1.5-3.9 times higher than the primary fluence rate for 630 nm. In general, the fluence rate amplification factor decreased with increasing redness of the tissue and was less for shorter diffusers. Fluence rate variations in time were observed which coincided with patients coughing, movement, and esophageal spasms. These factors combined with inter patient variability of the fluence rate measured appears to justify the routine application of this technique in PDT of BE.

Monitoring photoproduct formation and photobleaching by fluorescence spectroscopy has the potential to improve PDT dosimetry with a verteporfin-like photosensitizer

In current clinical practice, photodynamic therapy (PDT) is carried out with prescribed drug doses and light doses as well as fixed drug-light intervals and illumination fluence rates. This approach can result in undesirable treatment outcomes of either overtreatment or undertreatment because of biological variations between different lesions and patients. In this study, we explore the possibility of improving PDT dosimetry by monitoring drug photobleaching and photoproduct formation. The study involved 60 mice receiving the same drug dose of a novel verteporfin-like photosensitizer, QLT0074, at 0.3 mg/kg body weight, followed by different light doses of 5, 10, 20, 30, 40 or 50 J/cm2 at 686 nm and a fluence rate of 70 mW/cm2. Photobleaching and photoproduct formation were measured simultaneously, using fluorescence spectroscopy. A ratio technique for data processing was introduced to reliably detect the photoproduct formed by PDT on mouse skin in vivo. The study showed that the QLT0074 photoproduct is stable and can be reliably quantified. Three new parameters, photoproduct score (PPS), photobleaching score (PBS) and percentage photobleaching score (PBS%), were introduced and tested together with the conventional dosimetry parameter, light dose, for performance on predicting PDT-induced outcome, skin necrosis. The statistical analysis of experimental results was performed with an ordinal logistic regression model. We demonstrated that both PPS and PBS improved the prediction of skin necrosis dramatically compared to light dose. PPS was identified as the best single parameter for predicting the PDT outcome.

Monte Carlo simulations for EndoBronchial Photodynamic Therapy: the influence of variations in optical and geometrical properties and of realistic and eccentric light sources

BACKGROUND AND OBJECTIVE

Light dosimetry for endobronchial photodynamic therapy is not very advanced to date. This study investigates the dependency of the fluence rate distribution in the bronchial wall on several parameters.

STUDY DESIGN/MATERIALS AND METHODS

A Monte Carlo model is employed for the illumination of a cylindrical cavity by a linear diffuser to compute the fluence rate distribution in the tissue. The influence of optical and geometrical properties (e.g., the absorption coefficient of the bronchial mucosa and the diameter of the treated lumen) have been investigated, as well as the consequences of varying output characteristics of the diffusers. The optical properties used are those of ex vivo pig bronchial mucosa.

RESULTS

With on-axis linear diffusers that can be modelled as a row of isotropic point sources, a constant fluence rate buildup factor can be employed for varying diffuser lengths and lumen diameters. Extreme off-axis placement of the diffuser causes a highly variable, considerable increase in the maximum fluence rate as well as a highly asymmetrical fluence rate profile on the circumference of the illuminated lumen. The fluence rate profiles resulting from illumination with realistic diffusers can be evaluated by implementing the measured radiance profiles of these diffusers in the model. The changes in fluence rate caused by variations in the optical properties of the bronchial mucosa could be accounted for by diffusion theory. This relationship can be used to extrapolate the ex vivo results to the clinical situation.

CONCLUSION

A set of practical rules of thumb is presented that can help to estimate fluence rate distributions in clinical practice.

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

The damage to normal pig bronchial mucosa caused by photodynamic therapy (PDT) using mTHPC and Photofrin as photosensitizers was evaluated. An endobronchial applicator was used to deliver the light with a linear diffuser and to measure the light fluence in situ. The applied fluences were varied, based on existing clinical protocols. A fluence finding experiment with short-term (1-2 days) response as an end point showed considerable damage to the mucosa with the use of Photofrin (fluences 50-275 J cm(-2), drug dose 2 mg kg(-1)) with oedema and blood vessel damage as most important features. In the short-term mTHPC experiment the damage found was slight (fluences 12.5-50 J cm(-2), drug dose 0.15 mg kg(-1)). For both sensitizers, atrophy and acute inflammation of the epithelium and the submucosal glands was observed. The damage was confined to the mucosa and submucosa leaving the cartilage intact. A long-term response experiment showed that fluences of 50 J cm(-2) for mTHPC and 65 J cm(-2) for Photofrin-treated animals caused damage that recovered within 14 days, with sporadic slight fibrosis and occasional inflammation of the submucosal glands. Limited data on the pharmacokinetics of mTHPC show that drug levels in the trachea are similar at 6 and 20 days post injection, indicating a broad time window for treatment. The importance of in situ light dosimetry was stressed by the inter-animal variations in fluence rate for comparable illumination conditions.

Monte Carlo simulation of light fluence in tissue in a cylindrical diffusing fibre geometry

The propagation of light emitted by a linear light diffuser in a cylindrical hollow organ was investigated by means of the Monte Carlo (MC) method. The height and radius of the cavity, scattering (mu(s)) (or reduced scattering, mu'(s)) and absorption (mu(a)) coefficients, anisotropy (g), and refractive indices of the media involved (n1, n2) are required as input data by the MC code, as are characteristics of the light diffuser (length, delivered power and emission profile). Results of our MC model were tested by measuring the light fluence rate in a tissue-simulating phantom (mu(a) = 0.5 cm(-1), mu(s) = 23 cm(-1) and g = 0.75) irradiated at 633 nm with a cylindrical diffuser. Since geometric and optical parameters determine the behaviour of light propagation in tissue, MC simulations with different sets of input parameters were carried out to provide qualitative as well as quantitative data useful in planning photodynamic therapy. Data are reported on light penetration and fluence rate build-up at mu(a) and mu'(s) values ranging between 0.1 and 5 cm(-1) and 2.5 and 50 cm(-1), respectively. Furthermore, results suggest that a shift and spread could occur in the isofluence curves along the symmetry axis, which depend on the diameter of the treated lumen as well as on the emission profile of the light diffuser. Using our data it is possible to estimate how inaccuracy in knowledge of the optical coefficients can affect (i.e. usually by increasing) the light dose scheduled at a certain depth into tissue.

Applicator for light delivery and in situ light dosimetry during endobronchial photodynamic therapy: First measurements in humans

This paper presents a design of an applicator for light delivery and light dosimetry during endobronchial photodynamic therapy (EB-PDT). The design incorporates a linear diffuser that is fixed in the centre of the lumen by a steel spring basket that does not block air flow. An isotropic light detector is included in this design, to measure the light fluence actually delivered to the bronchial mucosa surface. The applicator is designed for use with common bronchoscopy equipment, and can be used with bronchoscopes with a large biopsy channel ( approximately 3 mm). The first clinical measurements were performed and caused no additional discomfort to the (nonphotosensitized) patients. The data showed considerable inter-patient variability of the light fluence rate measured as a result of fixed output power of the diffuser. This fact and the expected strong dependence of the fluence rate on the lumen diameter stress the importance of in situ fluence rate measurement for a proper evaluation of the relationship between light fluence and the biological response of EB-PDT.

Light dosimetry in vivo

This paper starts with definitions of radiance, fluence (rate) and other quantities that are important with regard to in vivo light dosimetry. The light distribution in mammalian tissues can be estimated from model calculations using measured optical properties or from direct measurements of fluence rate using a suitable detector. A historical introduction is therefore followed by a brief discussion of tissue optical properties and of calculations using diffusion theory, the P3-approximation or Monte Carlo simulations. In particular the form of the scattering function is considered in relation to the fluence rate close to the tissue boundary, where light is incident. Non-invasive measurements of optical properties yield the absorption coefficient mu a and mu s(1 - g), where mu s is the scattering coefficient and g is the mean cosine of the scattering angle. An important question is whether this combination is sufficient, or whether g itself must be known. It appears that for strongly forward scattering, as in mammalian tissues, rather detailed knowledge of the scattering function is needed to reliably calculate the fluence rate close to the surface. Deeper in the tissue mu s (1 - g) is sufficient. The construction, calibration and use of fibre-optic probes for measurements of fluence rate in tissues or optical phantoms is discussed. At present, minimally invasive absolute fluence (rate) measurements seem to be possible with an accuracy of 10-20%. Examples are given of in vivo measurements in animal experiments and in humans during clinical treatments. Measurements in mammalian tissues, plant leaves and marine sediments are compared and similarities and differences pointed out. Most in vivo light fluence rate measurements have been concerned with photodynamic therapy (PDT): Optical properties of the same normal tissue may differ between patients. Tumours of the same histological type may even show different optical properties in a single patient. Treatment-induced changes of optical properties may also occur. Scattered light appears to contribute substantially to the light dose. All these phenomena emphasize the importance of in situ light measurements. Another important dosimetric parameter in PDT is the concentration and distribution of the photosensitizer. Apart from in vivo fluorescence monitoring, the photosensitizer part of in vivo PDT dosimetry is still in its infancy.

Calibration of isotropic light dosimetry probes based on scattering bulbs in clear media

Miniature light detectors with isotropic response (isotropic light probes) permit quantitative measurement of light energy fluence rates in turbid media such as biological tissues. These isotropic probes are, for example, applied in photodynamic therapy to correlate light fluence in tissue with (tumour) tissue response, in vitro and in vivo. After description of its construction, two methods of calibration of an isotropic probe in air are discussed, in collimated and in diffuse light. The probe was first calibrated in air in collimated light, after which its response to diffuse light was checked in a flat and in a spherical geometry. Subsequently, the probe's response to collimated light in clear media, for example, water or glycerine which have refractive indices larger than that of air, has been established experimentally. The diffusion approximation to the transport equation in a simple spherical geometry has been used to calculate the probe's response as a function of the refractive index of clear media. The extent of agreement between theory and experiment indicates that the physical mechanisms are understood and indirectly validates the theoretical models.

Light distribution by linear diffusing sources for photodynamic therapy

The distribution of the light emitted by linear light diffusers commonly employed in photodynamic therapy (PDT) has been investigated. A device is presented which measures the angular distribution of the exiting light at each point of the diffuser. With these data the fluence rate in air or in a cavity at some distance from the diffuser can be predicted. The results show that the light is scattered from the diffuser predominantly in the forward direction. Experiments and calculations show that the fluence rate in air and in a cavity of scattering tissue at some distance from the diffuser has a maximum near the tip of the diffuser, instead of near the middle. However, the fluence rate resulting from an interstitial diffuser in a purely scattering tissue phantom shows a maximum in the bisecting plane of the diffuser as would be predicted when the diffuser emitted light isotropically. The scattering nature of the tissue is expected to cancel the anisotropy of the diffuser.