Absorbed photodynamic dose from pulsed versus continuous wave light examined with tissue-simulating dosimeters

Effective fluence and dose at skin depth of daylight and lamp sources for PpIX-based photodynamic therapy

SIGNIFICANCE

Skin-based photodynamic therapy (PDT) is used for the clinical treatment of actinic keratosis (AKs) and other skin lesions with continued expansion into the standard of care. Due to the spectral dependency of photosensitizer activation and skin optical fluence, there is a need for more accurate methods to estimate the delivered dose at depth from different PDT light sources and treatment regimens.

AIM

Develop radiometric methods for calculating photosensitizer-effective fluence and dose at depth and determine differences between red-lamp, blue-lamp, and daylight-based PDT treatments.

METHODS

Radiometric measurements of FDA-approved PDT lamp sources, outdoor daylight, and indoor daylight were performed for clinically relevant AK treatments. The protoporphyrin IX (PpIX) equivalent irradiance, fluence, and dose for each light source were calculated from the PpIX absorption spectrum and a 7-layer skin fluence model. The effective fluence and dose at depth was estimated by combining the spectral attenuation predicted at each wavelength and depth with the source fluence at each wavelength.

RESULTS

The red-lamp source had the highest illuminance (112,000 lumen/m²), but lowest PpIX-effective irradiance (9.6 W/m²), and highest effective fluence at depth (10.8 W/m² at 500 µm). In contrast, the blue light source had the lowest illuminance (2300 lumen/m²), but highest PpIX effective irradiance (37.0 W/m²), and ultimately the lowest effective fluence at depth (0.18 W/cm² at 500 µm). The daylight source had values of (outdoor | indoor) illuminance of (49,200 | 37,800 lumen/m²), effective irradiance of (19.2 | 10.7 W/m²), and effective fluence of (1.50 | 1.08 W/m² at 500 µm). The effective fluence and dose at depth facilitated the comparison of treatment regimens, for example, calculating an equivalent dose for a 2 hr indoor daylight treatment and a 10 min red-light treatment for the 300-1000 μm depth range.

CONCLUSIONS

The consideration of PpIX-effective fluence at varying depths is necessary to provide adequate comparisons of the delivered dose from PDT light sources. Methods for calculating radiometric fluence and delivered dose at depth were introduced, with open source MATLAB code, to help overcome the limitations of commonly used photometric and irradiance-based reporting.

J, M.D.P. DE. C, RAJENDRAM S. Photodynamic Therapy : An Overview and Insights into a Prospective Mainstream Anticancer Therapy

Photodynamic therapy (PDT) procedure has minimum invasiveness in contrast to conventional anticancer surgical procedures. Although clinically approved a few decades ago, it is not commonly used due to its poor efficacy, mainly due to poor light penetration into deeper tissues. PDT uses a photosensitizer (PS), which is photoactivated on illumination by light of appropriate wavelength and oxygen in the tissue, leading to a series of photochemical reactions producing reactive oxygen species (ROS) triggering various mechanisms resulting in lethal effects on tumor cells. This review looks into the fundamental aspects of PDT, such as photochemistry, photobiological effects, and the current clinical applications in the light of improving PDT to become a mainstream therapeutic procedure against a broad spectrum of cancers and malignant lesions. The side effects of PDT, both early and late-onset, are elaborated on in detail to highlight the available options to minimize side effects without compromising therapeutic efficacy. This paper summarizes the benefits, drawbacks, and limitations of photodynamic therapy along with the recent attempts to achieve improved therapeutic efficacy via monitoring various cellular and molecular processes through fluorescent imagery aided by suitable biomarkers, prospective nanotechnology-based targeted delivery methods, the use of scintillating nanoparticles to deliver light to remote locations and also combining PDT with conventional anticancer therapies have opened up new dimensions for PDT in treating cancers. This review inquires and critically analyses prospective avenues in which a breakthrough would finally enable PDT to be integrated into mainstream anticancer therapy.

PDT with genetically encoded photosensitizer miniSOG on a tumor spheroid model: A comparative study of continuous-wave and pulsed irradiation

BACKGROUND

Therapeutic effects of PDT depend on many factors, including the amount of singlet oxygen, localization of photosensitizer and irradiation protocol. The present study was aimed to compare the cytotoxic mechanisms of PDT under continuous-wave (CW) and pulsed irradiation using a tumor spheroid model and a genetically encoded photosensitizer miniSOG.

METHODS

1O2 detection in miniSOG and flavin mononucleotide (FMN) solutions was performed. Photobleaching of miniSOG in solution and in HeLa tumor spheroids was analyzed. Tumor spheroid morphology and growth and the cell death mechanisms after PDT in CW and pulsed modes were assessed.

RESULTS

We found a more rapid 1O2 generation and a higher photobleaching rate in miniSOG solution upon irradiation in pulsed mode compared to CW mode. Photobleaching of miniSOG in tumor spheroids was also higher after irradiation in the pulsed mode. PDT of spheroids in CW mode resulted in a moderate expansion of the necrotic core of tumor spheroids and a slight inhibition of spheroid growth. The pulsed mode was more effective in induction of cell death, including apoptosis, and suppression of spheroid growth.

CONCLUSIONS

Comparison of CW and pulsed irradiation modes in PDT with miniSOG showed more pronounced cytotoxic effects of the pulsed mode. Our results suggest that the pulsed irradiation regimen enables enhanced 1O2 production by photosensitizer and stimulates apoptosis.

GENERAL SIGNIFICANCE

Our results provide more insights into the cellular mechanisms of anti-cancer PDT and open the way to improvement of light irradiation protocols.

Light Technology for Efficient and Effective Photodynamic Therapy: A Critical Review

Photodynamic therapy (PDT) is a cancer treatment with strong potential over well-established standard therapies in certain cases. Non-ionising radiation, localisation, possible repeated treatments, and stimulation of immunological response are some of the main beneficial features of PDT. Despite the great potential, its application remains challenging. Limited light penetration depth, non-ideal photosensitisers, complex dosimetry, and complicated implementations in the clinic are some limiting factors hindering the extended use of PDT. To surpass actual technological paradigms, radically new sources, light-based devices, advanced photosensitisers, measurement devices, and innovative application strategies are under extensive investigation. The main aim of this review is to highlight the advantages/pitfalls, technical challenges and opportunities of PDT, with a focus on technologies for light activation of photosensitisers, such as light sources, delivery devices, and systems. In this vein, a broad overview of the current status of superficial, interstitial, and deep PDT modalities-and a critical review of light sources and their effects on the PDT process-are presented. Insight into the technical advancements and remaining challenges of optical sources and light devices is provided from a physical and bioengineering perspective.

Higher pulse frequency of near-infrared laser irradiation increases penetration depth for novel biomedical applications

BACKGROUND

The clinical efficiency of laser treatments is limited by the low penetration of visible light used in certain procedures like photodynamic therapy (PDT). Second Harmonic Generation (SHG) PDT is an innovative technique to overcome this limitation that enables the use of Near Infrared (NIR) light instead of visible light. NIR frequency bands present an optical window for deeper penetration into biological tissue. In this research, we compare the penetration depths of 405 and 808 nm continuous wave (CW) lasers and 808 nm pulsed wave (PW) laser in two different modes (high and low frequency).

METHODS

Increasing thicknesses of beef and chicken tissue samples were irradiated under CW and PW lasers to determine penetration depths.

RESULTS

The 808 nm CW laser penetrates 2.3 and 2.4 times deeper than the 405 nm CW laser in beef and chicken samples, respectively. 808 nm PW (pulse frequency-500 Hz) penetrates deeper than CW laser at the same wavelength. Further, increasing the pulse frequency achieves higher penetration depths. High frequency 808 nm PW (pulse frequency-71.4 MHz) penetrates 7.4- and 6.0-times deeper than 405 nm CW laser in chicken and beef, respectively.

CONCLUSIONS

The results demonstrate the higher penetration depths of high frequency PW laser compared to low frequency PW laser, CW laser of the same wavelength and CW laser with half the wavelength. The results indicate that integrating SHG in the PDT process along with pulsed NIR light may allow the treatment of 6-7 times bigger tumours than conventional PDT using blue light.

Mutual impact of clinically translatable near-infrared dyes on photoacoustic image contrast and in vitro photodynamic therapy efficacy

Photodynamic therapy (PDT), a spatially localized phototoxic therapy that involves irradiation of a photosensitizer (PS) with specific wavelengths of light, has shown exceptional promise in impacting cancer treatment outcomes, particularly oral cancer. To reduce PDT outcome variability, attempts toward image-guided personalized PDT are being pursued by monitoring PS uptake either via fluorescence or photoacoustic imaging (PAI), a nonionizing modality dependent on optical absorption properties of the tissue. PAI-guided PDT requires a near-infrared contrast agent for deep tissue imaging with minimal photobleaching effect. We evaluate the impact of PDT agent, benzoporphyrin derivative (BPD), on PAI agent indocyanine green (ICG) and vice versa, given that they have different optical absorption properties and singlet oxygen quantum yields for PDT. Specifically, we demonstrate in two oral squamous cell carcinoma lines (FaDu and SCC4) that ICG has minimal effect on BPD PDT efficacy when irradiated with either a continuous or pulsed laser. Furthermore, the impact of BPD on ICG photodegradation was monitored with PAI in tissue-mimicking phantoms. These studies inform us that the combination of BPD and ICG can be utilized for PAI-guided PDT. However, researchers need to consider the photodegradation effects of ICG in the presence of BPD when designing their drug delivery strategies for PAI-guided PDT.

Empirical Modeling of Physiochemical Immune Response of Multilayer Zinc Oxide Nanomaterials under UV Exposure to Melanoma and Foreskin Fibroblasts

Carcinogenesis is a complex molecular process starting with genetic and epigenetic alterations, mutation stimulation, and DNA modification, which leads to proteomic adaptation ending with an uncontrolled proliferation mechanism. The current research focused on the empirical modelling of the physiological response of human melanoma cells (FM55P) and human foreskin fibroblasts cells (AG01518) to the multilayer zinc oxide (ZnO) nanomaterials under UV-A exposure. To validate this experimental scheme, multilayer ZnO nanomaterials were grown on a femtotip silver capillary and conjugated with protoporphyrin IX (PpIX). Furthermore, PpIX-conjugated ZnO nanomaterials grown on the probe were inserted into human melanoma (FM55P) and foreskin fibroblasts cells (AG01518) under UV-A light exposure. Interestingly, significant cell necrosis was observed because of a loss in mitochondrial membrane potential just after insertion of the femtotip tool. Intense reactive oxygen species (ROS) fluorescence was observed after exposure to the ZnO NWs conjugated with PpIX femtotip model under UV exposure. Results were verified by applying several experimental techniques, e.g., ROS detection, MTT assay, and fluorescence spectroscopy. The present work reports experimental modelling of cell necrosis in normal human skin as well as a cancerous tissue. These obtained results pave the way for a more rational strategy for biomedical and clinical applications.

Beyond the Barriers of Light Penetration: Strategies, Perspectives and Possibilities for Photodynamic Therapy

Photodynamic therapy (PDT) is a photochemistry based treatment modality that involves the generation of cytotoxic species through the interactions of a photosensitizer molecule with light irradiation of an appropriate wavelength. PDT is an approved therapeutic modality for several cancers globally and in several cases has proved to be effective where traditional treatments have failed. The key parameters that determine PDT efficacy are 1. the photosensitizer (nature of the molecules, selectivity, and macroscopic and microscopic localization etc.), 2. light application (wavelength, fluence, fluence rate, irradiation regimes etc.) and 3. the microenvironment (vascularity, hypoxic regions, stromal tissue density, molecular heterogeneity etc.). Over the years, several groups aimed to monitor and manipulate the components of these critical parameters to improve the effectiveness of PDT treatments. However, PDT is still misconstrued to be a surface treatment primarily due to the limited depths of light penetration. In this review, we present the recent advances, strategies and perspectives in PDT approaches, particularly in cancer treatment, that focus on increasing the 'damage zone' beyond the reach of light in the body. This is enabled by a spectrum of approaches that range from innovative photosensitizer excitation strategies, increased specificity of phototoxicity, and biomodulatory approaches that amplify the biotherapeutic effects induced by photodynamic action. Along with the increasing depth of understanding of the underlying physical, chemical and physiological mechanisms, it is anticipated that with the convergence of these strategies, the clinical utility of PDT will be expanded to a powerful modality in the armamentarium for the management of cancer.

Towards PDT with Genetically Encoded Photosensitizer KillerRed: A Comparison of Continuous and Pulsed Laser Regimens in an Animal Tumor Model

The strong phototoxicity of the red fluorescent protein KillerRed allows it to be considered as a potential genetically encoded photosensitizer for the photodynamic therapy (PDT) of cancer. The advantages of KillerRed over chemical photosensitizers are its expression in tumor cells transduced with the appropriate gene and direct killing of cells through precise damage to any desired cell compartment. The ability of KillerRed to affect cell division and to induce cell death has already been demonstrated in cancer cell lines in vitro and HeLa tumor xenografts in vivo. However, the further development of this approach for PDT requires optimization of the method of treatment. In this study we tested the continuous wave (593 nm) and pulsed laser (584 nm, 10 Hz, 18 ns) modes to achieve an antitumor effect. The research was implemented on CT26 subcutaneous mouse tumors expressing KillerRed in fusion with histone H2B. The results showed that the pulsed mode provided a higher rate of photobleaching of KillerRed without any temperature increase on the tumor surface. PDT with the continuous wave laser was ineffective against CT26 tumors in mice, whereas the pulsed laser induced pronounced histopathological changes and inhibition of tumor growth. Therefore, we selected an effective regimen for PDT when using the genetically encoded photosensitizer KillerRed and pulsed laser irradiation.

Photodynamic effects of zinc oxide nanowires in skin cancer and fibroblast

Cytotoxic effects of zinc oxide (ZnO) nanomaterials, individual and conjugated with a photosensitizer (protoporphyrin IX), were studied in the presence and absence of ultraviolet light exposure (240 nm of light wavelength for a very short time exposure) in cell cultures of human normal and cancerous skin models. Zinc Oxide nanowires (ZnO NWs) were grown on the capillary tip and conjugated with protoporphyrin IX (PpIX). This coated tip was used as tool/pointer for intracellular drug delivery protocol in suggested normal as well as carcinogenic cellular models. After true delivery of optimal drug, the labelled biological model was irradiated with UV-A, which led to a loss of mitochondrial membrane potential, as tested by neutral red assay (NRA).

Necrosis response to photodynamic therapy using light pulses in the femtosecond regime

One of the clinical limitations of the photodynamic therapy (PDT) is the reduced light penetration into biological tissues. Pulsed lasers may present advantages concerning photodynamic response when compared to continuous wave (CW) lasers operating under the same average power conditions. The aim of this study was to investigate PDT-induced response when using femtosecond laser (FSL) and a first-generation photosensitizer (Photogem) to evaluate the induced depth of necrosis. The in vitro photodegradation of the sensitizer was monitored during illumination either with CW or an FSL as an indirect measurement of the PDT response. Healthy liver of Wistar rats was used to evaluate the tissue response. The photosensitizer was endovenously injected and 30 min after, an energy dose of 150 J cm(-2) was delivered to the liver surface. We observed that the photodegradation rate evaluated via fluorescence spectroscopy was higher for the FSL illumination. The FSL-PDT produced a necrosis nearly twice as deep when compared to the CW-PDT. An increase of the tissue temperature during the application was measured and was not higher than 2.5 °C for the CW laser and not higher than 4.5 °C for the pulsed laser. FSL should be considered as an alternative in PDT applications for improving the results in the treatment of bulky tumors where higher light penetration is required.

Determining the optical properties of a gelatin‑TiO(2) phantom at 780 nm

Tissue phantoms play a central role in validating biomedical imaging techniques. Here we employ a series of methods that aim to fully determine the optical properties, i.e., the refractive index n, absorption coefficient μ(a), transport mean free path [Formula: see text], and scattering coefficient μ(s) of a TiO(2) in gelatin phantom intended for use in optoacoustic imaging. For the determination of the key parameters μ(a) and [Formula: see text], we employ a variant of time of flight measurements, where fiber optodes are immersed into the phantom to minimize the influence of boundaries. The robustness of the method was verified with Monte Carlo simulations, where the experimentally obtained values served as input parameters for the simulations. The excellent agreement between simulations and experiments confirmed the reliability of the results. The parameters determined at 780 nm are [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]The asymmetry parameter g obtained from the parameters [Formula: see text] and [Formula: see text] is 0.93, which indicates that the scattering entities are not bare TiO(2) particles but large sparse clusters. The interaction between the scattering particles and the gelatin matrix should be taken into account when developing such phantoms.

Light-emitting diodes (LEDs) in dermatology

Light-emitting diode photobiomodulation is the newest category of nonthermal light therapies to find its way to the dermatologic armamentarium. In this article, we briefly review the literature on the development of this technology, its evolution within esthetic and medical dermatology, and provide practical and technical considerations for use in various conditions. This article also focuses on the specific cell-signaling pathways involved and how the mechanisms at play can be put to use to treat a variety of cutaneous problems as a stand-alone application and/or complementary treatment modality or as one of the best photodynamic therapy light source.

Monitoring singlet oxygen in situ with delayed chemiluminescence to deduce the effect of photodynamic therapy

Singlet oxygen ((1)O(2)) is an important factor mediating cell killing in photodynamic therapy (PDT). We previously reported that chemiluminescence (CL) can be used to detect (1)O(2) production in PDT and linked the signal to the PDT-induced cytotoxicity in vitro. We develop a new CL detection apparatus to achieve in vivo measurements. The system utilizes a time-delayed CL signal to overcome the interference from scattered excitation light, thus greatly improving the accuracy of the detection. The system is tested on healthy skin of BALB/ca mouse for its feasibility and reliability. The CL measurement is made during a synchronized gating period of the irradiation light. After each PDT treatment and in situ CL measurement, the skin response is scored over a period of 2 weeks. A remarkable relationship is observed between the score and the CL, regardless of the PDT treatment protocol. Although there are many issues yet to be addressed, our results clearly demonstrate the feasibility of CL measurement during PDT and its potential for in vivo PDT dosimetry. This requires further investigations.

A golden bullet? Selective targeting of Toxoplasma gondii tachyzoites using antibody-functionalized gold nanorods

Conjugates of gold nanoparticles and antibodies have useful functionalities. Here, we show how they can be used to selectively target and destroy parasitic protozoans. Gold nanorods were conjugated with an anti-Toxoplasma gondii antibody and used to target the extracellular tachyzoite which is an infectious form of an obligate parasite Toxoplasma gondii. Subsequent laser irradiation was used to kill the targeted protozoans. This concept provides a new paradigm for the treatment of parasitic protozoans.

Review of tissue simulating phantoms for optical spectroscopy, imaging and dosimetry

Optical spectroscopy, imaging, and therapy tissue phantoms must have the scattering and absorption properties that are characteristic of human tissues, and over the past few decades, many useful models have been created. In this work, an overview of their composition and properties is outlined, by separating matrix, scattering, and absorbing materials, and discussing the benefits and weaknesses in each category. Matrix materials typically are water, gelatin, agar, polyester or epoxy and polyurethane resin, room-temperature vulcanizing (RTV) silicone, or polyvinyl alcohol gels. The water and hydrogel materials provide a soft medium that is biologically and biochemically compatible with addition of organic molecules, and are optimal for scientific laboratory studies. Polyester, polyurethane, and silicone phantoms are essentially permanent matrix compositions that are suitable for routine calibration and testing of established systems. The most common three choices for scatters have been: (1.) lipid based emulsions, (2.) titanium or aluminum oxide powders, and (3.) polymer microspheres. The choice of absorbers varies widely from hemoglobin and cells for biological simulation, to molecular dyes and ink as less biological but more stable absorbers. This review is an attempt to indicate which sets of phantoms are optimal for specific applications, and provide links to studies that characterize main phantom material properties and recipes.

Modeling of a type II photofrin-mediated photodynamic therapy process in a heterogeneous tissue phantom

We present a quantitative framework to model a Type II photodynamic therapy (PDT) process in the time domain in which a set of rate equations are solved to describe molecular reactions. Calculation of steady-state light distributions using a Monte Carlo method in a heterogeneous tissue phantom model demonstrates that the photon density differs significantly in a superficial tumor of only 3 mm thickness. The time dependences of the photosensitizer, oxygen and intracellular unoxidized receptor concentrations were obtained and monotonic decreases in the concentrations of the ground-state photosensitizer and receptor were observed. By defining respective decay times, we quantitatively studied the effects of photon density, drug dose and oxygen concentration on photobleaching and cytotoxicity of a photofrin-mediated PDT process. Comparison of the dependences of the receptor decay time on photon density and drug dose at different concentrations of oxygen clearly shows an oxygen threshold under which the receptor concentration remains constant or PDT exhibits no cytotoxicity. Furthermore, the dependence of the photosensitizer and receptor decay times on the drug dose and photon density suggests the possibility of PDT improvement by maximizing cytotoxicity in a tumor with optimized light and drug doses. We also discuss the utility of this model toward the understanding of clinical PDT treatment of chest wall recurrence of breast carcinoma.

Intracellular kinetics of ATX-S10.Na(II) and its correlation with photochemical reaction dynamics during a pulsed photosensitization process: effect of pulse repetition rate

Although photodynamic therapy with pulsed light excitation has interesting characteristics, its photosensitization mechanism has not been fully elucidated. In this study, we showed that the intracellular kinetics of ATX-S10.Na(II), a lysosomal sensitizer, was closely related to photochemical reaction dynamics during photodynamic treatment of A549 cells with nanosecond pulsed light. Fluorescence microscopy revealed that at high frequencies of 10 and 30 Hz the sensitizer initially localized mainly in lysosomes but that it started to be redistributed to the cytosol in certain ranges of radiant exposures. These ranges were found to coincide with a regime of fluorescence degradation with limited oxygen consumption. On the other hand, at 5 Hz, there was no such a discontinuous behavior in the sensitizer redistribution characteristics throughout the period of irradiation; this was consistent with the fact that no reaction switching was observed. Two possible reasons for the appearance of the regime with limited oxygen consumption are discussed: participation of an oxygen-independent reaction and change in the microenvironment for the sensitizer caused by lysosomal photodamage. The pulse frequency-dependent intracellular kinetics of the sensitizer also explains our previous results showing higher cytotoxicity at 5 Hz than at 10 and 30 Hz.

Application of lower fluence rate for less microvasculature damage and greater cell-killing during photodynamic therapy

During the process of photodynamic therapy (PDT), problems arise such as stasis or occlusion of microvasculature, tumor oxygen depletion, and photosensitizer bleaching. This study shows that the first problem could be reduced by using a lower fluence rate light source in PDT. Microvasculature damage was studied experimentally in hematoporphyrin derivative-mediated PDT against light fluence rate, and, to some extent, less microvasculature damage was induced under 75 mW/cm2 illumination than under 150 mW/cm2. Histology of vessels at the end of PDT showed that vessel damage could be observed in both groups, indicating that the microvasculature would eventually be damaged as long as the administration of light fluence was sufficient and regardless of the illuminating fluence rates. Thus microvasculature damage induced by low fluence rate illumination could also be effective in tumor control after PDT. The cell-killing experiment was performed in vitro and designed so that cell-killing rate was influenced only by light characteristics. The higher cell-killing rate caused by 75 mW/cm2 illumination indicated that lower fluence rate light could enhance the light absorbency or decrease the bleaching of photosensitizer.

Application of lower fluence rate for less microvasculature damage and greater cell-killing during photodynamic therapy

During the process of photodynamic therapy (PDT), problems arise such as stasis or occlusion of microvasculature, tumor oxygen depletion, and photosensitizer bleaching. This study shows that the first problem could be reduced by using a lower fluence rate light source in PDT. Microvasculature damage was studied experimentally in hematoporphyrin derivative-mediated PDT against light fluence rate, and, to some extent, less microvasculature damage was induced under 75 mW/cm(2) illumination than under 150 mW/cm(2). Histology of vessels at the end of PDT showed that vessel damage could be observed in both groups, indicating that the microvasculature would eventually be damaged as long as the administration of light fluence was sufficient and regardless of the illuminating fluence rates. Thus microvasculature damage induced by low fluence rate illumination could also be effective in tumor control after PDT. The cell-killing experiment was performed in vitro and designed so that cell-killing rate was influenced only by light characteristics. The higher cell-killing rate caused by 75 mW/cm(2) illumination indicated that lower fluence rate light could enhance the light absorbency or decrease the bleaching of photosensitizer.

Excitation-and-collection geometry insensitive fluorescence imaging of tissue-simulating turbid media

We present an imaging technique for the correction of geometrical effects in fluorescence measurement of optically thick, turbid media such as human tissue. Specifically, we use the cross-polarization method to reject specular reflection and enhance the diffusive backscattering of polarized fluorescence excitation light from the turbid media. We correct the nonuniformity of the image field caused by the excitation-and-collection geometry of a fluorescence imaging system by normalizing the fluorescence image to the cross-polarized reflection image. The ratio image provides a map of relative fluorescence yield, defined as the ratio of emerging fluorescence power to incident excitation, over the surface of an imaged homogeneous turbid medium when fluorescence excitation-and-collection geometries vary in a wide range. We investigate the mechanism of ratio imaging by using Monte Carlo modeling. Our findings show that this technique could have a potential use in the detection of early cancer, which usually starts from a superficial layer of tissue, based on the contrast in the tissue fluorescence of an early lesion and of the surrounding normal tissue.

Transient absorption changes in vivo during photodynamic therapy with pulsed-laser light

High intensity pulsed-laser light can be used to excite absorbing molecules to transient states in large proportions. The laser-induced spectral changes can be characterized by transient changes in light propagation; through the tissue provided the excited states of these molecules have altered absorption spectra. Characterization of these transient changes may then be used to exploit new mechanisms in photosensitization and/or to optimize photobiological effects. In this study, transmittance and reflectance were measured as a function of laser pulse energy, from tissue-simulating media as well as in rat muscle and liver slices, both with and without the photosensitizer benzoporphyrin derivative monoacid (BPD-MA) present. There was a transient decrease in absorption from the photosensitizer at peak pulse irradiance in the range of 100-1000 W cm(-2). The depth of photodynamic treatment-induced tissue necrosis was measured in a subcutaneous prostate cancer model in Copenhagen rats. A comparison between continuous wave irradiation and pulsed irradiation with the same average incident irradiance showed no statistically significant difference in the depth of necrosis at 48 h after irradiation. These results indicate that photosensitizer population-state changes are measurable in tissues and may provide a method for measuring triplet-state properties of photosensitizer in vivo, but for BPD-MA at clinically used concentrations these changes do not significantly affect the depth of photodynamically-induced tissue damage.

Implicit and explicit dosimetry in photodynamic therapy: a New paradigm

Dosimetry for photodynamic therapy (PDT) is becoming increasingly complex as more factors are identified which may influence the effectiveness of a given treatment. The simple prescription of a PDT treatment in terms of the administered photosensitizer dose, the incident light and the drug-light time interval does not account for patient-to-patient variability in either the photosensitizer uptake, tissue optical properties or tissue oxygenation, nor for the interdependence of the photosensitizer-light-tissue factors. This interdependence is examined and the implications for developing adequate dosimetry for PDT are considered. The traditional dosimetric approach, measuring each dose factor independently, and termed here 'explicit dosimetry', may be contrasted with the recent trend to use photosensitizer photobleaching as an index of the effective delivered dose, termed here 'implicit dosimetry'. The advantages and limitations of each approach are discussed, and the need to understand the degree to which the photobleaching mechanism is linked, or 'coupled', to the photosensitizing mechanism is analysed. Finally, the influence of the tissue-response endpoints on the optimal dosimetry methods is considered.