Photodynamic therapy (PDT) relies on the interactions between light, photosensitizers, and tissue oxygen to produce cytotoxic reactive oxygen species (ROS), primarily singlet oxygen (¹O₂) through Type II photochemical reactions, along with superoxide anion radicals (O₂^•−), hydrogen peroxide (H₂O₂), and hydroxyl radicals (^•OH) through Type I mechanisms. Accurate dosimetry, accounting for all three components, is crucial for predicting and optimizing PDT outcomes. Conventional dosimetry tracks only light fluence rate and photosensitizer concentration, neglecting the role of tissue oxygenation. Reactive oxygen species explicit dosimetry (ROSED) quantifies the reacted oxygen species concentration ([ROS]_rx) by explicit measurements of light fluence (rate), photosensitizer concentration, and tissue oxygen concentration. Here we determine tissue oxygenation from non-invasive diffuse correlation spectroscopy (DCS) measurement of tumor blood flow using a conversion factor established preclinically. In this study, we have enrolled 24 pleural PDT patients into the study. Of these patients, we are able to obtain data on 20. Explicit dosimetry of light fluence, Photofrin concentration, and tissue oxygenation concentrations were integrated into the ROSED model to calculate [ROS]_rx across multiple sites inside the pleural cavity and among different patients. Large inter- and intra-patient heterogeneities in [ROS]_rx were observed, despite identical 60 J/cm² light doses, with mean [ROS]_rx,meas of 0.56 ± 0.26 mM for 13 patients with 21 sites, and [ROS]_rx,calc1 of 0.48 ± 0.23 mM for 20 patients with 76 sites. This study presented the first comprehensive analysis of clinical ROSED in pleural mesothelioma patients, providing valuable data on future ROSED based pleural PDT that can potentially produce uniform ROS and thus improve the PDT efficacy for Photofrin-mediated pleural PDT.

This study investigates the effect of fractionated (two-part) PDT on the long-term local control rate (LCR) using the concentration of reactive oxygen species ([ROS]_rx) as a dosimetry quantity. Groups with different fractionation schemes are examined, including a 2 h interval between light delivery sessions to cumulative fluences of 135, 180, and 225 J/cm². While the total treatment time remains constant within each group, the division of treatment time between the first and second fractionations are explored to assess the impact on long-term survival at 90 days. In all preclinical studies, Photofrin is intravenously administered to mice at a concentration of 5 mg/kg, with an incubation period between 18 and 24 h before the first light delivery session. Fluence rate is fixed at 75 mW/cm². Treatment ensues via a collimated laser beam, 1 cm in diameter, emitting light at 630 nm. Dosimetric quantities are assessed for all groups along with long-term (90 days) treatment outcomes. This study demonstrated a significant improvement in long-term survival after fractionated treatment schemes compared to single-fraction treatment, with the optimal 90-day survival increasing to 63%, 86%, and 100% vs. 20%, 25%, and 50%, respectively, for the three cumulative fluences. The threshold [ROS]_rx for the optimal scheme of fractionated Photofrin-mediated PDT, set at 0.78 mM, is significantly lower than that for the single-fraction PDT, at 1.08 mM.

The direct detection of singlet-state oxygen (¹O₂) constitutes the holy grail dosimetric method for type-II photodynamic therapy (PDT), a goal that can be quantified using multispectral singlet oxygen near-infrared luminescence dosimetry (MSOLD). The optical properties of tissues, specifically their scattering and absorption coefficients, play a crucial role in determining how the treatment and luminescence light are attenuated. Variations in these properties can significantly impact the spatial distribution of the treatment light and hence the generation of singlet oxygen and the detection of singlet oxygen luminescence signals. In this study, we investigated the impact of varying optical properties on the detection of ¹O₂ luminescence signals during Photofrin-mediated PDT in tissue-mimicking phantoms. For comparison, we also conducted Monte Carlo (MC) simulations under the same conditions. The experimental and simulations are substantially equivalent. This study advances the understanding of MSOLD during PDT.

SignificanceDirect detection of singlet-state oxygen (O12) is a critical objective in Type II photodynamic therapy (PDT) due to its pivotal role in mediating therapeutic effects. Although multispectral singlet oxygen dosimetry (MSOLD) has demonstrated the capability to detect O12 luminescence both in vitro and in vivo, there remains no standardized method for accurately quantifying reactive singlet oxygen, [O2]rx1, based on these measured signals. By contrast, the singlet oxygen explicit dosimetry (SOED) model offers a robust framework for calculating [O2]rx1. Demonstrating that O12 luminescence obtained through MSOLD can reliably quantify [O2]rx1, as achieved by the SOED model, is essential for advancing the accuracy and applicability of PDT dosimetry.AimWe aim to evaluate the accuracy and reliability of MSOLD in quantifying O12 concentrations from measured O12 luminescence in benzoporphyrin derivative (BPD)-mediated PDT. The performance of MSOLD is assessed by comparing its results with those derived from the SOED model.ApproachA continuous-wave 690 nm laser was used to excite a nanoparticle formulation of BPD, tradename Visudyne® in methanol at varying concentrations (2 to 6 mg/L). The singlet oxygen luminescence was measured using an InGaAs spectrometer and analyzed using a singular value decomposition algorithm. Near-infrared singlet oxygen emission at ∼1270 nm was extracted as O12 luminescence. Real-time singlet oxygen spectra were collected over 900 s using a 1.5 mm diameter fiber optic. Ground-state oxygen concentration was measured with a commercial oxygen probe, photosensitizer concentration was determined with a custom-made contact probe, and photon fluence rate was assessed with an isotropic detector. [O2]rx1 was then calculated based on the SOED model.ResultsThe extracted singlet oxygen (O12) luminescence exhibited clear concentration-dependent trends, with higher BPD concentrations producing stronger O12 luminescence. Over time, the O12 luminescence decayed due to photosensitizer bleaching. In addition, a strong linear correlation was observed between the O12 luminescence measured via MSOLD and the reactive oxygen species (ROS) concentrations calculated using the SOED model. We also investigated the impact of tissue optical properties on singlet oxygen luminescence detection and developed correction factors to account for their variations.ConclusionsWe demonstrate that singlet oxygen (O12) detected through MSOLD can reliably quantify ROS concentrations in BPD-mediated PDT with accuracy comparable to the SOED model, which requires separate measurements of light fluence rate, photosensitizer concentration, and oxygen availability, followed by modeling to estimate the amount of reactive singlet oxygen. By contrast, MSOLD can also be a more cost-effective, simpler, and faster alternative to SOED as it directly measures the singlet oxygen luminescence to quantify reactive singlet oxygen. Under appropriate correction for tissue optical properties, MSOLD presents a promising, robust, and direct dosimetry solution for clinical PDT applications.

SignificancePleural photodynamic therapy (PDT) faces significant dosimetry challenges due to complex light distribution patterns within the pleural cavity, where integrating sphere effects dominate light propagation. Accurate prediction of light fluence rate distributions is essential for optimizing treatment protocols and improving therapeutic outcomes in this emerging clinical application.AimThe aim is to quantitatively analyze light fluence rate distributions in pleural PDT using Monte Carlo (MC) simulations in various cavity geometries and tissue optical properties, providing essential data for treatment planning.ApproachGraphics processing unit-accelerated MC simulations (108 photons) using MCmatlab analyzed light distribution in spherical cavities (radii 0.2 to 10 cm) and anatomically realistic lung cavity models (volume = 2 L) with point sources. Simulations include a range of tissue optical properties (μa: 0.1 to 1.0 cm−1; μs′: 5 to 40 cm−1) for a flat-cut fiber source inside a realistic three-dimensional (3D) lung geometry, including realistic thoracotomy access openings and different fill media (air versus saline). Experimental validation is made using isotropic detectors in the same 3D-printed lung phantom with varying optical properties.ResultsMC statistical uncertainties averaged 1.9% across all voxels. Spherical cavities (r=4 cm) demonstrated highly uniform scattered light distribution along cavity–tissue boundaries (distribution uniformity 4.9%), whereas anatomically realistic lung phantoms showed greater heterogeneity (49.9%). Scattered light fluence rate per source power (ϕs/S) strongly correlated with tissue optical properties, particularly scattering coefficients. Source position minimally affected scattered light patterns, though direct components remained position-dependent. Side openings reduced scatter fluence near access points, with saline-filled cavities showing slightly higher fluence rates than air-filled cavities.ConclusionsWe demonstrate that patient-specific factors including cavity geometry, tissue optical properties, and surgical access considerations significantly influence light distribution in pleural PDT. The quantitative relationships established between these parameters and fluence patterns provide essential data for developing personalized treatment planning protocols to optimize therapeutic light delivery.

Direct detection of singlet-state oxygen (1O2) constitutes the holy grail dosimetric method for type-II photodynamic therapy (PDT), a goal that can be quantified using multispectral singlet oxygen near-infrared luminescence dosimetry (MSOLD). The optical properties of tissues, specifically their scattering and absorption coefficients, play a crucial role in determining how the treatment and luminescence light are attenuated. Variations in these properties can significantly impact the spatial distribution of the treatment light and hence the generation of singlet oxygen, and the detection of singlet oxygen luminescence signals. In this study, we investigated the impact of varying optical properties on the detection of 1O2 luminescence signals during Photofrin-mediated PDT in tissue-mimicking phantoms. For comparison, we also conducted Monte Carlo (MC) simulations under the same conditions. The experimental and simulations are substantially equivalent. This study advances understanding of MSOLD dosimetry during PDT.