Photodynamic therapy (PDT) is an evolving treatment modality that has gained approval from the U.S. Food and Drug Administration for the treatment of various cancers, including microinvasive lung cancer, obstructing esophageal cancer, and non-small cell lung cancer. It is also utilized for premalignant conditions, such as Barrett’s esophagus, and non-oncologic conditions, such as age-related macular degeneration.^{1 – 4} PDT offers valuable advantages, such as minimal invasion, low systemic toxicity, versatility, and repeatability. However, the optimal clinical outcomes of PDT have been hindered by the lack of a reliable dosimetry metric that accurately quantifies the effective treatment dose, which is essential for predicting and guiding PDT.⁵ The challenges in achieving accurate dosimetry for PDT stem from its dynamic and complex nature, involving intricate interactions between light, the photosensitizer (PS), and tissue oxygen (${}^3{\mathrm{O}}_2$).⁶ In a typical PDT process, the PS is excited by specific wavelengths of treatment light, causing it to transition from its ground state to the excited singlet state. Subsequently, the PS undergoes intersystem crossing, leading to its transition to the triplet state. Type II PDT, the most clinically relevant process, occurs when the triplet state transfers energy to ground-state oxygen, ${}^3{\mathrm{O}}_2$, generating singlet oxygen, ${}^1{\mathrm{O}}_2$. This singlet oxygen induces photodamage in the photosensitized area.^{7 – 11}

We have developed a novel noncontact setup to implement a cone shell illumination and detection configuration using axicon lenses for depth-sensitive fluorescence measurements. The setup was demonstrated experimentally to be capable of detecting fluorescence from a two-layered turbid agar phantom with a larger sensitivity to the deep layer and a larger range of sensitivity to either layer than a conventional cone configuration implemented by a microscope objective lens. Furthermore, the axicon lens-based setup eliminates the need of moving the objective lens up or down to achieve depth-sensitive measurements, which effectively improves the consistency of optical coupling and thus would be preferred in a clinical setting.

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.