New Concept of Intraoperative Fluorescence Diagnostics for Photodynamic Dosimetry of Tumors Using Chlorin-Based Photosensitizers

Photodynamic therapy (PDT) with chlorin-based photosensitizers requires real-time monitoring of photosensitizer distribution and photobleaching to guide treatment, yet existing endoscopic fluorescence systems rely on short-wavelength (375–450 nm) excitation that is spectrally incompatible with 660 nm chlorin e6 (Ce6)-PDT, forcing interruption of therapy for diagnostic imaging. We developed VENERA-red-660, a dual-channel endoscopic system using a single 660 nm laser for simultaneous white-light and Ce6 fluorescence imaging (>670 nm detection) without interrupting irradiation. System sensitivity was validated on optical phantoms (Ce6 0.1–5.0 mg/L), and phototheranostics was performed in patients with basal cell carcinoma (n = 5), vulvar intraepithelial neoplasia grade III (n = 3), and peritoneal carcinomatosis (n = 10) using intravenous chlorin-based photosensitizer. Phantom testing showed a linear fluorescence response (R2 = 0.998) down to the lowest tested concentration of 0.1 mg/L. Pre-PDT fluorescence contrast indices confirmed photosensitizer selectivity in all three tumor types, decreasing by 57%, 66%, and 60% after treatment, consistent with photobleaching; continuous intra-PDT monitoring confirmed progressive photobleaching kinetics, and confocal microscopy verified cellular-level Ce6 selectivity. Increased backscattered laser signal after PDT was consistent with treatment-induced changes in tissue optical properties. These pilot findings demonstrate that single-source phototheranostic monitoring is feasible across external and laparoscopic access routes, supporting further evaluation of VENERA-red-660 as a tool for intraoperative PDT dosimetry.