# Transscleral photodynamic therapy with a chlorin e6: An experimental study of exposure parameters and therapeutic window

## TL;DR

This study identifies safe and effective laser settings for transscleral photodynamic therapy using chlorin e6 in rabbits, avoiding damage to the retina and sclera.

## Contribution

The study defines a therapeutic window for transscleral photodynamic therapy with chlorin e6 using precise laser parameters and real-time thermal monitoring.

## Key findings

- Therapeutic laser settings (0.14–0.17 W for 5 mm probe and 0.48–0.6 W for 10 mm probe) caused selective choroidal damage without retinal or scleral injury.
- Suprathreshold settings led to retinal necrosis and scleral coagulation, indicating a clear safety threshold for treatment.
- Real-time thermal monitoring helped maintain safe temperature changes (ΔT ≤ 4.5°C) during treatment.

## Abstract

To define optimal exposure parameters and the therapeutic window for transscleral photodynamic therapy (TSPDT) with chlorin e6 by evaluating clinical, histological, and thermal effects of subthreshold, therapeutic, and suprathreshold settings in rabbit eyes.

The study was conducted on 21 healthy rabbits. TSPDT was performed using a 660 nm laser and chlorin e6 (2.5 mg/kg). Transscleral probes (5 mm: 0.1 W, 0.17 W, 0.3 W; 10 mm: 0.3 W, 0.6 W) with integrated thermosensors were used. Enucleation and histological analysis were performed 14 days post-irradiation.

Fundus examination on day 14 revealed distinct treatment zones correlating with laser settings. The therapeutic window was defined as 0.14–0.17 W (5 mm probe; power density: 0.693–0.866 W/cm²; energy density: 415.8–519.6 J/cm²) and 0.48–0.6 W (10 mm probe; 0.611–0.764 W/cm²; 366.6–458.4 J/cm²) with 600 s exposure time, achieving selective choroidal damage without scleral or retinal injury (ΔT ≤ 4.5°C). Suprathreshold settings (≥0.3 W for 5 mm; ≥ 0.6 W for 10 mm) induced retinal necrosis (up to 50%) and scleral coagulation (ΔT ≥ 8°C) with power densities exceeding 0.866 W/cm² (5 mm) and 0.764 W/cm² (10 mm).

TSPDT with chlorin e6 enables selective targeting of intraocular pathological tissues while preserving scleral and retinal integrity. Defining the therapeutic window and using real-time thermal monitoring enhances treatment safety. These findings lay a foundation for clinical protocols for uveal melanoma and other intraocular tumors.