Light Dosimetry at Tissue Surfaces for Oblique Incident Circular Fields

Area of Science:

• Biomedical Optics
• Photodynamic Therapy
• Medical Physics

Background:

• Oblique light incidence is common in photodynamic therapy (PDT) for skin cancers, especially on uneven surfaces.
• Understanding light propagation under these conditions is crucial for effective treatment planning.

Purpose of the Study:

• To investigate the impact of oblique light incidence on light fluence distribution in PDT.
• To develop and validate analytical models for predicting light fluence under oblique incidence.

Main Methods:

• Monte Carlo simulations were performed for circular light fields with varying radii and optical properties.
• An analytical solution for a slanted pencil beam was derived and approximated using diffusion or P3 theory.
• Measurements in a liquid phantom were conducted using an isotropic detector to validate simulation results.

Main Results:

• Oblique incidence increased peak fluence rate along the central axis compared to normal incidence.
• The effective attenuation coefficient decreased slightly for slanted beams.
• Beam profiles became asymmetrical, with higher fluence on the lateral side of incidence.
• Analytical solutions showed good agreement with Monte Carlo simulations and experimental data.

Conclusions:

• Oblique light incidence significantly alters light distribution in PDT.
• The developed analytical solutions provide accurate predictions for light fluence under oblique incidence.
• These findings can improve PDT treatment planning and dosimetry for complex geometries.