6.3 Light delivery systems and dosimetry in PDT

Photodynamic therapy relies on precise light delivery to activate photosensitizers in target tissues. Lasers and LEDs, coupled with fiber optics, enable tailored light application for various treatment scenarios. Understanding these systems is crucial for effective PDT implementation.

Dosimetry in PDT involves careful calculation of light intensity and exposure time. Factors like tissue optics and light penetration depth must be considered to optimize treatment outcomes. Proper dosimetry ensures sufficient photosensitizer activation while minimizing damage to healthy tissue.

Light Sources

Laser and LED Systems for PDT

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• Laser systems generate coherent, monochromatic light with high power output
• Diode lasers commonly used in PDT produce specific wavelengths (630-690 nm) matching photosensitizer absorption
• LED arrays provide cost-effective alternative to lasers for superficial treatments
• LED systems emit broader spectrum light with lower power density than lasers
• Both laser and LED systems can be pulsed or operated in continuous wave mode

Fiber Optic Light Delivery

• Fiber optics transmit light from source to treatment site with minimal loss
• Single-mode fibers carry one light path, ideal for precise targeting
• Multi-mode fibers allow multiple light paths, suitable for broader illumination
• Diffuser tips attached to fibers spread light evenly over treatment area
• Fiber optic delivery enables minimally invasive light application to internal organs

Light Delivery Techniques

Interstitial and Fractionated Light Delivery

• Interstitial light delivery involves inserting optical fibers directly into tissue
• Allows treatment of deep-seated or large volume tumors
• Multiple fibers can be arranged in grid patterns for uniform light distribution
• Fractionation splits total light dose into multiple shorter exposures
• Fractionated PDT enhances treatment efficacy by allowing tissue reoxygenation between exposures
• Can reduce pain and improve tolerability in some treatment protocols

Treatment Planning and Optimization

• Treatment planning software models light distribution in tissue
• Incorporates tissue optical properties and photosensitizer distribution
• Optimizes fiber placement and light dosage for target coverage
• Real-time monitoring of tissue oxygenation guides light delivery
• Adaptive planning adjusts parameters during treatment based on tissue response
• Patient-specific planning accounts for individual anatomy and tumor characteristics

Dosimetry Parameters

Fluence Rate and Light Dose Calculations

• Fluence rate measures light intensity at a point within tissue (mW/cm²)
• Light dose (J/cm²) calculated by multiplying fluence rate by exposure time
• Fluence rate decreases exponentially with tissue depth due to absorption and scattering
• Optimal fluence rates balance treatment efficacy with photodynamic oxygen consumption
• Total light dose typically ranges from 50-300 J/cm² depending on application

Tissue Optics and Light Penetration

• Tissue optical properties (absorption, scattering, anisotropy) affect light distribution
• Absorption coefficient (μa) determines light attenuation by chromophores (hemoglobin, melanin)
• Scattering coefficient (μs) characterizes light deflection by tissue structures
• Anisotropy factor (g) describes angular distribution of scattered light
• Effective attenuation coefficient (μeff) combines absorption and scattering effects
• Penetration depth defined as distance where light intensity falls to 1/e of surface value
• Red and near-infrared light (600-1000 nm) penetrate tissue more deeply than visible light