# 2.2 Selective Photothermolysis (선택적 광열분해) --- ## 1. Definition - Coined by **Anderson & Parrish (1983, Harvard)**. - “Use of a **specific wavelength and pulse duration** to selectively destroy a target structure (chromophore) with minimal damage to surrounding tissue.” --- ## 2. Key Principles ### A. Chromophore Selectivity - Different tissue components absorb light differently depending on **wavelength**. - **Main chromophores** in dermatology: - **Melanin** (pigment in epidermis, hair, nevi, tattoos). - **Hemoglobin** (vascular lesions). - **Water** (skin ablation, resurfacing). ### B. Thermal Relaxation Time (TRT, 열이완시간) - The time required for a structure to lose 50% of its heat. - To selectively destroy target: - **Pulse duration ≤ TRT of target**. - Prevents heat diffusion to surrounding tissues. ### C. Energy (Fluence) - Must be sufficient to **raise target to destructive temperature**. - Too low → ineffective. - Too high → collateral damage, scarring, PIH. --- ## 3. Clinical Examples ### A. Pigment Treatment - **Q-switched Nd:YAG (532/1064 nm)** - Melanin TRT ≈ nanoseconds → requires **ns or ps pulse**. - Used for freckles, lentigines, tattoos. ### B. Hair Removal - Hair follicle TRT ≈ 10–100 ms. - **Long-pulse lasers (alexandrite, diode, Nd:YAG, ms range)** → selectively heat follicle while sparing epidermis. ### C. Vascular Lesions - Capillaries TRT ≈ 1–10 ms. - **Pulsed dye laser (PDL, 585–595 nm, ms pulse)** → hemoglobin absorption, vessel coagulation without epidermal damage. ### D. Skin Resurfacing - **CO₂ laser (10,600 nm)** → absorbed by water. - Pulse duration tailored to avoid unnecessary dermal damage. --- ## 4. Histologic Outcomes - **Precise destruction of target** (melanin granules, blood vessels, hair follicles). - **Minimal collateral injury** due to pulse duration ≤ TRT. - Preserved surrounding dermis/epidermis → faster healing. --- ## 5. Clinical Pearls 1. **Three variables must be aligned**: - Wavelength (target) - Pulse duration (≤ TRT) - Fluence (sufficient energy) 2. **Different targets = different TRT** → explains why pigment, hair, and vessels require different laser types. 3. **Asian skin (Fitzpatrick III–V)** → higher melanin → need longer wavelengths (1064 nm) and lower fluence to minimize PIH. 4. Misapplication (e.g., long pulse on melanin target) → ineffective or damaging. --- ✅ **Summary for practice:** **Selective photothermolysis** is the foundation of dermatologic laser therapy: delivering energy of the **right wavelength, at sufficient fluence, with a pulse duration matched to the TRT of the target**, allows destruction of unwanted structures (pigment, hair, vessels) while sparing surrounding tissue. ---