A 72-year-old woman with a 20-year history of sun exposure presents with a 1.5-cm tan-brown macule with irregular borders on her left temple. The lesion has been present for 5 years and has slowly enlarged. Dermoscopy shows asymmetrical pigmentation with a rhomboidal pattern and follicular involvement. Excision biopsy reveals atypical melanocytes confined to the epidermis and superficial dermis (Clark level II) with a Breslow thickness of 0.8 mm. Mitotic rate is <1/mm². What is the most likely melanoma subtype, and what is the key prognostic implication?

Explanation

## Diagnosis: Lentigo Maligna Melanoma with Excellent Prognosis **Key Point:** Lentigo maligna melanoma (LMM) arises from lentigo maligna on sun-exposed skin, grows slowly over years, and has excellent prognosis when thin and completely excised with clear margins. ### Clinical Features Diagnostic of Lentigo Maligna Melanoma | Feature | Lentigo Maligna Melanoma | Superficial Spreading | Nodular | Acral Lentiginous | |---------|--------------------------|----------------------|---------|-------------------| | **Location** | Face, ears, neck (sun-exposed) | Trunk, limbs | Anywhere | Palms, soles, nails | | **Duration** | Years to decades (slow) | Months to years | Months (rapid) | Months to years | | **Appearance** | Flat tan/brown macule, irregular | Nodular/asymmetric, variegated | Nodular, uniform | Flat or nodular, acral | | **Breslow thickness** | Often <1 mm | Variable | Often >2 mm | Often >2 mm | | **Clark level** | II–III | II–V | III–V | III–V | | **Mitotic rate** | Low (<1/mm²) | Variable | High | High | | **Prognosis (thin)** | Excellent (>95% 5-yr survival) | Good (85–90%) | Poor (60–70%) | Poor (60–70%) | **High-Yield:** This patient's 5-year slow growth on the temple (sun-exposed face), flat macule with irregular borders, Clark level II, Breslow 0.8 mm, and low mitotic rate are pathognomonic for lentigo maligna melanoma. ### Why This Is Lentigo Maligna Melanoma 1. **Sun-exposed location** — Temple (face) is classic for LMM 2. **Slow growth** — 5 years for 1.5 cm lesion indicates indolent course 3. **Flat macule** — No nodular component; radial growth pattern 4. **Low mitotic rate** — <1/mm² indicates low-grade biology 5. **Shallow invasion** — Clark II, Breslow 0.8 mm (thin melanoma) 6. **Follicular involvement** — Characteristic of LMM on face ### Prognostic Implications **Excellent prognosis** because: - **Thin melanoma** (Breslow <1 mm) — Stage IA (AJCC 8th ed.) - **No ulceration** — Not mentioned - **Low mitotic rate** — <1/mm² is favorable - **Complete excision with clear margins** — Standard of care - **5-year survival** — >95% for stage IA melanoma **Clinical Pearl:** Lentigo maligna (LM) is the precursor lesion; when invasive melanoma develops within LM, it is called lentigo maligna melanoma. The distinction is histologic: LM = intraepidermal only; LMM = invasion into dermis (Clark II or deeper). **Mnemonic: LMM Features = SLOW FACE** — Slow growth, Flat appearance, Acral/sun-exposed, Clark II–III, Elderly patients, <1 mm Breslow thickness. **High-Yield:** LMM has the best prognosis of all melanoma subtypes when diagnosed at thin thickness because: 1. It grows slowly, allowing early detection 2. It is usually diagnosed before deep invasion 3. Complete surgical excision with clear margins is curative in most cases 4. Adjuvant therapy is rarely needed for stage IA–IB disease ### Why <5% Recurrence with Clear Margins? Lentigo maligna melanoma confined to the epidermis and superficial dermis (Clark II) with complete excision and negative margins has: - No lymph node involvement at this stage - No systemic metastatic risk - Recurrence risk primarily from incomplete excision (residual LM at margins) - Proper histologic margin assessment (0.5–1 cm) ensures cure in >95% of cases