A 58-year-old man presents with a 2 cm dark brown lesion on his left shin that has been present for 8 years without change. On dermoscopy, the lesion shows a reticular (net-like) pattern with brown globules and no ulceration. Histopathology reveals melanoma cells confined to the epidermis and superficial dermis, with a Breslow thickness of 0.6 mm and no mitotic figures. What is the most likely subtype and what is the primary prognostic significance of this presentation?

Explanation

## Clinical Diagnosis: Superficial Spreading Melanoma ### Key Features Pointing to SSM **Key Point:** Superficial spreading melanoma (SSM) is characterized by a long radial growth phase (horizontal spread in epidermis and superficial dermis) before vertical invasion, which explains the 8-year stable appearance and low Breslow thickness. **Clinical Pearl:** The reticular/net-like dermoscopic pattern with brown globules is classic for SSM. The lack of ulceration and low mitotic count further support early-stage disease. ### Prognostic Significance of Breslow Thickness | Breslow Thickness | Stage | 5-Year Survival | |---|---|---| | < 0.5 mm | IA | > 95% | | 0.5–1.0 mm | IB | 90–95% | | 1.0–2.0 mm | IIA | 80–90% | | 2.0–4.0 mm | IIB | 60–75% | | > 4.0 mm | IIC | < 50% | **High-Yield:** Breslow thickness is the single most important prognostic factor in cutaneous melanoma. A thickness of 0.6 mm places this patient in stage IB with excellent prognosis. ### Why SSM Has Better Prognosis Than Other Subtypes 1. **Long radial growth phase** — allows detection at thinner depths 2. **Typically lower Breslow thickness at diagnosis** — compared to nodular or acral lentiginous subtypes 3. **Lower mitotic rate** — indicates slower progression **Mnemonic: SSMR** — Superficial Spreading Melanoma = Radial growth = Relatively thin at diagnosis = Relatively favorable ### Melanoma Subtypes Comparison | Subtype | Location | Growth Pattern | Breslow at Dx | Prognosis | |---|---|---|---|---| | Superficial Spreading (70%) | Any site, often trunk/limbs | Long radial phase | 0.5–1.5 mm | Best | | Nodular (15%) | Any site | Rapid vertical growth | 2–4 mm | Worst | | Lentigo Maligna (10%) | Face/neck (sun-exposed) | Very long radial phase | Variable | Intermediate | | Acral Lentiginous (5%) | Palm/sole/nail | Radial then vertical | 2–3 mm | Poor | **Clinical Pearl:** SSM on the shin (non-acral, non-facial location) with 8 years of stability is the classic presentation. [cite:Robbins 10e Ch 25] [cite:Harrison 21e Ch 75]