A 58-year-old man from Mumbai presents with a 2-month history of a slowly enlarging dark brown lesion on his left shin. On examination, the lesion measures 8 mm × 6 mm with irregular borders, colour variegation (brown, black, and tan), and a slightly raised surface. The lesion has been present for approximately 3 years but has recently started to grow. Dermoscopy shows asymmetric pigmentation with irregular streaks. Excisional biopsy reveals a melanoma with a Breslow thickness of 1.2 mm, Clark level III, and 2 mitotic figures per mm². There is no evidence of ulceration. What is the most likely subtype of melanoma in this patient?

Explanation

## Diagnosis: Superficial Spreading Melanoma (SSM) ### Clinical Features Supporting SSM **Key Point:** Superficial spreading melanoma is the most common subtype (60–70% of all melanomas) and typically presents with a history of slow growth over months to years, followed by a period of rapid growth. The clinical presentation in this patient is classic for SSM: - **Long duration before recent growth** (3 years, then rapid change in past 2 months) - **Irregular borders and colour variegation** (brown, black, tan) - **Moderate thickness** (Breslow 1.2 mm, Clark III) - **Location on lower extremity** (shin — a common site for SSM) - **Asymmetric dermoscopic pattern** with irregular streaks ### Comparison of Melanoma Subtypes | Feature | SSM | Nodular | Lentigo Maligna | Acral Lentiginous | | --- | --- | --- | --- | --- | | **Frequency** | 60–70% | 15–20% | 5–10% | 5–10% | | **Growth phase** | Radial (months–years), then vertical | Vertical from onset | Radial (years–decades) | Radial then vertical | | **Typical duration before diagnosis** | 1–5 years | Months | 10–20 years | Variable | | **Thickness at diagnosis** | 0.75–1.5 mm (intermediate) | Often >4 mm (thick) | Variable | Often thick | | **Ulceration** | Uncommon | Common | Rare | Variable | | **Common sites** | Trunk, extremities | Trunk, head, neck | Face, neck (sun-exposed) | Palms, soles, nail beds | | **Prognosis** | Intermediate | Poor (thick at diagnosis) | Better (caught earlier) | Poor (thick, acral location) | **High-Yield:** The **radial growth phase** (slow, lateral spread) followed by **vertical growth phase** (rapid deepening) is the hallmark of SSM and explains the 3-year stable period followed by recent rapid growth. ### Why This Is NOT Other Subtypes **Nodular melanoma** would present as a rapidly growing nodule from the outset, typically >4 mm thick at diagnosis, without a prolonged stable phase. **Lentigo maligna melanoma** occurs almost exclusively on the face and neck in elderly patients with chronic sun damage; the radial phase lasts 10–20 years. **Acral lentiginous melanoma** arises on palms, soles, or under nails — not on the shin. ### Prognostic Implications **Clinical Pearl:** Breslow thickness is the single most important prognostic factor in melanoma. A thickness of 1.2 mm places this patient in **Stage IB** (AJCC 8th edition), with a 5-year survival of approximately 90%. The absence of ulceration and low mitotic rate (2/mm²) are favourable features. **Mnemonic for SSM features: "RADIAL THEN DEEP"** - **R**aised, **A**symmetric, **D**ark (variegated) - **I**rregular borders - **A**ltered (changed) over time - **L**ong history before rapid growth - **T**hen **D**eep (vertical growth phase) - **E**ventually **E**xcised - **P**rognosis intermediate [cite:Robbins 10e Ch 25]