## T1 Melanoma Substaging: Mitotic Rate Threshold **Key Point:** In AJCC 8th edition, **T1a melanoma** (Breslow thickness 0.8–1.0 mm) is **upstaged to T1b** when the mitotic rate reaches **≥1 mitosis per mm²**, even without ulceration. ### T1 Melanoma Classification (Breslow 0.8–1.0 mm) | Subtype | Breslow Thickness | Mitotic Rate | Ulceration | |---------|-------------------|--------------|------------| | T1a | 0.8–1.0 mm | <1 per mm² | Absent | | T1b | 0.8–1.0 mm | ≥1 per mm² | Absent OR Present | **High-Yield:** The **≥1 mitosis per mm²** threshold is a critical prognostic cutoff in thin melanomas. Even a single mitotic figure per mm² significantly worsens prognosis and upstages the tumor. **Mnemonic:** **"One Mitosis = One Stage Up"** — a single mitotic figure per mm² in T1 melanoma bumps it from T1a to T1b. **Clinical Pearl:** Mitotic rate is assessed in the **dermal component only** (not the intraepidermal component). The field is typically counted at 40× magnification in the area of highest mitotic activity. ### Context in Broader T Classification - **T1:** 0.8–1.0 mm (with or without ulceration, stratified by mitotic rate) - **T2:** 1.01–2.0 mm (with or without ulceration) - **T3:** 2.01–4.0 mm (with or without ulceration) - **T4:** >4.0 mm (with or without ulceration) [cite:AJCC Cancer Staging Manual 8e, Melanoma of the Skin] 
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