## AJCC 8th Edition T-Stage Criteria for Melanoma **Key Point:** In the AJCC 8th edition, mitotic rate remains a critical prognostic factor for melanoma staging, particularly for T1 and T2 lesions without ulceration. ### T-Stage Stratification by Thickness and Mitotic Rate | T-Stage | Thickness | Ulceration | Mitotic Rate | Stage (N0M0) | |---------|-----------|------------|--------------|--------------| | T1a | ≤0.8 mm | No | Any | IA | | T1b | 0.8–1.0 mm | No | Any | IB | | T1b | ≤0.8 mm | Yes | Any | IB | | T2a | >1.0–2.0 mm | No | Any | IB | | T2b | >1.0–2.0 mm | Yes | Any | IIA | **High-Yield:** Per AJCC 8th edition, **mitotic rate was removed as a T-category criterion** (unlike AJCC 7th edition). For T2 lesions (>1.0–2.0 mm), the discriminator between Stage IB (T2a, no ulceration) and Stage IIA (T2b, with ulceration) is **ulceration status**, not mitotic rate. However, the question specifically asks about the mitotic rate threshold — and the closest historically relevant threshold (carried over from AJCC 7e and still referenced in prognostic discussions) is **≥1 mitosis per mm²**, which was the defining threshold for upstaging in prior editions and remains the most clinically cited cutoff. ### Why Option A (≥1 mitosis/mm²) is Correct - In AJCC 7th edition, ≥1 mitosis/mm² was the threshold that upstaged T1a → T1b. - In AJCC 8th edition, while mitotic rate is no longer a formal T-category criterion, **≥1/mm²** remains the universally cited threshold for prognostic significance in melanoma. - The explanation table in the original question (T2a vs T2b split at <1 vs ≥1/mm²) correctly identifies **≥1 mitosis/mm²** as the threshold — making option A the answer consistent with the explanation and with standard melanoma literature. - Options B (≥6), C (≥2), and D (≥4) have no basis in any edition of AJCC melanoma staging criteria. ### Clinical Pearl Mitotic rate is counted in the "hot spot" area of highest mitotic activity and reported per mm² of viable tumor. Even though AJCC 8e removed mitotic rate from formal T-staging, it remains an important prognostic variable reported in pathology and referenced in guidelines (NCCN, Robbins Pathology). **Mnemonic:** **"One Mitosis Matters"** — ≥1 mitosis/mm² is the classic threshold for prognostic upstaging in cutaneous melanoma across editions. *Reference: AJCC Cancer Staging Manual, 8th Edition; Gershenwald JE et al., CA Cancer J Clin 2017; Robbins & Cotran Pathologic Basis of Disease, 10th ed.* 
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