Thin (T1) melanomas, defined in the 8th edition of the American Joint Committee on Cancer (AJCC) Cancer Staging Manual for melanoma as any primary cutaneous melanoma of Breslow thickness 1.0 mm or thinner,1 account for more than 70% of newly diagnosed cutaneous melanoma in the United States.2 (The AJCC Cancer Staging Manual, 8th edition, staging system for melanoma mandates reporting of Breslow thickness measurements to the nearest 0.1 mm, not the nearest 0.01 mm, and this convention will be used in this editorial. The AJCC recommends measuring thin melanomas to the nearest 0.01 mm if practical but then reporting the thickness rounded to the nearest 0.1 mm. With this convention, T1 melanomas include those as thick as 1.04 mm, and T1b melanomas are those reported as 0.8 to 1.0 mm, regardless of mitotic count or ulceration. Melanomas 0.75 to 0.84 mm in thickness would all be reported as 0.8 mm and included in T1b.) It is estimated that in 2017, 87 110 individuals will be diagnosed as having invasive melanoma of any thickness,3 and it is quite possible this represents an underestimate owing to the many thin melanomas that are treated in dermatologists’ offices and other settings where they would not be routinely reported to regional or national registries.4,5 Nevertheless, even assuming that this figure is correct, this represents more than 60 000 thin melanoma cases in 1 year alone, and virtually all of them present initially with clinically negative regional lymph nodes. While the average individual with a thin melanoma may have a very low chance of developing distant metastases and dying of their disease, owing to the large number of cases in aggregate 29% of melanoma deaths now come from the T1 patient population, more than from the T2, T3, or T4 melanoma categories.2
Sondak VK, Messina JL, Zager JS. Selecting Patients With Thin Melanoma for Sentinel Lymph Node Biopsy—This Time It’s Personal. JAMA Dermatol. 2017;153(9):857–858. doi:10.1001/jamadermatol.2017.2496
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