Tumor invasion in the hyponychium is associated with distant metastasis and poor prognosis in subungual melanoma: A histologic landscape of 44 cases

Background Subungual melanoma (SUM) has a poor prognosis because of delayed diagnosis. Its progression, consensus on surgical treatment, and correlation with clinical outcomes remain unclear. Objective We aimed to identify the pattern of dermal invasion in different locations of the nail apparatus and its relationship with prognosis. Methods In this retrospective review of surgically treated SUM patients between January 2011 and April 2019, the nail apparatus was divided into 5 anatomic subunits: the dorsal roof of proximal nail fold, ventral floor of proximal nail fold, germinal matrix, nail bed, and hyponychium. Invasions in the subunits were categorized using 3 criteria: no tumor, in situ tumor, or invasion. Results Among 44 cases of SUM, dermal invasion occurred mostly in the distal areas, with 11, 30, 18, 7, and 4 in the hyponychium, nail bed, germinal matrix, ventral floor of proximal nail fold, and dorsal roof of proximal nail fold, respectively. The patients with hyponychial invasion showed a significantly greater Breslow depth (P = .009), a higher rate of lymph node metastasis (P = .019), distant metastasis (P = .036), and shorter disease-free survival (P = .001). Conclusion Hyponychial invasion is an important prognostic predictor of SUM, given its strong association with invasion depth, metastatic progression, and disease-free survival. Patients with invasion in the hyponychium should undergo more strict workup, treatment, and surveillance.