The first Chinese case of pediatric subungual melanoma: A case report and literature review

Dear Editor, Longitudinal melanonychia (LM) presents as vertical linear hyperpigmented streaks in the nail plate. Etiologies include benign diseases (eg, melanocytic nevus) and malignancies (eg, subungual melanoma). Pediatric subungual melanoma, however, is rarely reported in any race. Here, we present a case of subungual melanoma in a Chinese girl. A 13-year-old girl presented with a painless black macule affecting her left thumbnail. It started as a 1-mm wide black nail streak 4 years ago, which progressively widened to 6 mm. Four months ago, a pale area appeared in the middle of the proximal nail plate. She had no prior trauma, other diseases, or family history of nail pigmentation or melanoma. Physical examination revealed an 8-mm black macule on her left thumb, with a hypomelanotic area on the proximal nail plate. Irregular pigmentation extended to the hyponychium (Figure 1). There was no palpable lymphadenopathy. An excisional biopsy was performed. Pathology revealed an increased number of atypical melanocytes mostly in the basal layer, with single cells predominating over nests. The atypical melanocytes stained positive for S100, Melan-A, and HMB-45 (Figure 2). A diagnosis of subungual melanoma was made after review by three dermatopathologists. We performed slow Mohs micrographic surgery to excise the tumor with a 5-mm margin. Histopathological studies confirmed the diagnosis of a subungual melanoma with a Breslow depth of 0.7 mm, and all margins were clear. One-year follow-up showed no signs of local recurrence or metastasis. LM is a frequent finding in individuals with skin of color and usually represents a benign process. Subungual melanoma is exceedingly rare in young patients. Therefore, primary physicians and dermatologists frequently encounter diagnostic and therapeutic dilemmas managing pediatric LM. To summarize the current findings on pediatric subungual melanoma, we performed a literature search in the PubMed database. References of each article were also reviewed. From 1967 to 2019, only 19 cases of pediatric subungual melanoma were reported (Table 1). The mean age at diagnosis was 6.7 years and the mean duration between the age of onset and biopsy was 4.7 years. All patients underwent surgical treatment. Postoperative follow-up was reported in eight cases. Regional lymph node metastasis was reported in one case, and no local recurrence or distant organ metastasis was reported. Indication and timing of biopsy in children with LM remain an area of controversy. First, various clinical features (width >3 mm, Hutchinson sign, and regression) proposed as predictors for subungual melanoma in adults may not apply in children. For instance, Hutchinson sign (melanonychia with pigmentation of the proximal nail fold) can be a normal finding in pediatric subungual nevi. Furthermore, histopathological features of melanoma, especially in the early stage, can be challenging to recognize. To further complicate the diagnostic dilemma, cytologic atypia, an important diagnostic clue for subungual melanoma, does not equate to malignancy. Spitz nevus, for example, displays cytologic atypia yet runs a benign course. Finally, nail plate damage should be taken into consideration before biopsying young patients. While there is currently no standard regarding whether and when to biopsy a child with LM, factors to consider include the patient's age, rapid expansion, pigmentary change, and Hutchison sign. Our patient was previously followed up annually, until the appearance of irregular hyponychial pigmentation and recent pigmentary change,