A sheep in wolf's clothing: Agminated blue naevi masquerading as in‐transit melanoma metastases

Blue naevi are benign dermal melanocytic tumours that commonly occur in isolation.1, 2 Rarely, multiple blue naevi occur in crops in a phenomenon called agminated blue naevi (ABN).2 These crops can have a similar clinical appearance to in-transit melanoma metastases, a diagnosis that requires exclusion. Furthermore, metastatic melanoma can occasionally mimic a benign blue naevus histologically presenting a further challenge in diagnosis. We present a case of ABN that was clinically suspicious for in-transit melanoma metastases to highlight this potential pitfall and provide guidance for diagnosis and management. A 67-year-old male presented with a crop of approximately 20 1- to 3-mm blue-black macules on his left upper chest (Figure 1a). This cluster was thought to have developed over the preceding 6 months and was clinically concerning for in-transit melanoma metastases. The patient was otherwise well with no history of melanoma, no preceding inflammatory dermatoses or treatment to the area. Further examination revealed a 1-cm pigmented submammary nodule (Figure 2a) but no other suspicious cutaneous or mucosal lesions and no palpable lymphadenopathy. The submammary nodule was excised, the microscopy of which revealed a pigmented basal cell carcinoma, in keeping with its dermoscopic appearance (Figure 2b). Of the chest macules, three representative lesions were biopsied or excised. Each demonstrated subtle populations of bland, spindled and epithelioid melanocytes admixed with melanophages within the superficial dermis. No junctional melanocytic component was seen. There was no significant cytological atypia, no mitoses, and no inflammatory response. The lesional melanocytes stained positive for SOX10, HMB45 and BAP1. All biopsies were histologically in keeping with blue naevi. A differential diagnosis of a blue naevus-like melanoma was considered given the clinical presentation, however, based on the histopathology, this was deemed unlikely. Following clinicopathological correlation and multidisciplinary discussion, a diagnosis of ABN was favoured. However, given the possibility for melanoma to show blue naevus-like morphology (which can be histologically indistinguishable from blue naevi),3 PET-CT was performed which demonstrated no evidence of metastatic melanoma. The patient has received ongoing clinical monitoring of the cluster (Figure 1b–f), with no progression over the subsequent 6 months. This case highlights the rare phenomenon of ABN that may present clinically as suspicious for metastatic melanoma. Occasionally, melanoma metastases involving the skin may resemble blue naevi histologically termed blue naevus-like metastatic melanoma (BNLMM) and can be challenging to diagnose.4 Nevertheless, there are often subtle clues such as cytological atypia, occasional mitoses, and an accompanying lymphoid infiltrate that should raise suspicion for BNLMM. None of these features were present in our case. Furthermore, BAP1 inactivation occurs in approximately 50% of BNLMM, and immunostaining for BAP1 may assist in diagnosis in problematic cases.5 Our case illustrates an uncommon diagnostic challenge and highlights the importance of clinicopathological correlation to ascertain an accurate diagnosis and inform management. Open discourse between the dermatologist and pathologist was required to establish the diagnosis of ABN while recognising the limitations of conventional microscopy and special stains to definitively exclude BNLMM. The case also highlights the value of short and longer-term dermoscopic monitoring to demonstrate an absence of change supporting a benign diagnosis. None. The authors gratefully acknowledge support from the BB and A Miller Foundation through the Jani Haenke Melanoma Pathology Fellowship (to ECP). RAS is supported by a National Health and Medical Research Council of Australia (NHMRC) Practitioner Fellowship (APP1141295). RAS has received fees for professional services from MetaOptima Technology Inc., F. Hoffmann-La Roche Ltd, Evaxion, Provectus Biopharmaceuticals Australia, Qbiotics, Novartis, Merck Sharp & Dohme, NeraCare, AMGEN Inc., Bristol-Myers Squibb, Myriad Genetics, GlaxoSmithKline. The patient involved in this report has given informed consent for their relevant clinical information and images to be used in its production.