The clinico‐pathological spectrum of plaque‐type blue naevi and their potential for malignant transformation

Aims Plaque‐type blue naevi are rare melanocytic tumours presenting as large, pigmented plaques at birth or during childhood. There is a risk for malignant transformation, but no larger comprehensive studies exist and the diagnosis is challenging, especially on limited biopsy material. The aim is to describe the clinicopathological features and behaviour of the disease more comprehensively. Methods and results We retrieved eight plaque‐type blue naevi, presenting as large, pigmented plaques (median = 7 cm; range = 3–26) most frequently affecting the scalp (four) followed by the cheek, arm, abdominal wall and gluteal cleft (one each), with a slight female predilection. Median age at time of biopsy was 39.5 years (range = 15–90), but three tumours had been present at birth and one since childhood. Histopathologically, the tumours were poorly circumscribed and composed of cellular fascicles of uniform spindle cells in a background of variably prominent pigmented dendritic cells affecting dermis and subcutaneous tissues. The majority had mutations in GNAQ. One tumour showed malignant transformation, characterised by an expansile nodule of pleomorphic epithelioid melanocytes with rhabdoid morphology, high mitotic activity and areas of necrosis. This patient developed metastatic melanoma to lymph nodes. All patients are alive with a median follow‐up of 60 months. Conclusion Plaque‐type blue naevi are diagnostically challenging tumours with risk for malignant transformation. Awareness and familiarity with the salient clinicopathological features are necessary for reliable diagnosis, and long‐term clinical follow‐up is required to monitor for malignant transformation. Plaque‐type blue naevi are rare melanocytic tumours in the spectrum of blue naevi, presenting during childhood as large, pigmented plaques with a predilection for the head and neck and trunk. They may undergo malignant transformation, characterised by epithelioid cell morphology, nuclear pleomorphism, high mitotic rate and necrosis, and patients require life‐long clinical follow‐up.