Late-onset bulky naevocytoma of the perineum masquerading as a malignant melanoma.

Bulky naevocytoma of the perineum is a very rare variant of giant congenital melanocytic naevus (GCMN). It presents as a bulky naevocytic tumour in the perineal region with characteristic histological findings, such as extensive areas with a neural appearance called 'lames foliacees', formation of a pseudofollicular structure and extension of naevus cells between collagen bundles in a row called 'Indian-file' pattern. We report a case of late-onset bulky naevocytoma of the perineum in a 13-year-old girl. The patient presented with two bulky, pedunculated, heavily pigmented masses in the vulvar area that developed in a pre-existing GCMN lesion, which began around puberty and caused severe gait disturbance. Given the possibility of malignant transformation, we conducted staged reduction surgery of the tumour masses, which were found to be intradermal naevi without evidence of malignancy. The patient's gait disturbance improved markedly after surgery.