CPC10 A waxy wonder: unveiling a rare dermatological mystery

Abstract A 75-year-old woman was seen in the dermatology clinic for a skin lesion on the right shin of unknown duration. The lesion’s appearance was slightly atypical, making it stand out from the other benign moles. On examination, there was a nontender, brown waxy papule on the right shin. Dermoscopy revealed a pink homogeneous lesion, with no specific network identified. An excisional biopsy of this lesion revealed a multifocal nodular bland spindle-cell dermal tumour, with the neoplastic cells positive for both S100 and melan-A on immunohistochemistry. The morphology and the immunophenotype were consistent with a benign pigmented neurofibroma (PN). PN, also known as melanotic neurofibroma, is a rare tumour, constituting only 1% of cases of neurofibroma. It is a variant of neurofibroma containing melanin-producing cells and has been observed in patients both with and without neurofibromatosis. PN typically presents during the second and third decades of life, occasionally later (in the sixth decade), and is more prevalent in female patients. It frequently affects the scalp, as well as other locations such as the neck, hands, buttocks, thighs and legs. Morphologically, the lesions are tumours that, in most cases, lack visible pigmentation; only 16% of tumours show macroscopically visible pigmentation. The consistency of the lesions is usually rubbery, with sizes ranging from 1 to 50 cm in diameter. There are case reports of associations with hypertrichosis. Microscopically, PN exhibits characteristic histological features of neurofibroma. The tumour cells are spindled and may occasionally display a storiform arrangement. Many lesions contain abnormal nerve segments with disorganized Schwann cells and Wagner–Meissner corpuscles. Additional findings can include an increased number of mast cells and sporadically distributed multinucleated giant cells within the stroma. PN is unique among neurofibroma subtypes in containing melanin-producing cells, typically located in the deep dermis and subcutaneous tissue. This distinct feature is a useful differentiator between PN and other pigmented lesions. The primary differential diagnoses of PN include melanocytic schwannoma, pigmented dermatofibrosarcoma protuberans (Bednar tumour), cellular blue naevus, giant congenital melanocytic naevus with neuroid features, and cutaneous neurocristic hamartoma. These considerations underscore the crucial role of biopsy, as histopathological findings are often essential for a definitive diagnosis. Recurrence of PN has been observed and documented in the literature; however, no cases of malignant transformation have been reported. If PN occurs in an individual with a confirmed diagnosis of neurofibromatosis, clinical observation alone is generally sufficient. However, if PN occurs spontaneously, it is essential to assess for other cutaneous signs to determine whether the PN is associated with undiagnosed neurofibromatosis.