CPC07: Solving a SOX10 riddle: an unusual and rare case of malignant triton tumour mimicking melanoma

W. Khan,¹ G. Leopold,² N. Ponnambath¹ and R. Goodwin¹

¹Dermatology Department, St Woolos Hospital, Newport, UK and ²Pathology Department, Morriston Hospital, Swansea, UK

Malignant triton tumour (MTT), a highly aggressive soft-tissue tumour, is a variant of malignant peripheral nerve sheath tumours (MPNST) and consists of malignant Schwann cells coexisting with malignant rhabdomyoblasts. It often manifests in individuals with neurofibromatosis type 1 disease, in postradiotherapy tissues and, rarely, as a primary cutaneous tumour. We report an unusual case of cutaneous MTT mimicking malignant melanoma on immunohistochemistry, which presented a diagnostic conundrum. Detailed analysis of MTT markers, its management and review of literature is presented. A 50-year-old man with no past medical history presented with a painful dusky red–purple, firm, fungating growth (10 × 8 cm) on his chest. The lesion had an inflammatory appearance, extending into the surrounding skin. The patient’s own photographs showed a pink pea-sized nodule 2 months prior. The lesion was initially treated by the surgical team as an abscess; however, a punch biopsy was also taken owing to its suspicious nature at the time of surgery. Initial immunohistochemistry stains were inconclusive, but further investigation demonstrated a poorly differentiated, highly anaplastic malignant tumour that focally expressed SRY-related HMG-box 10 (SOX10) and was negative for all other immunomarkers, leading to a preliminary diagnosis of malignant melanoma. In the context of a massive tumour with no epidermal component, a strong possibility of metastasis was suggested. BRAF was also found to be positive. Clinically, there was no evidence of other primary cutaneous melanoma. The patient was referred to plastic surgeons for a wide local excision. A computed tomography scan reported a lobulated left chest wall mass involving the subcutaneous fat and abutting the underlying muscle; however, no muscle invasion or lymphadenopathy was noted. Initial histological diagnosis of the whole tumour remained obscure. Following further clinicopathological discussion and additional immunohistochemical analysis, positive staining to desmin (rhabdomyoblast cytoplasm), nuclear myoblast determination protein 1 (MYOD1), focal scanty staining to S100 (nerve sheath differentiation of the spindle cells) and partial positivity to SOX10 was demonstrated, providing conclusive evidence of primary MTT. This case highlights the clinical and pathological features of triton tumours, which are rare but biologically aggressive. Nerve sheath differentiation is usually confirmed by S100 protein positivity and rhabdomyoblastic differentiation by immunohistochemical positivity for desmin. SOX10 is only rarely positive, where it shows focal or patchy staining in such tumours. BRAF mutations are readily identified in MPNST, but their therapeutic significance has yet to be demonstrated in clinical trials. The diagnostic workup for poorly differentiated neoplasms may include SOX10 staining but needs close clinicopathological correlation.