Case Report: Cutaneous melanocytic schwannoma with concomitant melanocytoma in a canine

TL;DR

This case report describes a rare occurrence of melanocytic schwannoma and melanocytoma in a dog's eyelids.

Contribution

The report adds to the limited knowledge of cutaneous melanocytic schwannoma in canines with a novel case.

Findings

A 7-year-old dog had two pigmented eyelid lesions diagnosed as melanocytic schwannoma and melanocytoma.

Histological analysis revealed distinct cellular arrangements in the two lesions.

The case highlights the diagnostic challenge and guarded prognosis of canine melanocytic schwannoma.

Abstract

Schwannoma is a nerve sheath tumour arising from differentiated Schwann cells, and melanocytic schwannoma (MS) is a rare variant where the Schwan cells produce melanin pigment. MS is typically associated with spinal nerve roots and there have been only ~20 reports of cutaneous or subcutaneous MS to-date in humans. In canines, there have only been two reports of MS, both associated with spinal root nerves. In this report, we describe a 7-year-old Weimaraner cross breed dog that presented with two pigmented lesions on the eyelids. The lesions were surgically removed and histological analysis revealed well-circumscribed, non-encapsulated, expansile, neoplasms that were displacing most of the dermis and adnexa. The first lesion was composed of spindloid cells arranged in short interlacing streams with large amounts of pale eosinophilic cytoplasm that sometimes contained fine melanin granules. In areas there were spindle cells arranged in verocay bodies which led to a diagnosis of MS. In contrast, the second lesion was composed of polygonal cells arranged in thick sheets with large amounts of pale eosinophilic cytoplasm that sometimes contained fine melanin granules. The diagnosis was melanocytoma (which is one of the macroscopic differential diagnoses for MS). Whilst melanocytoma is a commonly occurring cutaneous lesion in canines and surgical removal is considered curative, due to little being known about MS in dogs, the outcome remained guarded, as MS in humans has an unpredictable nature, and recurrence and metastasis have been reported.

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