Melanocytoma-acanthoma in a Dog

Abstract

Background: Melanocytic neoplasms are skin tumors that often occur in dogs. However, melanocytoma-acanthoma, also called melanoacanthoma, is a benign melanocytic neoplasm rarely reported in this species, which has been described only three times in the veterinary literature. Briefly, this tumor is characterized by a single, painless, darkly pigmented and firm cutaneous papule or nodule. Histologically, it is composed of mixed populations of well-differentiated melanocytes and keratinocytes, unlike traditional melanocytic tumors (melanoma and melanocytoma). These cells are arranged in lobules surrounded by collagenous stroma. Melanocytes are large epithelioid cells containing varying amounts of melanin. Keratinocytes form anastomosing trabeculae with peripheral palisading, and small cysts containing amorphous or laminated keratin. The definitive diagnosis of melanocytoma-acanthoma is based on histopathological findings. This report describes a case of melanocytoma-acanthoma in a dog in Brazil. Case: A 9-year-old female miniature Schnauzer dog was examined at the Veterinary Hospital of the Federal University of Santa Maria, where a single, firm, pigmented papule was found in the auricle. The lesion had started 15 days earlier.Hematological tests and serum biochemistry profile were normal. An excisional biopsy of the papule was surgically removed and subjected to histopathological examination. The tissue was fixed in 10% neutral buffered formalin, processed routinely and embedded in paraffin wax. Sections were stained with hematoxylin and eosin (HE). A histopathological examination revealed a nonencapsulated, well-defined, extensive, densely cellular proliferation located in dermis. This proliferation was composed of lobules and nests of well-differentiated stratified squamous epithelium closely associated with neoplastic melanocytes, surrounded by thin bundles of fibrous stroma. A diagnosis of melanocytoma-acanthoma was established based on these histological features. Discussion: The first description of melanocytoma-acanthoma in humans was as melano-epithelioma, classified into subtypes I and II. Both subtypes are benign neoplasms composed of well-differentiated melanocytes and keratinocytes, which are distinguished from one another based on the amount and distribution of melanocytes. Type I melano-epithelioma is characterized by proliferative lobules of melanocytes and keratinocytes, including melanocytes scattered diffusely among keratinocytes. Type II melano-epithelioma involves only the proliferation of keratinocytes, while melanocytes are limited to the basal layer of keratinocyte lobules. To clarify this condition, some authors use the term "melanoacanthoma" to indicate the above-described type I melano-epithelioma, and seborrheic keratosis to indicate type II melano-epithelioma. However, other authors use the term melanoacanthoma to denote the two conditions (types I and II melano-epithelioma). On the other hand, veterinary medicine does not recognize subtypes, instead using the term melanocytoma-acanthoma...

What is your diagnosis? Fine‐needle aspirate biopsy from a skin mass on the distal metatarsus of a dog

“…Melanoacanthoma, or melanocytoma‐acanthoma, is a rare neoplasm in the dog, with only four previously described cases in the literature 1‐4 . However, gross and histologic lesion descriptions are available, and detailed descriptions of the clinical and histologic features have been published 5 .…”

“…Melanoacanthoma, or melanocytoma-acanthoma, is a rare neoplasm in the dog, with only four previously described cases in the literature. [1][2][3][4] However, gross and histologic lesion descriptions are available, and detailed descriptions of the clinical and histologic features have been published. 5 This phenomenon is included in the International Histologic Classification of Skin tumors of domestic animals, 6 and the tumors have features of both compound melanocytic tumors and benign epithelial neoplasia.…”

“…Given the infrequency of reports about these lesions, age, breed, and predilection sites have yet to be ascertained. However, in the four previously reported cases, three occurred on the head, [1][2][3] and one occurred on the dorsal trunk. 4 In the case of this report the tumor occurred in the distal metatarsal area.…”