October 2019




Signalment (JPC # 2785784): Miniature schnauzer


HISTORY: Firm, pedunculated dermal mass

HISTOPATHOLOGIC DESCRIPTION: Haired skin: Expanding the dermis and elevating the overlying hyperplastic and focally ulcerated epidermis is an unencapsulated, pedunculated, moderately cellular neoplasm composed of spindle cells arranged in short, interlacing streams and bundles separated by moderate amounts of collagenous matrix. Neoplastic cells have indistinct cell borders, moderate amounts of eosinophilic cytoplasm that occasionally contains brown, globular pigment (melanin) and an oval to elongate nucleus with finely stippled chromatin and 1 variably distinct nucleolus. Anisocytosis and anisokaryosis are mild and mitotic rate is 1 per 10 HPF. Multifocally, there are individual or small nests of neoplastic cells within the epidermis (junctional activity). Multifocally within the dermis there are scattered macrophages with intracytoplasmic melanin (melanomacrophages). The epidermis is focally ulcerated with an overlying serocellular crust composed of cellular debris, degenerate neutrophils, hemorrhage, fibrin, and colonies of basophilic cocci. Subadjacent to the crust within the superficial dermis there are increased numbers of small caliber blood vessels lined by reactive endothelium oriented perpendicular to the ulcer and perpendicular to abundant surrounding hypertrophied fibroblasts (granulation tissue). Rare apocrine glands are ectatic and surrounded by moderate numbers of lymphocytes, plasma cells, and fewer transmigrating neutrophils and macrophages (hydradenitis). The epidermis is hyperplastic, characterized by acanthosis, spongiosis, and rete ridge formation.


MORPHOLOGIC DIAGNOSIS: Haired skin: Melanocytoma, miniature schnauzer, canine.



Signalment (JPC # 2327682): Age and breed unspecified dog.


HISTORY: A mass on the digit


HISTOPATHOLOGIC DESCRIPTION: Haired skin and bone, digit: Expanding the dermis, elevating a focally extensively ulcerated epidermis, and infiltrating the bone of the third phalanx (P3) is an unencapsulated, poorly demarcated, moderately cellular, infiltrative neoplasm composed of polygonal to spindle cells arranged in lobules, nests, and packets supported on a moderate fibrovascular stroma. Neoplastic cells have variably distinct cell borders, a scant to moderate amount of eosinophilic, granular cytoplasm that often contains brown pigment globules (melanin), and a round to oval nucleus with finely stippled chromatin and 1 to 2 prominent nucleoli. Anisocytosis and anisokaryosis are moderate and there are 2 to 3 mitotic figures per 40x HPF. Multifocally, there are individual or small nests of neoplastic cells within the follicular epithelium (junctional activity). Within the neoplasm there is frequent single cell necrosis, moderate numbers of melanomacrophages, and multifocal areas of lytic necrosis with replacement by hemorrhage, fibrin, edema, moderate numbers of viable and degenerate neutrophils, and fewer hemosiderin-laden macrophages. There is multifocal bone loss (osteolysis) with replacement by neoplastic cells. There are often scalloped trabeculae of immature woven bone extending from the remaining lamellar mature bone, basophilic reversal lines, and osteoclasts within Howship’s lacunae (bony remodeling).


MORPHOLOGIC DIAGNOSIS: Haired skin and bone, digit: Melanoma, breed unspecified, canine.



·       Melanocytes:

·       Originate from neuroectoderm, and migrate to the skin and hair bulbs during fetal development

·       Mature, dendritic, pigment-producing cells found between basal keratinocytes of the epidermis and hair bulb; contain E-cadherin cell surface markers which serve as adhesion mechanism between melanocytes and keratinocytes

·       Produce melanin, which is stored within melanosomes, and transferred to keratinocytes and serve to protect the skin from UV radiation

·       Melanoblasts which fail to reach the epidermis will reside in the dermis as intradermal melanocytes

·       Melanocytic tumors are most common in dogs, horses, and some pig breeds



·       Unknown

·       Vasculogenic mimicry was reported in a palpebral melanocytoma in a dog, a mechanism of tumour angiogenesis recognized in human melanocytomas (not veterinary species); the neoplasm contained numerous lacunar and slit-like spaces filled by erythrocytes and interspersed throughout the neoplastic melanocytes; spaces were lined by PLN-2 and factor VIII positive cells, but the cells were negative for CD31 (Nordio L, J Comp Path 2018)



·       Mitotic index (MI) is indicative of clinical behavior, and should be determined in all melanocytic neoplasms; nuclear atypia, ulceration, and deep infiltration beyond the dermis are also associated with a poor outcome; see Smedley (2011) publication for detailed information; degree of pigmentation and histologic pattern are not correlated with prognosis according to several studies

·       Oral and lip melanocytic neoplasms: Newer studies have shown a subpopulation of oral and lip melanocytic neoplasms with a favorable diagnosis, though historically, these locations were considered to be “almost invariably malignant”; oral and lip neoplasms composed of heavily pigmented, well-differentiated melanocytes with a low mitotic index (<4/10HPF) have a favorable prognosis

·       Nailbed melanocytic neoplasms: often malignant, and

·       Cutaneous melanocytic neoplasms: are often benign; canine melanocytic neoplasms with a MI of >3/10HPF are expected to have shorter survival times

·       Malignant melanoma:

·       Often occur in older animals, though are reported in young animals of many species

·       Metastasis most often occurs via lymphatics to regional lymph nodes and lungs, though has been reported in unusual locations such as the brain, heart and spleen

·       Metastasis of an oral melanoma to the pituitary gland has been reported (Miller MA, Vet Pathol 2018)



·       Melanocytoma (benign melanoma, melanotic nevus): Usually solitary, circumscribed, alopecic, gray to brown to black, cutaneous nodules that range from 1 to 4 cm in diameter; smooth or slightly papillated to pedunculated; ulceration is uncommon

·       Malignant melanoma: Sessile, variably circumscribed, unencapsulated; polypoid, dome-shape, or plaque-like; gray to brown to black; range from 1 to 10 cm in diameter; ulceration is common



·       General features:

·       Amount of pigmentation is variable in melanocytic neoplasms; with a variety of cell shapes from spindle cell, balloon cell (clear cell), epithelioid, and signet-ring cell

·       Junctional activity (proliferation of neoplastic melanocytes in small nests, within the epidermis at the epidermal/dermal or follicular/dermal junction) is a helpful diagnostic feature

·       Breslow thickness: A measurement from the top of the granular layer of the epidermis to the deepest invasive cell across the broad base of the tumor; prognostic factor (utilized in humans), and the measurement is greater in animals (dogs) with shorter overall survival and disease-free outcome; tumor thickness can be measured with a ocular micrometer or a ruler; cutoffs of 0.95 cm and 0.75 cm defined a higher hazard for an unfavorable outcome and to develop recurrence/metastasis (Seilvestri S, Vet Pathol 2019)

·       Melanocytoma (benign melanoma, melanotic nevus):

·       Histological classification: Junctional (confined to the epidermis and dermoepidermal junction), compound (involving both dermis and epidermis), or dermal; compound and dermal are the most common in domestic animals

·       Can be composed of any melanocytic cell type; most common are spindle cells (whorls or fingerprint patterns in the dermis), epithelioid cells (nests in the dermis or epidermis, and along the dermo-epidermal junction) or a combination

·       Degree of pigmentation can vary; epithelioid cells are often darkly pigmented, and spindle cells are often lightly pigmented to amelanotic

·       Mitotic figures are rare; MI <3 is reported to predict benign behavior

·       Balloon cell melanocytoma: Variant; dermal mass composed of large epithelioid to polygonal cells with abundant amphophilic to eosinophilic cytoplasm with a granular appearance; few neoplastic cells will contain fine melanin granules; nuclei are small, uniform and hyperchromatic

·       Malignant melanoma:

·       Can be composed of a variety of melanocytic cell morphologies; most common are epithelioid cells, spindle cells or a combination

·       Epithelioid cells form clusters and/or nests

·       Spindle cells form sheets, bundles, and/or whorls

·       Cells may be heavily pigmented or amelanotic; epithelioid and spindle cell form typically pigmented; balloon cell form and signet-ring cell form are usually poorly pigmented to amelanotic

·       Cytoplasm is moderate to abundant

·       Variable nuclear pleomorphism

·       3 or more mitotic figures/10 HPFs (for cutaneous location), with mitotic atypia

·       Varying degrees of junctional activity

·       Variants:

·       Balloon cell malignant melanoma: Dermal masses, sometimes multilobulated; junctional activity absent; large cells with large, vesicular nuclei, prominent nucleoli, low MI, abundant clear cytoplasm without visible melanin; rarely, few cells will contain pale, dust-like granules; rare multinucleated cells

·       Signet-ring malignant melanoma: Round to polygonal cells with eccentric nuclei compressed by abundant pale eosinophilic or amphophilic cytoplasm without visible melanin; rarely, few cells contain pale granules; rare multinucleated cells



·       Immunohistochemistry:

·       Melanocyte specific markers:

·       Melan-A (currently the most specific marker – melanocyte-differentiating protein recognized by cytotoxic T cells) useful in many species, including dogs, cat and horse (although included in a primary text, the utility of Melan-A in horses is refuted by Ramos-Vera Vet Pathol 2014)

·       PNL2, TRP-1 and TRP-2 used alone or as a cocktail with Melan-A

·       HMB-45: Specific marker for melanoma cells

·       Neural crest markers: S-100 and neuron specific enolase (NSE) are sensitive markers for melanocytes, but are not specific

·       Mesenchymal markers: Vimentin

·       Prognostic markers:

·       FoxP3 (transcription factor forkhead box protein P3) and IDO (indoleamine 2,3-dioxygenase): An increased risk of death due to canine melanoma is associated with a higher number of FoxP3+/HPF, higher percentage of CD3+ cells that were also FoxP3+ infiltrating and surround the tumor, and increased IDO+ cells/HPF, though stratification by diagnosis (oral melanoma, cutaneous melanoma, or cutaneous melanocytoma) resulted in loss of significance except for IDO+/HPF in oral melanomas (Porcellato I, Vet Pathol 2019)

·       MCAM/CD146 (melanoma cell adhesion molecule): Overexpression of MCAM/CD146 was reported in canine oral melanomas, weak in cutaneous melanoma, and negative in ocular melanoma (Asa SA, J Comp Path 2017)

·       C-kit receptor (responsible for transmission of promigration signals to melanocytes); expressed in all canine benign dermal melanocytomas; expression anywhere within malignant mucosal melanocytic tumors correlated with significantly longer survival times (Newman SJ, Vet Pathol 2012)

·       Nuclear survivin expression is associated with histological features of malignancy, metastasis and death (Bongiovanni L, Vet Dermatol 2015)

·       COX-2 highly expressed in canine oral melanocytic neoplasms and low in cutaneous ones (MartinezAW CM, Vet Pathol 2011)

·       Ki67 (which assesses tumor growth fraction) is a useful prognostic factor in melanomas of dogs and cats (Bergin IL, Vet Pathol 2011)

·       Histochemical stains:

·       Fontana-Masson silver stain: Highlights the small amounts of melanin in amelanotic melanomas

·       Bleach: Can be utilized to determine cytomorphologic characteristics in heavily pigmented melanocytes

·       Cytologic findings:

·       Benign and malignant neoplasms contain pleomorphic cells, ranging from epithelioid to spindle, or less often are discrete and round; well-differentiated cells contain abundant, fine, black-green cytoplasmic granules which may obscure the nucleus; benign forms contain small, uniform nuclei; malignant melanomas contain increased anisocytosis and anisokaryosis, coarse nuclear chromatin and prominent nucleoli

·       Staging lymph nodes: Consensus between routine cytology and histopathology for lymph node staging in patients with melanocytic neoplasms is poor and does not correlate with survival (Grimes, Vet Pathol 2017)



·       Other melanocytic lesions:

·       Melanocytoma-acanthoma (melanoacanthoma): Rare, reported in dogs; circumscribed, unencapsulated, solitary, pigmented nodule, < 1 cm in diameter, combination of junctional melanocytoma with a benign epithelial tumor; epithelial cells forms nests and cords +/- cystic structures containing keratin, and melanocytic cells forms nests in the epidermis and sometimes within the epithelial cell cords, or may have a spindle shape and form whorls between epithelial cords and

·       Melanocytic hyperplasia: In dogs, found on nipple; in cats, found on lips, eyelid, pinna, frequently in orange, cream or silver coat colors; hyperplasia of non-neoplastic melanocytes within the basal layer of the epidermis, with melanomacrophages in the superficial dermis

·       Oculoderma melanocytosis (nevus of Ota) (case report): a dermal melanocytic hamartoma presenting as cutaneous facial hyperpigmentation that corresponds to the distribution of the ophthalmic and maxillary branches of the trigeminal nerve, often with ipsilateral ocular involvement; marked panuveal melanocytosis with extension into the sclera, bulbar conjunctiva, and connective tissue surrounding the optic nerve; melanocytes have a small nucleus with uniform nuclear morphology and absence of mitotic activity (Giannikaki, Vet Pathol 2019)

·       Biphasic malignant melanoma-adenocarcinoma (case report): recently reported in the digit of a dog; the melanocytic population was melan A positive and TRP2, PNL2, CKAE1/3, and CK7 negative; the epithelial population CKAE1/3 positive, and CK7, melan A, TRP2, and PNL2 negative (Needle, J Vet Diagn Invest 2018)

·       Spindle cell neoplasms:

·       Spindle cell melanoma: Composed entirely of amelanotic spindle cells and impossible to distinguish from fibrosarcoma or neurofibrosarcoma without IHC stains

·       Pigmented peripheral nerve sheath tumor (PNST): May require IHC stains in some cases; PNST is GFAP positive and melanomas are rarely positive

·       Fibrosarcoma: May require IHC stains in some cases; fibrosarcomas stain negative with S100, melan-A, HMB-45, tyrosinase, and TRP-2

·       Other neoplasms that display junctional activity: Epitheliotrophic lymphoma, rarely cutaneous histiocytoma and mast cell tumors



·       Cats:

·       Melanomas are uncommon and are often amelanotic; greatest incidence in domestic shorthair cats; often occur on head

·       A rare, cutaneous amelanotic signet-ring cell melanoma was reported in a 2-year-old cat, with melan A-positive signet ring cells, spindle cells, and myofibroblastic differentiation (Hirz M, J Vet Diagn Invest 2016)

·       Horses: Common in older gray horses, often on perineum, genital area and distal limbs; behavior is difficult to predict based on histological features, can have slow growth for years and then a sudden onset of malignant behavior; equine melanocytic tumors do not express Melan A, use PNL2 and S-100 instead

·       Pigs: Sinclair miniature swine and melanoblastoma-bearing Libechov minipig (Hormel crosses, MeLiM strain, used in research) develop congenital melanoma; high incidence of spontaneous malignant melanoma, with reports of spontaneous regression or malignant behavior with metastasis

·       Cattle:

·       Most are benign; may occur as a congenital lesion or at any age; Angus breeds overrepresented

·       Recent report an amelanotic melanoma in a crossbred heifer calf, with widespread metastases (Winslow CM, J Vet Diagn Invest 2017)

·       Sheep and goats: Uncommon and are generally pigmented

·       Avian species:

o   Malignant melanomas usually develop in the skin, adrenal glands, ovaries, lungs, or liver, and often develop on the facial skin near the beak in older birds (Veiga-Parga T, J Am Vet Med Assoc 2016)

o   Penguins: Malignant melanoma is relatively common in penguins compared to other avian species; often heavily pigmented and act aggressively with metastasis to multiple organs (Duncan AE, J Zoo Wild Med 2014)

·       Snakes: Chromatophoromas (I-N30) are neoplasms that arise from pigment-bearing cells (chromatophores) of the dermis; chromatophores include zanthophores, iridophores, and melanophores (containing melanosomes); melanocytic neoplasms are the most common; due to their dermal origin, there is neither junctional activity nor lentiginous spread; moderate to marked nuclear atypia appears consistently in neoplasms with a high risk of metastasis; San Francisco garter snake is overrepresented; S-100 + more reliable than PNL-2; sensitivity of melan A and HMB45 is extremely low in reptile melanocytic tumors; nuclear atypia is the most consistent feature associated with malignancy (Munoz-Gutierrez,Vet Pathol 2016)

·       Platyfish (Xiphophorus maculatus) and Swordtail fish (Xiphophorus helleri) hybrid crosses are used to study spontaneously developing melanomas

·       Fallow deer: Melanomas are common, often occurring on thin haired regions; periocular melanomas have an aggressive biologic behavior in white fallow deer (Hughes KL, J Vet Diagn Invest 2017)

·       Pigmy hippopotamus: Multiple cutaneous melanomas and melanocytomas were reported in a 36-year-old pigmy hippopotamus (pachyderms, similar to pigs); melanocytic neoplasms are reported sporadically in zoo species (Saunders RA, Vet Dermatol 2017)

·       Melanoma has been reported in golden and king wildebeests (Adetunji, J Zoo Wild Med 2018)


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