Melanocytic tumor is used as a diagnosis when it is not initially clear if the neoplasm is likely to be malignant or benign.
There are several characteristic features for diagnosis. These include intraepithelial nests and neoplastic cells, junctional activity where neoplastic cells bud from the basal region of the epidermis, presence of melanin in neoplastic cells, large central nucleoli (owl's eye cells) , packeting and combined spindle and epithelioid cells.
There is a never ending list of subtypes of melanoma - epithelioid, spindle, round, and angiomatoid.
Angiomatoid melanoma
These tumors have large blood filled spaces, that are not lined by endothelium. The lining cells are CD34 negative and SOX10 positive.
Ramírez-Hernández C, García-Márquez LJ, Ramírez-Romero R. Primary Angiomatoid Melanomas in Dogs. J Comp Pathol. 2022; 199: 8-11.
Melanoma acanthoma
This is a combination of acanthoma and melanoma
Pilar neurocristic melanoma
Pilar neurocristic hamartoma is a recognized entity in humans and development of melanoma from them is reported
In dogs, Anderson et al (1988) reported on 4 cases of pilar neurocristic melanoma.
Anderson WI, Luther PB, Scott DW. Pilar neurocristic melanoma in four dogs. Vet Rec. 1988; 123: 517-518.
These are melanocytic tumors of low malignant potential.
Characteristic include
mitotic count of 2 or less (oral is 3 or less)
Nuclear atypia of less than 30% of cells
Ki67 index of 19.5 or less is strictly an indicator of growth fraction and it is not strictly a proliferation indicator or marker.
depth of growth less than 0.95 cm indicates less likihood of metastasis and recurrence.
These are melanocytic tumors of high malignant potential. Some also call them just melanoma.
Characteristic include
mitotic count of 3 or more (oral is 4 or more)
Nuclear atypia of greater than 30% of 200 cells. Spangler and Kass (2006) reported nuclear atypia as "larger nucleoli of less-regular shape. They were eccentrically located in the nucleus and often multiple. In some cases, multiple nucleoli haphazardly connected to the inner surface of the nuclear membrane by thin strands of chromatin and gave the appearance of a coarsely vacuolated nucleus"
Depth of growth higher than 0.95 cm indicates less likihood of metastasis and recurrence. Extension beyond the dermis indicates potential malignancy.
Ki67 index of 19.5 or higher is strictly an indicator of growth fraction and it is not strictly a proliferation indicator or marker.
Bostock (1979) reported that 15 of 33 (45%) died in 2 years and median survival was 70 weeks (15 months)
Spangler and Kass (2006) reported that 12% of 227 dogs with cutaneous melanocytic tumors (both benign and malignant) and only 7% died of tumor related disease in 12 months.
Bostock DE. Prognosis after surgical excision of canine melanomas. Vet Pathol. 1979; 16: 32-40.
Spangler WL, Kass PH. The histologic and epidemiologic bases for prognostic considerations in canine melanocytic neoplasia. Vet Pathol. 2006; 43: 136-49.
Smedley RC, Bongiovanni L, Bacmeister C, Clifford CA, Christensen N, Dreyfus JM, Gary JM, Pavuk A, Rowland PH, Swanson C, Tripp C, Woods JP, Bergman PJ. Diagnosis and histopathologic prognostication of canine melanocytic neoplasms: A consensus of the Oncology-Pathology Working Group. Vet Comp Oncol. 2022; 20: 739-751.
Smedley RC, Spangler WL, Esplin DG, Kitchell BE, Bergman PJ, Ho HY, Bergin IL, Kiupel M. Prognostic markers for canine melanocytic neoplasms: a comparative review of the literature and goals for future investigation. Vet Pathol. 2011; 48: 54-72.
A Merkel cell tumor (also called Merkel Cell Carcinoma) is a very rare tumor of skin in dogs. Merkel cells are of a neuroendocrine type. They are identified by mass extending from the epidermis, with an epithelial appearance but with distinct packeting.
Immunohistochemically, these stain with cytokeratins (especially CK 18, 19, 20), synaptophysis and chromogranin.
Ito S, Chambers JK, Mori C, Sumi A, Omachi T, Nakayama H, Uchida K. Comparative In Vitro and In Vivo Studies on Feline, Canine, and Human Merkel Cell Carcinoma. Vet Pathol. 2021; 58: 276-287.