title: Metastatic balloon cell melanoma in a dog.
abstract: BACKGROUND
  Balloon cell melanoma is a rare variant of amelanotic melanoma that is difficult to differentiate from sebaceous cell carcinoma, liposarcoma, and other clear cell neoplasms without immunohistochemistry or ultrastructural evidence of melanin or melanosomes.
  
  
  OBJECTIVE
  The purpose of this report was to describe the clinical, cytologic, histologic, immunohistochemical, and ultrastructural findings in a dog with metastatic balloon cell melanoma.
  
  
  METHODS
  A 6-year-old female Golden Retriever was evaluated for a white, flocculent infiltrate in the anterior chamber of the left eye and an enlarged left prescapular lymph node. Cytologic evaluation of the eye and lymph node were performed following aqueocentesis and fine-needle aspiration, respectively. The affected lymph node was examined histologically and stained for cytokeratin, vimentin, S-100, and Melan A. Following euthanasia a necropsy was performed and samples of the affected lymph node were examined by electron microscopy.
  
  
  RESULTS
  Cytologic examination of the lymph node and aqueocentesis sample revealed round neoplastic cells that had abundant clear vacuolated cytoplasm. A tentative diagnosis of metastatic sebaceous cell carcinoma or clear cell neoplasm was made. Histologically, the affected lymph node had similar polygonal clear cells arranged in sheets and packets divided by delicate fibrovascular stroma. Immunohistochemical staining of the cells was negative for cytokeratin but positive for vimentin, weakly positive for S-100, and strongly positive for Melan A. At necropsy, metastatic lesions were identified in the diaphragm, heart, lung, kidneys, left eye, prescapular and sublumbar lymph nodes, and multiple skin sites. Ultrastructural examination of neoplastic lymph nodes revealed many membrane-bound vacuoles, myelin-like figures, and rare melanosomes.
  
  
  CONCLUSIONS
  Immunohistochemical staining and ultrastructural features of the neoplastic cells supported a diagnosis of metastatic balloon cell melanoma.
authors.affiliation: Department of Diagnostic Medicine/Pathobiology, College of Veterinary Medicine, Kansas State University, Manhattan, KS 66506, USA. wilkersn@vet.ksu.edu
authors.forename: Melinda J
authors.initials: MJ
authors.lastname: Wilkerson
authors.forename: Karen
authors.initials: K
authors.lastname: Dolce
authors.forename: Brad M
authors.initials: BM
authors.lastname: DeBey
authors.forename: Heather
authors.initials: H
authors.lastname: Heeb
authors.forename: Harriet
authors.initials: H
authors.lastname: Davidson
mesh_terms: D000818:Animals; D004283:Dog Diseases / Q000188:drug therapy / Q000473:pathology*; D004285:Dogs; D005260:Female; D008545:Melanoma / Q000188:drug therapy / Q000473:pathology* / Q000648:ultrastructure / Q000662:veterinary*; D009362:Neoplasm Metastasis / Q000188:drug therapy / Q000473:pathology* / Q000648:ultrastructure; D016896:Treatment Outcome