Atypical subcutaneous giant cells carcinoma in three dogs: histological, immunohistochemical and ultrastructural features.

Three subcutaneous solitary nodules, from dorsal, cervical, and cheek regions, respectively, of three dogs, were incidentally detected, excised and submitted for histology. The nodules, ranging from 0.8 to 1.5 cm, were indolent and notfixed to the surrounding tissues. For all three nodules histology reveals a well demarcated mass within the dermis and the hypodermis consisting of multiple irregular lobules associated with moderate stroma. Lobules are composed by pleomorphic cells arranged in sheets, in short bundles with mild palizading, and in a pseudoalveolar pattern; a central residual lumen is occasionally associated with mineralized PAS positive deposits. Polyhedral to spindle cells showed moderate to very large cytoplasm with marked irregular vacuolation, hyaline glassy material and/or granular PAS positive residues. Multifocally, discrete ballooning often multinucleated bizarre cells with abundant eosinophilic cytoplasm and occasional clear peripheral halo, is also present. Rarely, signet ring cellsand variably sized areas of squamous-like differentiation are detected. Morphological differential diagnosis include atypical forms of amelanotic melanoma, neuroendocrine tumor, peripheral nerve sheet tumor, and anaplastic sarcoma/carcinoma. Histochemistry, immunohistochemistry and electron microscopy were performed. Neoplastic cells are negative to Grimelius silver impregnation and Fontana stainings. Immunohistochemistry reveals a coalescing to diffuse intensively positive staining for the presence of pancytokeratins; only scattered cells are positive for calponin (myoepithelial and smooth muscle cells), CK14 (basal epithelial and myoepithelial cells), CK8/18 (luminal glandular epithelial cells), CK5/6 (basal cells of stratified squamous epithelia). Frequently a concomitant expression of vimentin and occasionally S100 is detected in neoplastic cells, particularly in atypical giant cells. Neoplastic cells are generally negative for smooth muscle actin, glial fibrillar acidic protein, - 158 - POSTERS neuron specific enolase,and chromogranin A. Electron microscopy allowed identification of numerous desmosomal structures associated with neoplastic cells. On the basis of these results, the lesions are classified as atypical carcinoma with pleomorphic multinucleated giant cells,possibly a new morphological type of sebaceous carcinoma.