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Image of the Week
Edited and moderated by Stacey Mills, MD, Pathology Network's Image of the Week Blog is a forum for the discussion of interesting and often diagnostically challenging pathology images.
Thursday, August 01, 2013
A 48-year-old with a thumb mass.
48-year-old man presented with a small subungual mass involving his thumb.  An excision was performed.
The resultant specimen, shown below, consisted of a mixture of gaping blood vessels and uniform to highly pleomorphic polygonal cells with prominent eosinophilic cytoplasm. Mitotic figures were not identified and there was no evidence of necrosis.
What is your diagnosis?
Immunohistochemical stains were performed.  The tumor was strongly positive for smooth muscle actin, shown below.  It was negative for desmin, cytokeratin, and factor XIII.  Ki67 showed a very low proliferation index.

This is an example of an atypical or symplastic glomus tumor.  It has been documented in multiple publications (eg.  Am J Surg Pathol 25:1, 2001) that glomus tumors may occasionally show marked nuclear pleomorphism, analogous to that seen in uterine leiomyomas and other stromal neoplasms.  In the absence of other features of malignancy, these tumors behave exactly like conventional glomus tumors. 
The presence of marked nuclear pleomorphism in a small, superficial lesion without increased mitotic activity should not lead to overdiagnosis of malignancy.
About the Author

Stacey E. Mills, MD
Stacey E. Mills, MD, a graduate of University of Virginia (UVA) and the UVA Medical Center, has authored nearly 230 articles, 20+ books, atlases and monographs—including the renowned Sternberg's Diagnostic Surgical Pathology. He has been a practicing Professor and Staff Pathologist at UVA for 30+ years and is Director of Surgical Pathology and Cytopathology. His clinical specialty is general surgical pathology with emphasis on neoplasms and neoplasm-like lesions. Dr. Mills is also Editor-in-Chief of The American Journal of Surgical Pathology.