GLOMUS TUMOR CONTAINING ATYPICAL CELLS (SYMPLASTIC GLOMUS TUMOR)
Ordinary glomus tumors and glomangiomas are composed of a monotonous population of small round cells that are associated with variably prominent vascular channels. These cells share immunohistochemical properties with smooth muscle and pericytes. Increased tissue mucin is found in the stroma of some glomus tumors or variants of glomus tumors, and this is of no prognostic significance. Smooth muscle differentiation is readily apparent in glomangiomyomas.

Occasional glomus tumors contain cytologically atypical cells that, as an isolated finding, are not indicative of malignancy. This type of atypia has been postulated to be on a 'degenerative' basis and may be analogous to the atypia found in ancient schwannomas, atypical smooth muscle tumors, and pleomorphic lipomas.

Criteria for the distinction between symplastic glomus tumors, glomus tumor of uncertain malignant potential, and malignant glomus tumors have been published by Folpe et alref1. You need to see the article or abstract because some of the criteria I summarized in the table below are only significant in combinations defined in the article.

 
Deep location Size > 2 cm. Atypical mitoses Moderate or high grade nuclear atypia. > or = 5 mitoses/50 HPF
MALIGNANT + + + + +
SYMPLASTIC - - - + -
UNCERTAIN MALIGNANT POTENTIAL + of this is the only abnormality + if this is the only abnormality  -
+/- plus one other criterion
 + or -. If +, must be superficially located tumor  only



A painful tumor of a finger from a 40 year old female:
 
Scan power view showing a tumor that is located in the lower dermis and subcutis. Neither necrosis nor mitoses were found.
A high power view. A monotonous population of small round or slightly oval cells are associated with small vascular channels. This combination is typical of a glomus tumor. Tissue mucin is slightly increased in this example.
A medium power view from another area. Ordinary glomus cells are mixed with a few cytologically atypical cells. There is a fascicle of spindle cells in the left side of the picture, but this degree of smooth muscle differentiation was minimal in this particular tumor.
A very high power view from the area shown above. Note the atypical cells.
A very high power view from another area. The atypical cell near the center of the picture is either multinucleated or polylobulated.

Reference: 1.  Folpe AL, Fanburg-Smith JC, Miettinen M, Weiss SW.
Atypical and malignant glomus tumors: analysis of 52 cases, with a proposal for the reclassification of glomus tumors.
Am J Surg Pathol. 2001 Jan;25(1):1-12.

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