In Folpe's study, glomus tumors not fulfilling criteria for malignancy but having at least one atypical feature other than nuclear pleomorphism were categorized as “glomus tumors of uncertain malignant potential”[6]. Tumors in this category are superficial tumors with high mitotic activity or they are large or deep. Glomangiomatosis was used for tumors with diffuse angiomatosis and excess glomus cells. Finally, the term “symplastic glomus tumor” was proposed for glomus tumors with high nuclear grade in the absence of any other malignant feature. Nine of the cases were classified as symplastic glomus tumor in the series. Two of them showed recurrence and none of them metastasized. They concluded that these tumors were benign similar to ordinary glomus tumors.
The marked nuclear atypia in glomus tumors is similar to that seen in some benign mesenchymal tumors of long duration, especially uterine leiomyoma and ancient schwannoma. These tumors behave like ordinary leiomyomas and schwannomas, therefore the nuclear atypia has been accepted as a degenerative change[1,6,14].
The tumor cells of the presented case showed striking nuclear atypia, some with bizarre nuclei. Although the pleomorphism and the epithelioid configuration of the tumor were alarming at the initial evaluation, malignancy was not in the foreground because neither mitotic activity nor necrosis was present and Ki-67 labeling index showed that the proliferative activity was undetectable. While typical glomus tumor areas were only focal, the punched out appearance of tumor cells could be identified even at the atypical areas. Type IV collagen pattern and smooth muscle actin staining were consistent with symplastic glomus tumor. The morphology together with the clinical symptoms confirmed the diagnosis.
The evaluation of symplastic glomus tumors needs careful histopathological examination[1,5,6]. Marked nuclear atypia of the tumor may be reminiscent of a malignant tumor. Epithelioid appearance of the tumor may be misleading. While the identification of areas of typical glomus tumor is the most important clue to the diagnosis, these areas may be minimal or absent in some cases. The branching capillaries and the perivascular arrangement of tumor cells should be noted. Immunohistochemical study showing smooth muscle actin expression and type IV collagen, a constituent of basal lamina, outlining the cells are the most beneficial in histopathological diagnosis especially when the typical glomus tumor areas are not distinctive[1]. Lack of expression by cytokeratins helps to eliminate a primary or a metastatic epithelial tumor and adnexial tumors. HMB45 and S100 negativity allow distinction from malignant melanoma. Despite its atypical morphology, symptoms and location are quite typical in most cases of symplastic glomus tumor, thus it is important to get thorough clinical history.
Symplastic glomus tumor is considered benign with occasional recurrences due to incomplete excision. Recognition of this rare morphologic deviation is critical, as the marked atypia can lead to an incorrect diagnosis of malignancy and overtreatment.
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