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A Polypoid Lesion on the Left Knee: Answer.

DISCUSSION Malignant glomus tumors (MGTs), or glomangiosarcomas, were first pathologically described by Lumley and Stansfeld1 in 1972. They are exceedingly rare soft-tissue neoplasms composed of modified smooth muscle cells of the normal perivascular glomus body. Most cases of cutaneous involvement of MGT present as solitary, painful nodules with a predilection for the lower extremities;2 however, there have been a significant number of reports of hand/digit involvement.3–10 Although rare, there have been reports of metastasis with primary cutaneous involvement.3,5,11–14 Histologically, it has been proposed that MGT be subclassified into 3 categories: locally infiltrative glomus tumors, glomangiosarcomas arising in a benign glomus tumor, and glomangiosarcomas arising de novo.4 Nearly all reports of metastasis have been from primary glomangiosarcomas arising de novo, with an estimated metastasis rate of 52.5%.8 More recently, a study of 52 cases of atypical and MGTs by Folpe et al suggested that the presence of at least one of 3 criteria be used to diagnose MGT because these features increased the metastatic propensity of the tumor. These included deep location and size greater than 2 cm, or atypical mitotic figures, or the combination of moderate-tohigh nuclear grade and mitotic activity (at least 5 mitotic figures per high-power field).3 The present tumor fits these diagnostic criteria based on size, deep location, and presence of atypical mitotic figures. MGTs stain positively with vimentin, membranous and cytoplasmic smooth muscle actin, and muscle-specific actin, whereas staining for desmin, S100, HMB-45, CD34, CD31, cytokeratin, type IV collagen, and epithelial membrane antigen (EMA) is typically negative.13,15 The histologic differential diagnosis for MGTs includes malignant nodular hidradenoma, which can demonstrate solid, glandular, and papillary patterns, and has positive staining for cytokeratin 5 and 6 and AE1/AE3, negatively for EMA, S-100, GCDFP-15, BLC-1, and BLC-2, and variable staining for carcinoembryonic antigen (CEA).16,17 Malignant eccrine spiradenoma can show 2 distinct patterns: an abrupt transition from benign-appearing spiradenoma to having frankly carcinomatous or sarcomatous areas or one where a lobular growth architecture is retained with areas of adenocarcinoma. At higher magnification, this tumor shows monomorphic tumor cells with hyperchromatic chromatin, distinct nuclei, marked pleomorphism, and mitotic figures in a sheet-like growth pattern.18,19 Merkel cell carcinoma is characterized by a sheet-like growth of monotonous, uniform, small, round, blue undifferentiated cells with large, lobulated nuclei and scant cytoplasm with finely dispersed chromatin.20 Numerous mitotic figures and areas of necrosis can be appreciated. Immunohistochemical staining of these tumors is typically positive for both epithelial markers, including paranuclear dot-like staining for cytokeratin 20, EMA, AE1/AE3, and CAM5.2, and also neuroendocrine markers, such as neuronspecific enolase, chromogranin A, synaptophysin, calcitonin, and vasoactive intestinal peptides. Pertinent negative stains include thyroid transcription factor-1, Melan-A, vimentin, and leukocyte common antigen.20 Hemangiopericytoma, more commonly known as solitary fibrous tumor, is histologically described as having a roundor spindle-shaped cell proliferation with varying degrees a characteristic “staghorn” configuration of thin-walled vascular spaces.21 Cellular pleomorphism and mitoses can be present. A reticulin stain will highlight a delicate reticulin sheath surrounding the vascular proliferation.22 Immunohistochemical evaluation of this tumor will show positive staining for CD34 and vimentin and negative staining for cytokeratin, desmin, S-100, and CD31.21 MGTs can be differentiated from leiomyosarcoma, which display larger, more hyperchromatic cells with more apparent cytoplasmic eosinophilia. In addition, cells in leiomyosarcoma can have perinuclear vacuoles and stain positively for desmin, which differs from MGT.23 Anecdotal reports suggest wide local excision as the preferred treatment option; however, there was one report of using Mohs micrographic surgery to ensure complete removal.9,15,24 As with any malignancy, close follow-up for recurrence is recommended because of the aggressive nature and risk for metastasis. There is typically a favorable prognosis if no metastasis has occurred at the time of diagnosis.2 From the *Department of Dermatology, Wayne State University, Detroit, MI; and †Pinkus Dermatopathology Laboratory, Aurora Diagnostics, Monroe, MI. The authors declare no conflicts of interest. Correspondence: Rachel E. Ward, MD, Department of Dermatology, Wayne State University, 18100 Oakwood Boulevard, Suite 300, Dearborn, MI 48124 (e-mail: rward@med.wayne.edu). Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.