Indian Journal of Pathology and Microbiology
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Year : 2016  |  Volume : 59  |  Issue : 2  |  Page : 254-256
Myoepithelioma of soft tissue in the knee: A case report of an unusual location

1 Department of Pathology, Soonchunhyang University Hospital, Bucheon, Gyeonggi-do, Korea, South Korea
2 Department of Pathology, Soonchunhyang University Hospital, Bucheon, Gyeonggi-do, South Korea

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Date of Web Publication9-May-2016

How to cite this article:
Moon A, Chin S, Kim HK, Kwak JJ. Myoepithelioma of soft tissue in the knee: A case report of an unusual location. Indian J Pathol Microbiol 2016;59:254-6

How to cite this URL:
Moon A, Chin S, Kim HK, Kwak JJ. Myoepithelioma of soft tissue in the knee: A case report of an unusual location. Indian J Pathol Microbiol [serial online] 2016 [cited 2023 Sep 26];59:254-6. Available from:


Myoepithelial tumors (METs) of soft tissue are rare and under-recognized. The location of these tumors is in the deep soft tissue of extremities, usually in the muscles of thigh, groin, calf, upper arm, or forearm. Only two cases of MET of soft tissue arising from the knee have been reported in English literature. Herein, we report a case of a myoepithelioma arising from the subcutaneous fat layer of the right knee. This uncommon site for a myoepithelioma in the soft tissue and the clinical and overlapping histopathological features of this tumor with other tumors, such as extraskeletal myxoid chondrosarcoma (EMC), confused us. However, immunohistochemical (IHC) profile confirmed this as a myoepithelioma. We report this unusual tumor with its morphological and IHC characteristics including its differential diagnoses.

MET is defined as tumor composed of myoepithelial cells with or without ductal structures. METs are well-characterized in the salivary glands. Recently, it is known that some of these tumors can primarily originate from the soft tissues, including myoepithelioma and myoepithelial carcinoma, along with benign and malignant mixed tumors.[1] In the recent World Health Organization (WHO) classification, parachordoma and MET have been included as a common entity. These tumors arise in patients of all ages with a peak incidence in the second through fourth decades of life. Here, we report a case of myoepithelioma arising from the knee that clinically and histologically mimicked an EMC.

A 41-year-old woman complained about palpable mass at the medial aspect of the right knee without pain that was first noticed 7 years ago. Magnetic resonance imaging (MRI) scan showed a well-demarcated mass of 2 cm size with an oval shape in the anteromedial aspect of the right knee [Figure 1]. Initial impression on MRI was either hemangioma or low-grade angiosarcoma. The patient underwent an excisional biopsy for her mass. The specimen consisted of a well-circumscribed ovoid firm mass with surrounding adipose tissue. The cut surface showed well-circumscribed whitish myxoid mass with encapsulation measuring at 2.0 cm × 1.2 cm × 1.0 cm. Histopathologically, the tumor showed a lobulated growth pattern with cords of epithelioid, ovoid, and spindle cells embedded in a chondromyxoid and collagenous stroma [Figure 2]a. No duct structure was identified. The tumor cells displayed only a mild degree of nuclear atypia (inconspicuous nucleoli, fine chromatin, and mild pleomorphism) with no identifiable mitosis [Figure 2]b. IHC studies showed strong, diffuse positive staining of tumor cells with epithelial membrane (EMA) [Figure 2]c and vimentin. Pan-cytokeratin demonstrated strong but very focal expression [Figure 2]d. Immunostaining for smooth muscle actin (SMA) showed relatively weak expression in the cytoplasm and aberrant strong expression in the nucleus of the tumor cells. The other myogenic markers (S-100 and p63 proteins) were negative. Glial fibrillary acid protein (GFAP) and CD31 were also negative.
Figure 1: Magnetic resonance imaging scan. (a) T1 coronal image showing a relatively well-defined oval shaped tumor in medial side of right knee; (b) T1 axial image showing the mass; and (c) T2 axial image also showing the mass in subcutaneous fat layer

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Figure 2: (a) H and E staining show a multilobular myxoid mass; (b) round to ovoid cells with reticular growth pattern identified in a myxoid stroma; (c) Immunohistochemical staining shows strong epithelial membrane expression in the tumor cells; and (d) cytokeratin (pan) expressed in very focal area of the tumor

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The location of soft tissue MET is in the deep soft tissues of the extremities, usually in the muscles of thigh, groin, calf, upper arm, or forearm. We report a case of a myoepithelioma arising in the subcutaneous fat layer of the right knee in a 41-year-old woman. This location is very rare. Hornick and Fletcher [1] published 101 soft tissue METs, out of which, only 2 (2.0%) tumors were located in the knee or popliteal area.

MET is defined as epithelial tumor containing myoepithelial cells with no or very limited ductal differentiation. Even though MET has been considered as one end of the spectrum of mixed tumors, only a subset of MET of the soft tissue shows ductal differentiation (approximately 20%). Furthermore, up to 45% of myoepitheliomas and myoepithelial carcinomas harbor EWSR1 gene rearrangements, unlike mixed tumor/chondroid syringoma that is characterized by PLAG1 gene rearrangement.[2] Therefore, myoepithelial neoplasm of the soft tissue has been more recently considered as a pathologically distinct neoplasm from mixed tumor/chondroid syringoma.[2],[3]

MET of soft tissue can be further classified as benign (myoepithelioma) or malignant (myoepithelial carcinoma). The most useful criterion for myoepithelial carcinoma of soft tissue is the presence of moderate cytologic atypia (prominent nucleoli, vesicular, or coarse chromatin, and pleomorphism) with or without mitotic counts exceeding 2–3/10 high power field. However in salivary glands, the invasive growth pattern constitutes the single most useful criterion.[1]

MET of the soft tissue must be first distinguished from an EMC. There is a significant degree of morphologic overlap between these two entities. Therefore, distinguishing MET from an EMC can be difficult if based on morphology alone as in our present case. Therefore, IHC stains are required for definitive diagnosis. Proper IHC panel for MET of soft tissue is debatable. Hornick and Fletcher [1] proposed epithelial markers (keratins and/or EMA) in conjunction with S-100 protein or myogenic markers (calponin or SMA). Rekhi et al. suggested EMA and S-100P as optimal markers supplemented with broad spectrum keratins, such as AE1/AE3, along with p63.[4] Our case showed diffuse strong expression of EMA with very focal expression of pan-cytokeratin. Therefore, we suggest that EMA is necessary to diagnose MET.

Ossifying fibromyxoid tumor is another differential diagnosis. This tumor is a lobulated neoplasm comprising cords or nests of uniform, pale-staining ovoid cells deposited in a variably myxoid or hyalinized stroma with a rim of metaplastic bone in 70% of cases. Epithelial marker positivity is very rare in this tumor. Some reports included parachordoma as a differential diagnosis of MET in soft tissue.[5] However, the WHO currently regards myoepithelioma and parachordoma as synonymous.

In conclusion, we describe a case of a myoepithelioma of soft tissue at an unusual site that might be misinterpreted as an EMC. We would like to alert the pathologists to be aware of this rare entity to distinguish it from other myxoid tumors.


This work was supported in part by the Soonchunhyang University Research Fund.

Financial support and sponsorship

Soonchunhyang University Research Fund.

Conflicts of interest

There are no conflicts of interest.

   References Top

Hornick JL, Fletcher CD. Myoepithelial tumors of soft tissue: A clinicopathologic and immunohistochemical study of 101 cases with evaluation of prognostic parameters. Am J Surg Pathol 2003;27:1183-96.  Back to cited text no. 1
Jo VY, Fletcher CD. Myoepithelial neoplasms of soft tissue: An updated review of the clinicopathologic, immunophenotypic, and genetic features. Head Neck Pathol 2015;9:32-8.  Back to cited text no. 2
Thway K, Fisher C. Myoepithelial tumor of soft tissue: Histology and genetics of an evolving entity. Adv Anat Pathol 2014;21:411-9.  Back to cited text no. 3
Rekhi B, Sable M, Jambhekar NA. Histopathological, immunohistochemical and molecular spectrum of myoepithelial tumours of soft tissues. Virchows Arch 2012;461:687-97.  Back to cited text no. 4
Abbes I, Sassi S, Mrad K, Dhouib R, Driss M, Ben Romdhane K. A myoepithelial tumour of the soft tissue of the thigh: A case report. Pathology 2008;40:541-2.  Back to cited text no. 5

Correspondence Address:
Jeong Ja Kwak
Department of Pathology, Soonchunhyang University Hospital, Bucheon, 170 Jomaru-ro, Wonmi-gu, Gyeonggi-do 420-767
South Korea
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0377-4929.182041

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