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Large prepatellar glomangioma: A case report

Highlights • Large glomangioma appearing inrare, anteroinferior aspect of knee in healthy 49 year old male.• MR imaging detected intermediate signal intensity on T1, mixed signal intensity on T2, and marked signal enhancement with gadolinium.• Histology demonstrated large, dilated vessels surrounded by normal endothelial cells and infiltrating glomus cells which stained positively for smooth muscle actin.


Introduction
Surgeons are occasionally confronted with complex soft tissue tumors where the diagnosis may be in doubt. A carefully considered approach to the diagnosis and initial management is essential. Glomangiomas are rare, benign tumors derived from the glomus body. They have been reported to account for 1.6% of all soft tissue tumors in the extremities [1]. Glomus tumors typically arise in the extremities, especially subungual, but they may also be found in atypical locations. Surgical removal of glomangiomas is usually effective and curative. Unfortunately, diagnosis is often delayed. There are few reports of glomus tumors in the knee area, often presenting with knee pain (see Table 1). In the present study, we report an unusual case of a large prepatellar glomangioma.

Presentation of case
An otherwise healthy 49 year-old man presented with a progressively enlarging, painful mass on the anteroinferior aspect of his left knee for 1 year. Over the year the mass had gradually increased in size and become more painful. More recently, the patient described a constant throbbing pain with intermittent "stabbing" sensations and limited range of motion. The patient was a diesel mechanic and spent many hours on his knees. He reported multiple episodes of minor penetrating injuries to the area. On physical exam, there was limited range of motion at the left knee joint; a heterogeneous mass (60 mm × 50 mm × 50 mm) appeared on the anteroinferior aspect of the left knee, which was exquisitely tender with surrounding erythema and warmth (Fig. 1a,b). The mass also demonstrated small areas of ulceration (Fig. 1a). No regional lymphadenopathy was palpated.
Plain radiograph of the left knee joint showed a mass superficial to the patellar tendon with a nonspecific focus of calcification.
Histopathology demonstrated a monomorphic population of small, round, eosinophilic cells with minimal atypia (Fig. 5). Immunohistochemistry was positive for smooth muscle actin (Fig. 5) and negative for cytokeratin, S-100, and CK-34. This was consistent with a glomangioma. Once final histopathology confirmed the benign nature of the lesion the wound was closed with a split thickness skin graft. The graft was harvested from the left lateral thigh using a 0.018 inch dermatome with a 3 inch guard and placed unmeshed on the 80 × 100 mm wound. Numerous slit incisions were then made in the graft (pie crusting) to allow for fluid drainage. As of this writing the patient had remained symptom free for 2.5 years with no recurrence.  The lobulated mass shows a semi cystic/fluid-fluid structure. The lesion is generally heterogeneous in signal strength. A moderate amount of subcutaneous edema is also present. The lesion is separated from the patella tendon by a thin fat plane.

Discussion
A glomus tumor is a neoplasm composed of the perivascular smooth muscle cells responsible for thermoregulation in the dermis. These cells are associated with arteriovenous anastomoses involved in the release of heat based on the degree of dilation in the dermis. Glomus tumors are most frequently found in the skin, particularly associated with the extremities. In approximately 65% of cases, the tumor presents in the subungual position and most commonly is diagnosed between 20 and 40 years of age [2]. While glomus cells are typically a dermal finding, there are case reports of glomus tumors found in tissues that are not normally associated with these cells: oral cavity, kidney, liver, GI tract, lung, genitals [3][4][5][6][7]. Extradigital glomangiomas involving the knee have previously been reported in the subcutaneous tissue, infrapatellar fat pad, the popliteal fossa and lateral femoral condyle as detailed in Table 1.
The typical presentation of glomus tumors is a red-purple cutaneous nodule, usually with pain out of proportion to size. The average duration of symptoms is between 7 and 11 years [8]. They have a deep red-purple color and are soft to palpation. They are typically far smaller than our case, usually between 0.5 and 2 cm. Glomus tumors frequently demonstrate cold-sensitivity resulting from reflex vasodilatation; this may be attributed to the role of glomus cells in thermoregulation. Clinically, glomus tumors present infrequently and make up less than 2% of soft tissue tumors. In typical presentations, such as subungual, clinical diagnosis may be sufficient. Atypical presentations, however, may necessitate further evaluation, including MR imaging and histology.
Glomus tumors may be subcategorized as solid type, glomangiomas, and glomangiomyoma [9]. Most common is the solid type, in which the glomus cells predominate, appearing in solid cell sheets. The solid type is found in 75% of cases. The glomangioma variant is associated with greater vascularity, and can be found in 20% of resected lesions. Lastly, the glomangiomyoma subset, in which both vascularity and smooth muscle cells are prominent, occurs in only about 5% of glomus tumors. While glomus tumors are primarily individual tumors, there is a tendency for multiplicity in 10% of presentations.
MR imaging can be used to identify the characteristics of glomus tumors prior to excision. Lesions have characteristic intermediate to low T1-weighted signal intensity and high T2-weighted signal intensity. Gadolinium enhancement is intense and diffuse, a characteristic identified in nearly all glomus tumors [10][11][12]. While glomus tumors demonstrate classic features with T1 and T2-weighting as well as contrast enhancement, prior literature has demonstrated a glomus tumor located in the tip of an index finger that lacked the classic MR features [13]. Furthermore, while MR imaging may be used to identify many characteristics of soft tissue tumors, the differentiation of benign from malignant lesions is not absolute [2,12]. Imaging findings may then be coupled with the size, location, borders, adjacent tissue involvement, and internal characteristics of the lesion to inform a differential diagnosis and surgical planning [11]. The images in this case could not definitively classify the lesion as either benign or malignant.
Glomus tumors, including solid type, glomangioma and glomangiomyoma, may be confirmed histologically. Microscopically, solid type glomus tumors consist of solid sheets of glomus cells. Glomangiomas consist of vascular malformations with large, often dilated vascular channels and surrounding nests of glomus cells. Glomangiomyoma demonstrate glomus cells, vascular channels, as well as smooth muscle cell differentiation. Glomus cells, thought to serve as thermoregulatory receptors, are small uniform cells with prominent eosinophilic cytoplasm and centrally located, rounded nuclei. They are positive for smooth muscle actin, myosin, vimentin, and desmin, all supporting a likely smooth muscle origin. Glomangiomas are typically less circumscribed than their better-known counterpart, solid type glomus tumors, which lack such prominent vessels. Mitotic figures are rare in both entities, which also lack zonal necrosis and significant cellular pleomorphism. Glomus tumors are primarily benign neoplasms which are usually "cured" by local excision with little risk of recurrence [14,15].
While glomus tumors are primarily a benign pathology, there is a 1% likelihood of malignancy. While this is rare, there are case reports of metastasis [16]. Features that increase the likelihood of malignancy include size greater than 2 cm, deep location, abnormal mitotic figures, mitotic rate greater than 5 per high-power field, and moderate to high nuclear grade. If all of these criteria are met, the risk of metastasis has been found to be 25%; however incomplete criteria cannot be used to confirm nor deny malignant potential [17]. Should malignancy be determined, there are three patterns of development: locally infiltrative glomus tumors, glomangiosarcoma arising from benign glomus tumor, or glomangiocarcinoma arising de novo. Management of a glomus tumor is primarily excisional. Sclerotherapy using sodium tetradecyl sulphate has been reported to be effective in patients with multiple hereditary glo-mangiomas but excision is the treatment of choice [18]. There is a recurrence rate of approximately 10%, [19] generally associated with incomplete excision. While there are examples of using Mohs surgery in the literature for cosmetically sensitive areas, the typical procedure is wide local excision. The typically small size makes primary closure more feasible than in our case, in which the large area of excision necessitated use of a split-thickness skin graft.

Conclusion
We report a case of a large glomangioma in rare location, anterior to the knee, presenting with sharp pain and a history of chronic minor trauma to the area. The diagnosis was suggested by clinical presentation and MR imaging, and the final diagnosis was confirmed by histological evaluation.

Consent
Written informed consent was obtained from the patient for publication of this case report and accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal on request

Author contributions
Richard Bell, Melissa Maxey, Katherine Mastriani and Fernando Navarro participated in the diagnosis and treatment of the present case. Richard Bell, Chase Houghton, Katherine Mastriani, Melissa Maxey, and Fernando Navarro drafted, critically revised, and approved the manuscript.