Glomus tumors are perivascular myoid neoplasms that originate from the glomus body, a specialized dermal arteriovenous structure with a rich nerve supply that is involved in thermoregulation. These tumors are typically found in the subungual region, although they can rarely be found in the visceral organs. Herein, we describe a case of a renal glomus tumor and report the computed tomography (CT), magnetic resonance imaging (MRI), and histopathological characteristics of this rare neoplasm.

A 68-year-old woman with a history of high grade T1 stage bladder cancer complicated by obstruction of the distal left ureter presented for routine follow-up CT urography (CTU). She had previously completed 6 cycles of intravesical Bacillus Calmette-Guérin (BCG) therapy and placement of a left-sided ureteral stent.

CTU incidentally revealed a 1.7 cm lesion in the lateral interpolar left renal cortex. Non-contrast CT images demonstrated that the lesion had slight hypoattenuation (19 HU) relative to the surrounding renal parenchyma (mean attenuation 29 HU) (Figure 1(a)). No foci of macroscopic fat or calcification were seen in the lesion. Arterial phase contrast-enhanced CT images revealed continuous peripheral rim enhancement (110 HU) and central hypoattenuation (37 HU) of the well-circumscribed lesion (Figure 1(b)). Excretory phase contrast-enhanced CT images acquired 10 minutes after intravenous contrast administration showed homogeneous hypoattenuation (62 HU) of the lesion relative to the surrounding renal parenchyma (Figure 1(c)). No prior cross-sectional imaging was available for comparison.

An abdominal MRI examination was then performed to further characterize the lesion. T1-weighted in-phase and opposed-phase gradient recalled echo images through the kidneys showed that the left renal lesion was

isointense to the renal parenchyma and did not have macroscopic or microscopic lipid or foci of susceptibility artifact (Figure 2(a) and Figure 2(b)). Fat-suppressed T2-weighted fast spin echo images demonstrated homogenous high signal intensity of the lesion relative to renal parenchyma, with a thin peripheral low signal intensity rim (Figure 2(c)). Low b-value and high b-value diffusion-weighted gradient recalled echo images revealed loss of high signal intensity of the lesion with increasing b-value (Figure 2(d) and Figure 2(e)), and the corresponding apparent diffusion coefficient parametric map images demonstrated high signal intensity of the lesion, indicating a lack of restriction of water molecule diffusion (Figure 2(f)). Fat-suppressed T1-weighted gradient recalled echo images obtained after intravenous administration of gadolinium-based contrast material revealed arterial phase continuous peripheral rim enhancement of the lesion, followed by venous phase persistent rim enhancement and partial centripetal fill in of enhancement of the lesion (Figure 2(g) and Figure 2(h)). No renal sinus involvement, left renal vein thrombosis, or lymphadenopathy was seen in the abdomen. Based on the CT and MRI findings, the major differential diagnostic considerations for this lesion included renal cell carcinoma, oncocytoma, and mesenchymal lesions such as juxtaglomerular cell tumor and hemangioma.

Subsequently, the patient underwent an uneventful open partial left nephrectomy. On gross pathological

examination, a 1.0 cm tan-white well-circumscribed soft tissue mass was identified in the left kidney. Histopathologically, the lesion was composed of sheets of small round to oval cells with moderate amount of eosinophilic to amphophilic cytoplasm. The cells were fairly uniform with no evidence of pleomorphism or increased mitotic activity. The nuclei revealed a finely speckled chromatin and smooth nuclear contours. The background stroma was composed of delicate fibrovascular stroma (Figure 3(a) and Figure 3(b)). A differential diagnosis of myoid predominant angiomyolipoma, leiomyoma, and the remote possibility of gastrointestinal stromal tumor, lymphangioma, or hemangioma was considered. Accordingly, a panel of immunohistochemical staining was performed. The tumor cells were strongly and diffusely positive for smooth muscle actin, and CD31 highlighted the capillary network. The tumor was negative for all other markers including HMB-45, CD10, RCC, EMA, CD117, synaptophysin, chromogranin, and D2-40. All together, these findings were in keeping with a diagnosis of a renal glomus tumor.

At follow-up clinical evaluation 7 months later, the patient was noted to be doing well. Follow-up cross- sectional imaging did not demonstrate the presence of tumor recurrence or of a new mass in either kidney.

The glomus tumor, first described in 1924 by Masson [1] , is a perivascular myoid neoplasm that originates from the modified smooth muscle cells found in the walls of the glomus body, a specialized arteriovenous structure with a rich nerve supply that is involved in thermoregulation [2] . Glomus bodies are found within the stratum reticularis of the dermis, predominantly in the subungual region, the lateral aspects of the digits, and the palm. Glomus bodies are also found in the precoccygeal soft tissue [3] .

Glomus tumors account for less than 2% of soft tissue tumors. The tumors occur in both sexes with equal frequency, primarily within the age range of 20 to 40 years. Ninety percent occur as solitary neoplasms, and familial cases have been described [4] . Most glomus tumors present as paroxysmally painful, subcutaneous blue-red nodules of the distal extremities, usually in the subungual region [5] [6] . About one quarter of glomus tumors are found in visceral organs that typically do not express glomus bodies [4] and have been reported to occur in the gastrointestinal tract [7] - [9] , bone [10] , lung [11] , liver [12] , pancreas [13] , oral cavity [14] , and sinonasal region [15] . Glomus tumors in the genitourinary tract are uncommon and primarily involve the clitoris, vagina, cervix, and periurethral soft tissue [16] - [18] . To date, there have been 11 previously reported cases of renal glomus tumors; our case is the 12th such case in the literature [2] [3] [5] [19] - [24] .

Histopathologically, glomus tumors are well circumscribed and composed of varying proportions of glomus cells, blood vessels, and smooth muscle. Based on the relative proportions of these cell types, glomus tumors are divided into three subgroups: glomus tumor proper, glomangioma, and glomangiomyoma [2] .

Glomus tumors are generally considered to be benign and are treated by local excision [6] . However, rare malignant glomus tumors have been reported [6] [25] [26] . These more aggressive tumors arise both de novo and

from preexisting benign glomus tumors [25] . Folpe et al. retrospectively analyzed 52 cases of atypical glomus tumors to establish histological criteria for malignancy. The authors proposed that deep location, size greater than 2 cm, presence of moderate to high grade nuclear atypia, or presence of 5 or more mitoses per 50 high- powered fields should be considered as criteria for malignancy [26] . However, each of the neoplasms reviewed arose in the extremity [26] ; the applicability of the criteria to glomus neoplasms of the visceral organs is therefore uncertain. Given the rarity of visceral glomus tumors and limited follow-up, the biological behavior of glomus tumors of the internal organs has not been fully characterized. Nine of the 11 previously described renal glomus tumors were considered to be benign, without recurrent or metastatic disease seen after short-term follow-up [2] [19] - [24] . One of the 11 cases was considered to be an infiltrative tumor of uncertain malignant potential [3] . Another one of the 11 was classified as malignant with metastatic disease identified at time of presentation [5] .

To our knowledge, the present case report is the first in the literature to describe the CT and MR imaging characteristics of a renal glomus tumor. Although the reported imaging findings are likely insufficient for the purpose of definitive prospective diagnosis (given their overlap with the features of other renal neoplasms), an interesting and potentially distinctive imaging feature of this lesion was the presence of centripetal fill in of enhancement from arterial phase to venous phase contrast-enhanced images. Although this pattern of enhancement is characteristic of hepatic and splenic hemangiomas [27] [28] , it has not been previously described in renal hemangiomas [29] . Renal mixed epithelial and stromal tumor (MEST) and renal leiomyosarcoma may occasionally contain areas of delayed enhancement on CT imaging; however, these tumors enhance heterogeneously and do not exhibit centripetal fill-in of enhancement from arterial phase to venous phase contrast-enhanced images [29] - [31] .

In conclusion, renal glomus tumor is a rare perivascular myoid neoplasm that may mimic other renal neoplasms. We describe a case of renal glomus tumor and report the CT, MRI, and histopathological characteristics of this rare neoplasm.

Jillian W. Lazor,Thomas J. Guzzo,Zhanyong Bing,Priti Lal,Parvati Ramchandani,Drew A. Torigian, (2016) Glomus Tumor of the Kidney: A Case Report with CT, MRI, and Histopathological Findings. Open Journal of Urology,06,80-85. doi: 10.4236/oju.2016.65015