A rare pericytic tumor of the kidney: The first case in Korea

Highlights • The family of pericytic tumors includes glomus tumors and variants, myopericytoma including myofibroma, and angioleiomyoma.• The renal pericytic tumor is extremely rare, and only few comprehensive discussions about this entity have been done.• We report the first documented case of renal pericytic tumor in a 58-year-old Korean male.


Introduction
Pericytic (perivascular) tumors are distinct mesenchymal neoplasms that rarely involve the kidney. Instead, pericytic tumors are morphologically related to the differentiation of perivascular myoid cells which invest blood vessels and function physiologically in vascular modification and thermoregulation. These include myopericytoma, myofibroma, angioleiomyoma, glomus tumors and variants [1].
The latest WHO classification of kidney tumors [2] does not address the pericytic tumor since the tumor has been exceptionally rare and has been limited in experiences for the risk prediction of clinical behaviors. Herein, we describe the first case of pericytic tumor of the kidney, considering its uncertain malignant potential, in a Korean male, and we offer a brief comment. This work has been reported in line with the SCARE criteria [3].

Case presentation
A 58-year-old man underwent abdominal ultrasonography for a health screening at a local clinic. The patient was transferred to our institute with suspicions of renal cell carcinoma. The kidney dynamic computed tomography scan showed a 3 cm sized solid mass in the upper pole of the right kidney ( Fig. 1A  serum creatinine was within normal range. Laparoscopic radical nephrectomy was performed due to the deep-seated mass.

Pathological findings
On gross examination, there was a well-defined, pale-tancolored, round and solid mass in the upper portion of the right kidney measuring 3.2 × 2.2 cm ( Fig. 2A). There was neither hemorrhage nor necrosis on the cut surface. The tumor was confined to the kidney; there was no invasion into the renal capsule.

Discussion
The pericytic (perivascular) tumors are mesenchymal neoplasms rarely encountered in visceral sites, but are considered to be same entities to their counterparts in skin and soft tissue because of morphological similarities [1]. The World Health Organization  (WHO) classification of tumors of soft tissue defines a group of pericytic (perivascular) tumors including glomus tumors and variants, myopericytoma, myofibroma, and angioleiomyoma [4]; however, the latest WHO classification of kidney tumors does not list pericytic tumors since those tumors were exceptionally rare in the kidney [2].
The current case considering the epithelioid morphology with characteristic vascularity should be differentiated from perivascular epithelioid cell tumor (PEComa), glomus tumor, juxtaglomerular cell tumor (JGCT), epithelioid solitary fibrous tumor (SFT), carcinoid tumor, and clear cell renal cell carcinoma (CCRCC). PEComas are often composed of triphasic components of angiomyolipoma, and they are immunopositive for melanocytic makers, such as HMB-45 and Melan-A. Glomus tumors consist of more rounded small tumor cells, but relatively lack the concentric orientation of tumor cells around vessels [5]. JGCTs usually occur in adolescents and young adults, and present with hypertension correlated to plasma renin activity [1]. The tumors demonstrate nested small round cells in vascular backgrounds with pathognomic rhomboid-shaped renin crystals on ultrastructural examination, and immunoreactivities for CD34 and CD117 [1]. SFTs are considered to be fibroblastic/myofibroblastic in origin, and are recently characterized of STAT6 immunopositivity resulting in NAB2/STAT6 in tumorigenesis [6,7]. Carcinoid tumors show nested epithelioid histology with immunopositivities for neuroendocrine markers. CCRCC with low histologic grade can be separated by morphologic features and immunohistochemical stains, e.g. cytokeratin, EMA, CD10, and carbonic anhydrase IX etc [1,8].
Although general biological behavior of published renal pericytic tumors is likely to be benign, the clinicopathologic experiences are very limited. Therefore, we should evaluate the malignant potential of the entity according to the parameters proposed for soft tissue tumors, including tumor location, tumor size, growth pattern, cellularity, cytological atypia, and mitotic figures with atypical forms. The current case shows several worrisome features, including an extremely rare tumor location, partially infiltrative growth, and a mildly increased proliferating index, which resulted in it being classified as an uncertain malignant potential.
In summary, we described the first case of renal pericytic tumor, addressing uncertain malignant potential, in a Korean male, which would be a distinct mesenchymal neoplasm differentiating from other groups of perivascular tumor families based on histological and immunohistochemical features.

Sources of funding
No sources to be stated.

Ethical approval
This study was approved by the Ethics Committee of the Kyungpook National University School of Medicine (IRB Number KNUH 2019-08-004).

Consent
Written informed consent was obtained from the patient for publication of this case report.

Author's contribution
Jae-Wook Chung: Surgeon of the patient's procedure described in the case report, concept and design of study.
Ghil Suk Yoon, Tae Gyun Kwon: Approving the final version of the manuscript.

Registration of research studies
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Provenance and peer review
Not commissioned, externally peer-reviewed.

Declaration of Competing Interest
There is no conflict to be declared.