Editors: Francesca Khani ( and Mahmut Akgul (

2022-03/January 17
Contributors: Anandi Lobo, Shivani Sharma, Sambit K Mohanty

A male patient in his 40s with 19 cm multiloculated cystic mass in his right kidney.


1. What is the correct diagnosis?

a. Synovial sarcoma

b. Cystic renal cell carcinoma

c. Solitary Fibrous Tumor

d. Sarcomatoid carcinoma


2. Which of the following parameters are included in the Modified Risk Stratification model for the prediction of metastatic risk in solitary fibrous tumors?

a. Tumor necrosis

b. Age of the patient

c. Tumor size

d. Mitotic count per 10 high power fields

e. All of the above

1. c; 2. e

Solitary Fibrous Tumor

A diagnosis of multiloculated and multicystic solitary fibrous tumor (SFT), intermediate risk class (score = 4) was rendered. SFT is a distinctive mesenchymal tumor with thin-walled vasculature and NAB2-STAT6 gene rearrangement. This can occur at any anatomic site including soft tissue and visceral locations; most commonly described as pleural based lesions. However, they can arise in any organ in the body with no gender predilection (1,2). Extrapleural locations reported include the upper respiratory tract, lung, orbit, paranasal sinuses, nasal cavity, mediastinum, breast, liver, retroperitoneum and urogenital organs1.According to the WHO classification, SFTs are considered tumors of intermediate malignancy with indolent behavior and infrequent metastatic potential (2).

SFT of the kidney is a rare neoplasm that has been described in patients ranging from 4 years to 85 years of age (3-9). SFT of the kidney was first defined by Gelb et al. in 1996 (4). Nearly 64 cases of SFT of the kidney have been reported so far in the literature (3-9). Among the tumors of renal origin, 15% were located in renal capsule, 6% in peripelvis, and 3% in renal pelvis, while 76% had an unknown site of origin (3-9). The tumor is usually incidentally detected and patients are usually asymptomatic or may present with abdominal pain and distension, secondary to a mass effect due to the size of the tumor or hematuria (3-9). Grossly, SFTs present as well-circumscribed, unencapsulated, and lobulated masses ranging from 2 cm to 25 cm in maximum dimension. The cut surfaces are usually solid, firm, and tan-white to yellow in appearance with foci of hemorrhagic, cystic, or myxoid degeneration. Although cystic SFTs have been reported in the orbit and pleura (10-12), a predominantly cystic SFT as a multiloculated and multicystic mass in the kidney has never been reported in the literature. Grossly, other cystic renal neoplasms such as clear cell renal cell carcinoma with cystic change, cystic nephroma, multilocular cystic renal neoplasm of low malignant potential, mixed epithelial and stromal tumor, tubulocystic carcinoma, and angiomyolipoma with epithelial cysts are considered as differentials.

SFT depict so called ‘patternless’ arrangement with haphazardly arranged spindle cells and varying amounts of collagen, with small- to medium-sized and thin-walled blood vessels having a staghorn appearance. Microscopic features associated with malignancy include nuclear atypia, increased cellularity, and greater than 4 mitoses/10 HPF. Traditionally, SFTs express CD34, CD99, BCL2, epithelial membrane antigen, and STAT6; the latter has emerged as a sensitive and specific marker which identifies the NAB2-STAT6 gene fusion product (1). A novel risk stratification scheme for non-meningeal SFT incorporating patient age, tumor size, and mitotic activity to predict risk of metastasis has been reported by Demicco EG et al. with a proposal to incorporate tumor necrosis as the fourth variable to improve the risk score (13).

The microscopic differential diagnosis of a spindle cell neoplasm in the kidney include renal cell carcinoma with sarcomatoid dedifferentiation (infiltrative atypical cells with at least focal renal cell carcinoma component; CD10+, PAX8+, RCC+/-, CAIX+/-), cellular mesoblastic nephroma [actin +, t(12;15)], monophasic synovial sarcoma (more infiltrative grossly and microscopically; atypical spindle cells, no dense collagen; CD99+, TLE1+, focal S-100+; CK+, t(X;18); CD 34±, STAT6-), leiomyosarcoma (usually no dense collagen; actin+, desmin+, h-caldesmon+), fibrosarcoma (herringbone and storiform pattern of spindle cells), nerve sheath tumors (morphology; SOX10+, S-100+, CD34+ in neurofibroma; STAT-), extragastrointestinal gastrointestinal stromal tumor (DOG1+, CD117+, CD34+), inflammatory myofibroblastic tumor (actin+, desmin+, ALK+), mixed epithelial and stromal tumor and adult cystic nephroma [endometrial type stroma (ER+/PgR+); actin+, desmin+, CD10+], and angiomyolipomas (co-expression of melanocytic [HMB45+, Melan A+, MITF+] and smooth muscle markers [actin+, calponin+, desmin+, h-caldesmon+]).

In conclusion, SFTs with predominantly cystic component are extremely rare renal neoplasms and a high index of suspicion is required to differentiate this entity from other cystic as well as spindle cell neoplasms of the kidney.

1. Ouladan S, Trautmann M, Orouji E, et al. Differential diagnosis of solitary fibrous tumors: A study of 454 soft tissue tumors indicating the diagnostic value of nuclear STAT6 relocation and ALDH1 expression combined with in situ proximal ligation assay. Int J Oncol. 2015;46(6):2595-605.

2. WHO Classification of Tumours, Soft Tissue and Bone Tumours (Beta Version). Available at: Accessed April 02, 2020.

3. Wang H, Qing L, Liao X, et al. Huge malignant solitary fibrous tumor of kidney: case report and review of literature. Diagnostic Pathology.2014;9(13).

4. Fursevich D, Derrick E, O’Dell MC, et al. Solitary Fibrous Tumor of the Kidney: A Case Report and Literature Review. Cureus. February 2016; 8(2): e490.

5. Gelb AB, Simmons ML, Weidner N. Solitary fibrous tumor involving the renal capsule. Am J Surg Pathol. 1996;20:1288-1295.

6. Demirtaş A, Sabur V, Akgün H, Akınsal EC, Demirci D. Solitary fibrous tumor of the kidney: a case report. Case Rep Urol. 2013;2013:147496. doi: 10.1155/2013/147496. Epub 2013 Apr 10.

7. Park SB, Park YS, Kim JK, Kim MH, Oh YT, Kim KA, Cho KS. Solitary fibrous tumor of the genitourinary tract. AJR Am J Roentgenol. 2011 Feb;196(2):W132-7.

8. Kouba E, Simper NB, Chen S, et al. Solitary fibrous tumour of the genitourinary tract: a clinicopathological study of 11 cases and their association with the NAB2-STAT6 fusion gene. J Clin Pathol. 2017 Jun;70(6):508-514.

9. Argani P, Kao YC, Zhang L, et al. BCOR Overexpression in Renal Malignant Solitary Fibrous Tumors: A Close Mimic of Clear Cell Sarcoma of Kidney. Am J Surg Pathol. 2019 Jun;43(6):773-782.

10. Feuerman JM, Flint A, Elner VM. Cystic Solitary Fibrous Tumor of the Orbit. Arch Opthalmol. 2010;128 (3):385-7.

11. Alam S, Backiavathy V, Mukherjee B, Subramanian K. A rare case of giant multicystic solitary fibrous tumor of the orbit. Orbit. 2018 Feb;37(1):69-72.

12. Watanabe T, Tanahashi M, Suzuki E, et al. Solitary fibrous tumor of the pleura with marked cystic degeneration: a case report. Surg Case Rep 6, 163 (2020).

13. Demicco EG, Wagner MJ, Maki RG, et al. Risk assessment in solitary fibrous tumors: validation and refinement of a risk stratification model. Mod Pathol. 2017 Oct;30(10):1433-42.

Anandi Lobo, Shivani Sharma, and Sambit K. Mohanty

Kapoor Path Labs, CORE Diagnostics, and Advanced Medical Research Institute, India


Solitary fibrous tumor; cystic tumor; spindle cell neoplasm; CD34; STAT6; kidney.