Case Reports in Urology
Volume 2016 (2016), Article ID 2426874, 8 pages
http://dx.doi.org/10.1155/2016/2426874

Case Report

Solitary Fibrous Tumor of the Kidney Developing Local Recurrence

Wataru Usuba,¹ Hideo Sasaki,¹ Hidekazu Yoshie,¹ Kazuki Kitajima,¹ Hiroya Kudo,¹ Ryuto Nakazawa,¹ Yuichi Sato,¹ Masayuki Takagi,² and Tatsuya Chikaraishi¹

¹Department of Urology, St. Marianna University, 2-16-1 Sugao, Miyamae, Kawasaki 216-8511, Japan
²Department of Pathology, St. Marianna University, 2-16-1 Sugao, Miyamae, Kawasaki 216-8511, Japan

Received 21 January 2016; Revised 30 March 2016; Accepted 7 April 2016

Academic Editor: Phillip M. Pierorazio

Copyright © 2016 Wataru Usuba et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Solitary fibrous tumor (SFT) of the kidney is a rare entity and usually displays a favorable prognosis. We herein report a second case of renal SFT developing local recurrence. A 50-year-old man was referred to our hospital because of a left renal mass. An abdominal CT detected a large renal tumor and radical nephrectomy was performed with a possible diagnosis of renal cell carcinoma. The resected tumor size was measured at 17 × 11 × 8 cm. Grossly, necrosis was observed in central lesion of the tumor but hemorrhage was not observed. Microscopically, the tumor consisted of spindle-shaped cells with scant cytoplasm accompanied by hyalinized collagenous tissue, which displayed hemangiopericytomatous patterns. The cellularity was normal and nuclear pleomorphism was not observed. Ki-67 labeling index was less than 3%. The pathological diagnosis of SFT was made without obvious malignant findings. Three years after the surgery, a follow-up CT scan detected a mass lesion in the tumor bed. Surgical resection was performed and the resected tumor was compatible with local recurrence of the SFT without obvious malignant findings. Renal SFT should be carefully monitored even in the absence of obvious malignant findings.

1. Introduction

Solitary fibrous tumor (SFT) is a clinical entity that was first reported as a tumor of the pleura in 1931 and usually arises in the pleura [1]. SFT is a rare spindle cell neoplasm and it is postulated that the tumor originated from mesenchymal tissue [2]. Histologically SFT shows hemangiopericytoma-like growth pattern and immunohistochemical staining for CD-34 and Bcl-2 is helpful for diagnosing the SFT. SFT typically is strong and diffusely positive for CD-34 and 70% of the SFT is positive for Bcl-2 [3]. The disease commonly arises from the thoracic cavity, yet it may arise from other sites including the kidney [2]. SFT of the kidney is an extremely rare and generally indolent tumor, unlikely to recur locally or distantly. Up to the present, only 81 cases of occurring renal SFT have been reported. SFT of the kidney usually displays a favorable prognosis and only two cases were reported to develop a distant metastasis. Furthermore local recurrence of SFT of the kidney had been reported in only one case [4]. Herein, we describe the second case of local recurrence of renal SFT after radical.

2. Case Presentation

A 50-year-old male was referred to our hospital because of a left renal mass, which had been incidentally detected by ultrasonography performed in a routine health check-up. A physical examination and blood chemical analysis were normal. Subsequent computed tomography (CT) scan detected a well-enhanced large left renal tumor (Figure 1(a)). He was diagnosed with left renal cell carcinoma preoperatively, and radical nephrectomy was performed. Grossly, the tumor was measured at 17 × 11 × 8 cm, was well-circumscribed, and displayed necrosis with a gray-white cut surface. Hemorrhage was not observed. Microscopically, the tumor was composed of spindle-shaped cells, which displayed hemangiopericytomatous patterns (Figure 2(a)). The tumor displayed normal cellularity without nuclear pleomorphism. Mitotic count was less than 1 per 10 high power fields. Immunohistochemical staining was positive for CD-34 (Figure 2(b)), Bcl-2 (Figure 2(c)), CD-99, and STAT-6, all of them representing conventional immunohistochemical markers for SFT. Meanwhile, SMA stain was negative and Ki-67 labeling index was less than 3% (Figure 2(d)). Thus, he was histologically diagnosed with SFT of the kidney without obvious malignant findings. Postoperatively, follow-up CT examination was performed regularly every 3-4 months. Three years after the operation, a mass lesion was detected in the tumor bed (Figure 1(b)). The mass lesion was increased in size after 3 months (Figure 1(c)). Fluorodeoxyglucose (FDG) positron emission tomography (PET) was ordered but the tumor did not accumulate FDG (Figure 1(d)). Nonetheless, as a local recurrence or lymph node metastasis could not be denied, we planned a surgical removal of the tumor. Although the recurrent tumor displayed spindle-shaped cells with hemangiopericytomatous patterns as in the original tumor, the cellularity was increased and cytological atypia was observed (Figure 2(e)). These results suggested an increased malignant potential of the tumor, but mitotic count was less than 4 mitoses per 10 high power fields. Immunohistochemical staining for CD-34 (Figure 2(f)), Bcl-2 (Figure 2(g)), and CD-99 all remained positive. Ki-67 labeling index was less than 15% (Figure 2(h)) and SMA stain was positive in the resected tissue from the tumor bed. Although an increased malignant potential was suggested, pathological findings did not meet the diagnostic criteria of malignant SFT [5]. The recurrent tumor was developed from an extra nodal connective tissue not from the lymph node (Figure 3). Therefore, we diagnosed local recurrence of renal SFT without evidence of obvious malignant findings. Twelve months after the second operation, the patient is followed up on the outpatient basis with no evidence of local recurrence or distant metastasis.

(a)

(b)

(c)

(d)

Figure 1: Radiological findings of the renal SFT. Enhanced abdominal CT revealed 17 × 11 × 8 cm tumor located in the left kidney (a). Follow-up plain CT revealed suspicions of recurrent tumor (1 × 0.7 cm) in the tumor bed at 3 years after the nephrectomy (b). Three months after the CT, which detected suspicions of recurrent tumor, follow-up CT scan and PET-CT were performed. (c, d) The mass lesion was increased in size (1.7 × 1.1 cm).

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Figure 2: Histological findings of the solitary fibrous tumor. The primary tumor displayed hemangiopericytomatous patterns ((a), HE ×40). Immunohistochemical staining of the primary tumor was positive for CD-34 and Bcl-2 ((b) and (c), ×40) and Ki-67 labeling index was less than 3% ((d), ×40). Cellularity was increased in the tumor that recurred at the hilar portion of the kidney (e). Immunohistochemical staining for CD-34 and Bcl-2 was positive ((f) and (g), ×40). Ki-67 labeling index was less than 15% ((h), ×40).

Figure 3: Histological finding of local recurrence of the solitary fibrous tumor. The recurrent tumor was developed from an extra nodal connective tissue (×10). L: lymph node; T: tumor.

3. Discussion

In 1931, SFT was firstly reported as a tumor of the pleura [1]. It is a rare tumor comprising spindle-shaped cells, which might originate from mesenchymal tissue [2]. Although SFT is commonly thought of as an intrathoracic tumor, it could arise from extrathoracic organs, including the kidney [2]. Surgical resection is a standard treatment and complete resection can be associated with a favorable prognosis, even if the SFT is histologically diagnosed as malignant [4, 6].

SFT of the kidneys is a rare neoplasm, and Sasaki et al. reviewed the 68 cases of SFT in 2013 [7], and additional 13 cases were reported up to now. All reported cases, including our case, are summarized in Table 1. Most of the tumors were incidentally found with no apparent clinical symptoms. Preoperatively, most of them were diagnosed as renal cell carcinoma, and 72 out of 82 cases underwent radical nephrectomy. Mean age at diagnosis was (3–85) years and mean tumor size was (2–29) cm. Histologically, 68 tumors showed a benign appearance, whereas 11 cases exhibited a malignant one. Most patients displayed a favorable prognosis with no evidence of recurrence during the follow-up period, ranging from 0.1 to 96 months. Only 4 patients experienced recurrence; 2 patients developed distant metastasis; and 2 patients, including the present case, developed local recurrence.

Table 1: Clinicopathological findings of renal solitary fibrous tumors in the literature.

Case
Year
Age
Sex
Symptom
Side
Affected site
Tumor size
(cm)
Treatment
Histology
Follow-up (month)
Outcome
CD-34
Authors and journals

1
1996
48
M
Back pain and macrohematuria
R
Renal capsule
3
Nephrectomy
BEN
0.1
DNOD
POS
Gelb et al. Am J Surg Pathol 20:1288

2
1996
45
F
Incidental
R
Kidney
6
Nephrectomy
BEN
8
NED
POS (2/3)
Fain et al. J Urol Pathol 4:227

3
1996
46
F
Incidental
R
Kidney
7.2
Nephrectomy
BEN
33
NED
POS (2/3)
Fain et al. J Urol Pathol 4:227

4
1996
51
M
Incidental
L
Kidney
4.5
Nephrectomy
BEN
2
NED
POS (2/3)
Fain et al. J Urol Pathol 4:227

5
1997
33
F
Abdominal pain
R
Peripelvis
3.5
Nephrectomy
BEN
89
NED
POS
Fukunaga et al. Histopathology 30:451

6
1997
36
F
Abdominal pain
L
Peripelvis
2
Nephrectomy
BEN
12
NED
POS
Fukunaga et al. Histopathology 30:451

7
1998
59
M
Incidental
L
Renal capsule
NA
Nephrectomy
BEN
NA
NA
POS
Ookouci S et al. Jpn J Radiol 58:539

8
1998
57
M
Incidental
L
Kidney
7
Tumorectomy
BEN
NA
NA
POS
Tanahashi C et al. Proc Jpn Soc Pathol 87:510

9
1999
64
M
Macrohematuria
R
Kidney
4.5
Nephrectomy
BEN
8
NED
POS
Hasegawa et al. Hum Pathol 30:1464

10
1999
71
F
Incidental
L
Kidney
9
Nephrectomy
BEN
NA
NA
NA
Kojima K et al. Jap-Deu Med Beriche 44:185

11
2000
66
F
Abdominal pain and macrohematuria
R
Kidney
9
Nephrectomy
BEN
9
NED
POS
Leroy et al. Urol Int 65:49

12
2000
72
F
NA
L
Kidney
8
Nephrectomy
BEN
10
NED
POS
Morimitsu et al. APMIS 108:617

13
2000
56
F
Incidental
L
Renal capsule
5
Tumor resection
BEN
NA
NA
NA
Ikeda A et al. J Hiroshima Med Assoc 53:640

14
2001
70
M
Incidental
R
Renal pelvis
6
Nephrectomy
BEN
60
NED
POS
Yazaki et al. Int J Urol 8:504

15
2001
28
F
Abdominal pain
L
Kidney
15
Nephrectomy
BEN
12
NED
POS
Cortes-Gutierrez et al. J Urol 166:60

16
2001
41
M
Macrohematuria
L
Kidney
14
Nephrectomy
BEN
48
NED
POS
Wang J et al. Am J Surg Pathol 25:1194

17
2001
72
M
Abdominal discomfort
R
Kidney
13
Nephrectomy
BEN
5
NED
POS
Wang J et al. Am J Surg Pathol 25:1194

18
2002
57
M
Incidental
L
Kidney
6
Nephrectomy
BEN
NA
NA
POS
Miyazaki N et al. Jpn Red Cross Med J 54:182

19
2002
58
M
Incidental
L
Kidney
NA
Nephrectomy
BEN
9
NED
NA
Inokawa E J Hiroshima Med Assoc 55:1057

20
2002
31
F
Flank pain
R
Kidney
8.6
Nephrectomy
BEN
8
NED
POS
Magro G Pathol Res Pract 198:37

21
2003
64
F
Microhematuria
R
Kidney
4
Nephrectomy
BEN
7
NED
POS
Li S et al. Hinyokika Kiyo 49:121

22
2003
51
F
NA
R/L
Kidney
25 and 2
Tumor resection
BEN
NA
NA
NA
Llarena Ibarguren et al. Arch Esp Urol 56:835

23
2003
35
M
NA
R
Kidney
17
Nephrectomy
BEN
6
NED
NA
Durand X et al. Prog Urol 13:491

24
2003
60
F
NA
R
Kidney
11
Nephrectomy
BEN
48
NED
NA
Bugel H et al. Prog Urol 13:1397

25
2004
67
M
Incidental
L
Kidney
4.5
Tumorectomy
BEN
5
NED
POS
Toriyama S et al. Hinyokika Kiyo　50:138

26
2004
83
M
NA
R
Kidney
9
Nephrectomy
BEN
18
NED
POS
Gres P et al. Prog Urol 14:65

27
2004
53
M
Flank pain and swelling
R
Renal capsule
14
Tumor resection
BEN
36
DNOD
POS
Kunieda K et al. Surg Today 34:90

28
2004
59
M
Incidental
L
Renal capsule
6.8
Nephrectomy
BEN
48
NED
POS
Yamada H et al. Pathol Int 54:914

29
2005
29
NA
Incidental
NA
Kidney
2.2
Nephrectomy
BEN
NA
NA
POS
Pierson DM et al. Mod Pathol 18:159

30
2005
NA
NA
Incidental
NA
Kidney
NA
Nephrectomy
BEN
NA
NA
POS
Pierson DM et al. Mod Pathol 18:160

31
2005
NA
NA
Incidental
NA
Kidney
NA
Nephrectomy
BEN
NA
NA
POS
Pierson DM et al. Mod Pathol 18:161

32
2005
NA
NA
Incidental
NA
Kidney
NA
Nephrectomy
BEN
NA
NA
POS
Pierson DM et al. Mod Pathol 18:162

33
2005
NA
NA
Incidental
NA
Kidney
NA
Nephrectomy
BEN
NA
NA
POS
Pierson DM et al. Mod Pathol 18:163

34
2005
NA
NA
Flank pain
NA
Kidney
NA
Nephrectomy
BEN
NA
NA
POS
Pierson DM et al. Mod Pathol 18:164

35
2005
79
NA
Flank pain
NA
Perirenal
10.1
Nephrectomy
BEN
NA
NA
POS
Pierson DM et al. Mod Pathol 18:165

36
2005
51
F
Flank pain
NA
Renal capsule
10
Nephrectomy
BEN
NA
NA
POS
Yamaguchi T Urology 65:175

37
2005
51
F
Fever elevation
R
Renal capsule
13
Nephrectomy
BEN
NA
NA
POS (focal)
Jhonson TR et al. J Comput Assist Tomogr 29:481

38
2005
83
F
Incidental
L
Kidney
11
Nephrectomy
BEN
NA
NA
POS
Kawagoe M Nishinihon J Urol 67:568

39
2006
76
M
Incidental
L
Kidney
12
Nephrectomy
MAL
4
Lung metastasis
POS (benign site)
Fine SW et al. Arch Pathol Lab Med 130:857

40
2006
18
F
Flank pain
L
Kidney
3
Nephrectomy
BEN
15
NED
POS
Koroku M et al. Hinyokika Kiyo 52:705

41
2006
4
M
NA
R
Kidney
8
Nephrectomy
BEN
NA
NA
NA
Provance et al. Clin Pediatr 45:871

42
2006
85
M
Flank pain
L
Kidney
4.5
Nephrectomy
BEN
NA
NA
POS
Kohl SK et al. Arch Pathol Lab Med 130:117

43
2006
54
M
Incidental
R
Kidney
NA
Nephrectomy
BEN
16
NED　
POS
Tanaka M et al. Hinyokika Kiyo 52:79

44
2006
36
M
Flank pain
R
Kidney
NA
Nephrectomy
BEN
NA
NA
NA
Alvarez Mugica M et al. Arch Esp Urol 59:195

45
2007
26
M
Incidental
R
Kidney
7
Nephrectomy
BEN
6
NED
POS
Constantinidis C et al. The Can J Urol 14:3583

46
2007
70
M
Flank pain and macrohematuria
L
Kidney
15
Nephrectomy
BEN
6
NED
POS
Znati K et al. Revies in Urol 9:36

47
2007
51
F
Flank pain
L
Kidney
4
Nephrectomy
BEN
10
NED
POS
Bozkurt SU et al. APMIS 115:259

48
2007
66
F
Abdominal mass and macrohematuria
R
Kidney
11
Nephrectomy
BEN
NA
NA
NA
Kakoi N et al. Japn J Urol Surg 20 supple 598

49
2007
60s
M
Incidental
R
Kidney
3
Nephrectomy
BEN
3
NED
NA
Yoshida T et al. Hinyokika Kiyo　53:745

50
2008
34
F
Flank pain
L
Kidney
9
Nephrectomy
MAL
21
NED
POS
Magro G et al. APMIS 115:1020

51
2008
67
M
Macrohematuria
L
Kidney
7
Nephrectomy
BEN
10
NED
POS
Amano T et al. Hinyokika Kiyo　54:357

52
2008
44
F
Incidental
L
Kidney
5.8
Nephrectomy
BEN
40
NED
POS
Hirabayashi J et al. Hinyokika Kiyo　54:357

53
2009
75
F
Incidental
L
Kidney
4.5
Nephrectomy
BEN
9
NED
POS
Hirano D et al. Mod Mol Morphol 42:239

54
2009
64
F
Cough
L
Kidney
2.5
Biopsy
BEN
12
NED
POS
Petrella F et al. Minerca Chir 64:669

55
2009
35
M
Incidental
R
Kidney
8
Partial nephrectomy
BEN
NA
NA
POS
Makris A et al. Can J Urol 16:4854

56
2009
72
F
Abdominal mass
L
Kidney
19
Nephrectomy
MAL
NA
NA
NA
Marzi M et al. Urologia 76:112

57
2009
76
F
Incidental
R
Kidney
2.5
Nephrectomy
BEN
48
NED
POS
Yoneyama T et al. Hinyokika Kiyo 55:479

58
2009
50
M
Incidental
L
Kidney
5.5
Nephrectomy
BEN
NA
NED
POS
Matsumoto T et al. Japn J Urol Surg 22:230

59
2009
63
M
Incidental
L
Kidney
5.3
Nephrectomy
MAL
NA
NA
POS
Murayama S et al. Japn J Urol Surg 22:230

60
2009
51
F
Incidental
R
Kidney
12
Nephrectomy
BEN
NA
NA
POS
Ogushi S et al. Japn J Urol Surg 22:230

61
2009
75
M
NA
L
Kidney
3
Nephroureterectomy
BEN
NA
NA
POS
Kobori Y et al. Hinyokika Kiyo 55:305

62
2010
39
M
Dysuria
L
Kidney
25
Nephrectomy
BEN
12
NED
POS
Taza L et al. Actas Urol Esp 34:568

63
2010
39
F
Abdominal fullness
L
Kidney
20
Embolization and nephrectomy
BEN
6
NED
POS
Yamaguchi Y et al. Hinyokika Kiyo 56:435

64
2011
44
M
Macrohematuria
L
Kidney
NA
Embolization and nephrectomy
BEN
NA
NA
NA
Saegusa M et al. Nishinihon J Urol 68:187

65
2011
52
F
Abdominal pain
R
Kidney
18
Nephrectomy and thrombectomy
BEN
6
NED
POS
Naveen HN et al. Urol Ann 3:158

66
2011
72
F
Abdominal mass
L
Kidney
19
Nephrectomy
MAL
15
NED
POS (focal)
Marzi M et al. Minerva Urol Nephrol 63:109

67
2011
50
F
Flank pain
R
Kidney
15
Nephrectomy
MAL
30
NED
POS
Tsan-Yu Hsieh Diag Pathol 6:96

68
2012
68
F
Flank pain
NA
Kidney
NA
Nephrectomy
MAL
NA
NA
POS
M. de Martino Aktuel Urol 2012; 43(01):59–62

69
2012
72
M
Flank pain
L
Kidney
7
Nephrectomy
MAL
45
NED
POS
Sfoungaristos S Prague Med Rep/Vol 113 No. 3, 246–250

70
2012
56
M
Shortness of breath
L
Kidney
10, 10
Nephrectomy
MAL
10
NED
POS
G. Zhao et al. Oncology Letters 4:993–995, 2012

71
2013
49
F
Dyspnea
L
Kidney
NA
Nephrectomy
BEN
23
SD
POS
J. Cuello et al. Case Rep Oncol Med 2013; 2013:564980

72
2013
48
M
Abdominal mass
R
Kidney
29
Nephrectomy
BEN
96
NED
POS (55%)
Sasaki H et al. Case Rep Nephrol Urol 3:1–8

73
2013
57
M
Lumbar pain
L
Kidney
14
Nephrectomy
BEN
26
NED
POS
Abdullah D et al. Case Report in Urol 147496:4

74
2013
3
M
NA
NA
Kidney
NA
Nephrectomy
NA
NA
NA
NA
Wu WW et al. Int J Surg Pathol 23(1):34–47

75
2013
49
F
Fever elevation and flank pain
R
Kidney
5
Nephrectomy
BEN
NA
NED
POS
Nazih K et al. Urol Int 2013; 91:373–383

76
2013
43
M
Acute recurrent pancreatitis
NA
Kidney
NA
NA
NA
NA
NA
NA
Patel YA et al. Pancreatology 13(6):631–3

77
2013
30
F
NA
NA
Renal pelvis
NA
Nephrectomy
BEN
NA
NA
NA
Pathak TB et al. JNMA Apr-Jun; 52(190):388–90

78
2014
66
F
Flank mass
R
Kidney
26
Nephrectomy
MAL
9
NED
POS > NEG
Wang et al. Diagnostic Pathol 9:13

79
2014
19
F
Hematuria
L
Kidney
14.5
Embolization and nephrectomy
MAL
30
NED
POS
Ettore M et al. Onco Targets and Therapy Jul 679–685

80
2014
35
F
Back pain
L
Kidney
3
Nephrectomy
BEN
15
NED
POS
Jie Ma et al. Int J Clin Exp Pathol 7(7):4268–4237

81
2014
55
NA
NA
NA
Kidney
NA
Nephrectomy
NA
NA
NA
NA
Tritschler P et al. JBR-BTR Sep-Oct; 97(5):298–300

82
our case
50
M
Incidental
L
Kidney
17
Nephrectomy
BEN
36
LR
POS

M, male; F, female; NA, not available; R, right; L, left; BE, benign; MAL, malignant; DNOD, died not of disease; NED, no evidence of disease; SD, stable disease; LR, local recurrence; POS, positive.

CD-34 immunoreactivity (the extent of positive area is shown in parentheses, if information is available).

As SFT commonly expresses CD-34, Bcl-2, and CD-99 [8], these surface antigens can serve as useful diagnostic markers [8]. And negativity in CD-34 and Bcl-2 reportedly represents increased malignant potential [8, 9]. Fine et al. documented a case of malignant renal SFT without expressing CD-34, which developed distant metastasis four months after surgery [10]. We also reported a similar case previously, which did not express CD-34 and went on to metastasize to the lung and liver [7]. In that case, half of the cross section area of the primary tumor was positive for CD-34, while the remaining area was negative for it. The patient developed distant metastases 8 years after nephrectomy. Resection of the metastatic tumors had revealed that CD-34 was totally absent in the tumors. Thus, the loss of CD-34 staining in SFT of the kidney may promote tumor metastasis to other organs [7]. Similarly to CD-34 staining, Bcl-2 staining was commonly observed in SFT and the loss of Bcl-2 staining was reported to be associated with malignant potential in retroperitoneal SFTs [9].

On the contrary, malignant potential is rather low in the present case, which developed local recurrence 3 years after nephrectomy. In this case, no obvious malignant findings were observed in either primary or recurrent tissue from the tumor bed. Furthermore, CD-34 and Bcl-2 were positive in the primary tumors and remained positive in the recurrent tissue. It seems that the local recurrence does not necessarily accompany the loss of expression of CD-34 and Bcl-2, and another explanation for unpredicted local recurrence would be incomplete resection at surgery [5]. However, from a different standpoint, the tumor in the present case may have had a great tendency to local recurrence, as the tumor accompanies multiple clinical features such as extrathoracic location, large tumor size, increased cellularity, and presence of necrosis among the risk factors for local recurrence described by Jason et al. [11].

Overall, we believe that there is no strict dichotomy between benign and malignant SFTs and that all tumors likely have some degree of metastatic potential, albeit quite low. Therefore, although renal SFT is thought to be a benign tumor, an adequate follow-up period is required to evaluate the precise clinical outcome of renal SFT, and the follow-up period in this report of 82 patients may not be sufficient (Table 1). Furthermore, most reported renal SFTs were large in size at the diagnosis and it might be leading cause of missing the malignant features in whole tumor tissue. We should also concern this issue for evaluating the real feature of renal SFTs in future.

FDG accumulation was not observed within the tumor on FDG-PET. To date, there is no reported association between SFTs and FDG accumulation, and our result suggests that PET-CT may be invalid. Further detailed examination is also required to clarify this point.

In conclusion, a case of SFT of the kidney exhibiting local recurrence was reported. In our case, no obvious malignant findings were observed in either the primary tumor or the recurrent tumor. Loss of expression in CD-34 and Bcl-2, which is closely associated with malignant potential, was not observed. Although SFT of the kidney usually displays a favorable clinical course, careful and sufficient follow-up may be required even in the absence of malignant findings.

Consent

The patient described in the case report has given his informed consent for the case report to be published.

Competing Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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