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Abstract A 70-year-old Japanese man was referred because of a right renal mass of 2 years in duration. Imaging studies, including magnetic resonance imaging, revealed an ovoid mass, with relatively abundant vascularity, in the right renal pelvis. Right radical nephrectomy was done and a tumor measuring 6.0 × 4.5 × 4.0 cm was found in the renal pelvis. Solitary fibrous tumor (SFT) was highly suspected by histology. Immunohistochemical study using a monoclonal antibody directed against the human hematopoietic progenitor cell antigen (CD34) stain confirmed SFT. This is the first case of SFT of the renal pelvis.
Although SFT is extremely rare in urogenital organs, this tumor must be included in the differential diagnosis when we encounter urogenital tumors consisting of mesenchymal elements.
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- Case report
A 70-year-old Japanese man was referred because of a painless right renal tumor of 2 years duration. The tumor was incidentally found by echography at the time of physical work-up at the referral hospital. The patient was reluctant to undergo a thorough checkup because he felt no symptoms. Physical examination at the first visit to our department was non-contributory. Laboratory examinations of blood including C-reactive protein, immunosuppresive acidic protein (IAP) and urinalysis were non-contributory. Urinary cytology was negative for malignancy.
Computed tomography (CT) scan was done because renal cell carcinoma was suspected. The CT scan revealed a contrast-enhanced ovoid mass occupying the right renal pelvis and compressing the upper collecting system outwards. The characteristics of the renal mass could not be defined. Excretory urography (IVU) and magnetic resonance imaging (MRI) were done. The IVU revealed similar findings to that of the CT scan (Fig. 1). Magnetic resonance imaging revealed that the mass and renal parenchyma had similar MR signals in T1- and T2-weighted images (Fig. 2a,b). The mass was found to be separate from the renal parenchyma. Even with MRI the characteristics of the mass could not be defined. Selective right renal arteriography was done to define the vascularity of the mass. Relatively abundant tumor vessels were seen. Although the characteristics and accurate localization of the tumor could not be defined, malignancy was suspected due to the presence of relatively abundant vascularity. Thus, right radical nephrectomy was done. On the cut section, a yellowish-brown mass measuring 6.0 × 4.5 × 4.0 cm was seen originating from the renal pelvis (Fig. 3). Microscopically, the tumor was composed of spindle cells without an obvious growth pattern, so-called ‘patternless pattern’ (Fig. 4a). These tumor cells were devoid of necrosis or mitosis. In some areas, tumor cells were surrounded by collagenous stroma. The non-invasive tumor originating from the renal pelvis was separated from the renal parenchyma by fibrous and fatty tissues. Electron microscopy of the tumor revealed that the spindle cells had irregularly shaped nuclei, well-developed rough endoplasmic reticulum, and variable numbers of mitochondria with a background of collagenous stroma (Fig. 4b). Solitary fibrous tumor (SFT) of the renal pelvis was the most probable pathological diagnosis. Immunohistochemical staining of CD34 was positive in the tumor cells (Fig. 5). Tumor cells were positive for vimentin, but negative for actin. The results of these pathological examinations suggested SFT. The postoperative course was uneventful and the patient was followed up for approximately 5 years without recurrence of the tumor.
Figure 2. Magnetic resonance imaging of the mass and renal parenchyma showed a similar MR signal in (a) T1-weighted and (b) T2-weighted images.
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Figure 4. (a) The renal pelvic tumor showed a patternless arrangement of spindle cells in collagenous stroma (original magnification × 100; H&E). (b) Electron microscopy of the tumor revealed spindle cells with irregular nuclei, rough endoplasmic reticulum and variable numbers of mitochondria.
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- Case report
In this case, preoperative diagnosis by imaging studies revealed an ovoid mass occupying the right renal pelvis. The characteristics and accurate localization of the mass could not be defined. Postoperative study revealed a mass arising from renal pelvis and separated from renal parenchyma by fibrous and fatty tissue. Pathology suggested a mesenchymal tumor that was most probably SFT.
Solitary fibrous tumor is a rare but well-established neoplasm most frequently arising in the pleura. In recent years, however, the occurrence of such a tumor at sites other than the pleura has been described. Extrapleural tumors have occurred in the upper respiratory tract, lung, nasal cavity, paranasal sinuses, orbits, mediastinum, major salivary glands, breast, meninges, liver and urogenital organs.1 Only 10 cases of SFT of the urogenital organs have been reported. These include the kidney,2 renal capsule,3 renal peripelvis,4 bladder,1 prostate and periprostatic tissue1 and spermatic cord.5 There has been no report of SFT in the renal pelvis in English or Japanese-language published work.
Solitary fibrous tumor of unknown etiology is predominant in middle-aged and older patients (40–80 years) and more common in women. Tumors vary in size. Although most cases of SFT are benign, some have malignant potential. The incidence of malignant SFT in a large series of pleural SFT has been reported to be 12–23%.2 The clinical characteristics of SFT of urogenital organs are shown in Table 1. Characteristic findings are not seen in imaging studies including CT and MRI. Final diagnosis of SFT is by means of pathology. Histology and electron microscopy are useful.1–4,6 Typical microscopic findings are proliferation of ovoid to spindle cells with small or inconspicuous nucleoli, fine nuclear chromatin and indistinct cytoplasm, with a haphazard or patternless arrangement in a collagenous stroma. Electron microscopy is not necessarily indispensable because there are no characteristic findings. Electron microscopy reveals fibroblast-like tumor cells with rough endoplasmic reticulum and scattered mitochondria, embedded in a collagenous matrix.2 Electron microscopy is useful because it is able to define tumor cells with fibroblastic features rather than muscular components. Immunohistochemistry is another useful method.1–4,6 CD34 immunoreactivity has been reportedly shown to be strongly and diffusely expressed in many cases of SFT. CD34 immunoreactivity of the neoplastic cells is not specific for SFT but strong CD34 reactivity is currently regarded as characteristic and an indispensable finding in the diagnosis of SFT. Other immunohistochemistries for differential diagnosis of SFT include vimentin, keratin, actin, desmin and S-100 protein.1–4,6 These studies can differentiate cells with muscle component from cells without.
Table 1. Clinical characteristic of urogenital solitary fibrous tumor
|No. cases||Years||Authors||Age||Sex||Site||Chief complaints||Tumor size (cm)||Imaging studies||Treatment||Follow up (months)||Malignancy|
|1||1994||Fisher||46||Male||Spermatic cord||Groin mass||10||—||Resection||30||No|
|2||1996||Gelb||48||Female||RT renal capsule||Flank mass||3.0 × 2.5 × 1.5||US, CT||Rad Nx||Died of respiratory failure, 3POD||No|
|3||1996||Fain||45||Male||RT kidney||Abdominal pain||6.0 × 5.0 × 3.5||CT, MRI||Rad Nx||8||No|
|4||1996||Fain||46||Female||RT kidney||Abdominal pain||7.2 × 6.0 × 5.5||US, IVU, CT||Rad Nx||33||No|
|5||1996||Fain||51||Male||LT kidney||Chest pain||4.5 × 4.0 × 2.5||US, CT||Rad Nx||2||No|
|9||1997||Fukunaga||33||Female||RT renal peripelvis||Abdominal pain||3.0 × 2.5 × 2.5||CT, IVU, angiography||RT Nx||90||No|
|10||1997||Fukunaga||36||Female||LT renal peripelvis||Abdominal pain||2.0 × 1.5 × 1.5||CT, MRI||LT Nx||12||No|
|11||2000||Yazaki||70||Male||RT renal pelvis||Abdominal mass||6.0 × 4.5 × 4.0||CT, IVU, MRI, angiography||Rad Nx||60||No|
In this case, results of pathological and electron microscopic studies of renal pelvic tumor indicated mesenchymal tumor mostly compatible with that of SFT. Blastic changes were not seen and mitotic activities were low showing the mature and benign nature of this tumor. Subsequent immunohistological studies of CD34 revealed positive cells. The final diagnosis was SFT of renal pelvis.
Although SFT of urogenital organs is extremely rare, this tumor must be included in the differential diagnosis when we encounter tumors consisting of mesenchymal elements.
Urogenital mesenchymal tumors that should be differentiated from SFT include inflammatory myofibromatous tumor, sarcomatoid renal cell carcinoma, inflammatory fibrosarcoma, malignant fibrous histiocytoma, low-grade neurogenic tumor and angiomyolipoma in renal tumors,2 pseudosarcomatous fibromyxoid tumor, postoperative spindle cell nodule, inflammatory pseudotumor and pseudosarcoma, myxofibroblastic tumor in bladder and prostate tumors.1