Malignant Solitary Fibrous Tumor of the Kidney: Report of the First Case Managed with Interferon

Solitary fibrous tumors of the kidney are extremely rare tumors with unpredictable behavior. We describe a case of a patient with a solitary fibrous tumor of kidney with malignant findings with distant metastasis and nephrectomy managed with subcutaneous interferon achieving 23 months of progression-free survival. To date there is no prospective evaluation of any specific modality of treatment, but the surgical management and long-term followup are the only ones so far recommended strategies in the management of these patients. Studies are awaited with more patients to evaluate the different strategies of systemic therapy reported so far to allow adding survival benefit.


Introduction
Solitary �brous tumors are rare mesenchymal neoplasms, considered a variant of hemangiopericytomas usually originates in the pleura; however, there have been reports of extrapleural origin (abdomen, retroperitoneum, upper extremities, orbit, cervix, meninges, mediastinum, parotid, nasal cavity, neck, etc.) [1][2][3][4]. e location is even more rare urogenital, and according to the literature, only 49 cases of solitary �brous tumor of the kidney have been reported in the literature [5]. e origin of the majority of cases formed in the kidney is found in the renal capsule tissue or connective tissue interstitial peripelvis. Most cases presented with suspected renal cell neoplasms; however, morphologically, solitary �brous tumors are characteri�ed by the proliferation of spindle cells with little pattern in their architecture, and the �nal diagnosis was made with immunohistochemical �ndings that show staining for CD34 [6][7][8][9].

Case Report
A 49 year old woman with no history of importance who enters the emergency room by 2-months of dyspnea at rest, associated with pleuritic chest pain on right chest, dry cough, no fever. e chest radiograph showed a right pleural effusion and multiple nodular lesions on the pleura, so we decided to perform a CT chest and abdomen in which evidenced free right pleural effusion occupying 80% of the right chest, at least two pleural masses with solid density that capture the contrast, multiple pulmonary nodules with so tissue density in both lungs and le kidney mass. Carried le nephrectomy with suspected metastatic renal carcinoma, shows renal mass plus liver metastases which were resected. e analysis of the tumor presented as a �rst option versus solitary �brous tumor angiomyolipoma. Immunohistochemical studies that showed cell reactivity with CD34, CD99, BCL-2, and vimentin are negative for HMB-45, AMS, CD68, cytokeratin cocktail, and S100, and the Ki67 is not assessable (Figure 1). is pro�le supports the histological diagnosis of solitary �brous tumor with origin in the kidney.
We decided to start treatment with interferon a2b subcutaneous dose, extrapolating the cases of patients with solitary �brous tumor of the pleura [12]. e dose was adjusted for �u symptoms and she is with stable disease at 23 months follow-up.

Discussion
Solitary �brous tumors are e�tremely rare tumors, arising mostly at the level of the pleura, and cases arising from the urogenital region are even more rare, with 49 cases reported so far in the literature. e histogenesis of this entity is still unknown, but recent studies suggest a primitive mesenchymal cells or level perivascular [10,11].
e differential diagnosis of these cases includes sarcomatoid variant of renal carcinoma, angiomyolipoma, �bromas, and �brosarcomas. Table 1 presents the cases so far published, reporting the primary source, and histologic variant outcomes.
ese results show a relatively rare entity, with peak presentation in the ��h decade of life, arising mostly in the renal parenchyma unilaterally ( Table 2).
About 14% of patients have aggressive behavior where common management strategy is nephrectomy with complete resection of the lesions. e pathological �ndings that have been correlated with aggressive behavior are pleomorphism, increased cellularity and mitotic activity (>4 mitosis/10 high-power �elds), necrosis, hemorrhage, and atypical sites (parietal pleura, lung parenchyma) [13]. However, even the clinical behavior can not accurately predict the histopathological �ndings, as some cases with results suggesting benign disease may show aggressive behavior and vice versa, so it is necessary that these patients have an inde�nite period of observation [14][15][16][17][18][19].
In relation to the management of this condition, there is as yet no prospective evaluation of any speci�c treatment modality; however, case reports and retrospective case series suggest that complete surgical resection and long-term followup are generally most recommended strategies. In cases such as the present, which present with metastatic disease, there is no clearly de�ned systemic therapy. Metastasectomy is thought to improve progression-free survival, but in many cases like this, this strategy is not feasible.
In case reports hemangiopericytomas, entity closely related to solitary �brous tumors, has achieved stable disease with the use of interferon with or without thalidomide. So far, this is the �rst case reported in the literature in which bene�t is demonstrated with the use of interferon in a patient with a malignant variant renal solitary �brous tumor with metastatic disease, achieving stable disease for about 20 months. Some authors suggest the use of antiangiogenic therapies (bevacizumab, sunitinib, pazopanib, etc.), based on the �ndings of high vascularity and a possible origin of pericytes at this entity [15]. e combination of bevacizumab associated with temozolomide is a potentially promising scheme for patients with solitary �brous tumors. A series of 14 patients with solitary �brous tumor unresectable or metastatic, were treated with temozolomide 150 mg/m2 orally on days 1-7 and days 15-21 and bevacizumab 5 mg/kg intravenously on days 8 and 22, with cycles every 28 days. In this study, 11 patients (79%) achieved partial response assessed by Choi criteria with 2 cases (14%) with stable disease. e median progression-free survival was 8.6 months [12,19,20].