More than 90% of masses in the kidney are renal cell carcinomas [4]. Renal sarcomas are rare neoplasm, accounting for < 1% of masses [5]. Ewing's sarcoma of the kidney is an extremely rare but highly aggressive neoplasm with a rapid increase in lesion size and metastasis, as seen in our case [6].
Ewing's sarcoma of the kidney is seen in the young age group (mean age of 28–34 years) [7]. They are initially asymptomatic and when large enough, usually present with symptoms like flank pain and hematuria [8].
No specific imaging features are seen in imaging and it is impossible to differentiate renal Ewing's from much more prevalent RCC by imaging alone. Ultrasound shows an ill-defined hypo to isoechoic mass lesion, with minimal or no internal vascularity. CECT shows a hypo-dense, hypo-enhancing (as compared to the normal renal parenchyma) mass [7]. Calcification is found only in approximately 10% of tumors and usually appears faint and amorphous. On magnetic resonance imaging (MRI), these tumors are classically iso to hyperintense to skeletal muscle on T1-weighted images and hyperintense on T2-weighted images. Although smaller tumors appear homogeneous, larger tumors typically show heterogeneity owing to internal hemorrhage and necrosis. They usually do not cross the midline [1], but have a strong tendency to extend into perinephric and renal sinus fat, cause lymphovascular invasion, and distant metastases to lungs and bones, similar to RCC [8].
On imaging, differential diagnosis of solid, aggressive, primary renal tumor includes RCC, transitional cell carcinoma, lymphoma, and other mesenchymal malignancies like osteosarcoma, rhabdomyosarcoma, leiomyosarcoma, etc. RCC and sarcomas can have a similar heterogeneous appearance with areas of necrosis, hemorrhage, and calcification. Although bulk fat is common in angiomyolipoma, fat-containing RCCs are also common. Renal osteosarcomas have areas of calcification or ossification within [9]. Transitional cell carcinoma maintains the renal shape and involves the pelvicalyceal system. Lymphoma typically shows homogeneous enhancement and calcification in untreated lymphoma is extremely rare [10].
Primary renal sarcomas are diagnosed on imaging only after exclusion of renal metastatic involvement from a primary sarcoma, secondary renal involvement from a retroperitoneal sarcoma, and ruling out sarcomatoid renal cell carcinoma [9]. However, definitive diagnosis of renal Ewing's is solely by post or pre-operative biopsy. Light microscopy shows the presence of small round blue cells, with high nucleo-cytoplasmic arranged in sheets and can form Homer-Wright Rosettes. The tumor cells are positive for neuro-ectodermal IHC markers like Neuron-specific enolase and Synaptophysin. Demonstration of reciprocal translocation t(11;22)(q24;q12) by cytogenetic studies, is considered to be specific to PNET and Ewing's sarcoma [11]. Unfortunately, karyotyping was not performed in our case as it was not available in our institution.
There is no consensus regarding the treatment of renal Ewing's sarcoma. Most cases have been treated with surgical resection and adjuvant chemotherapy. The effective chemotherapeutic agents are vincristine, doxorubicin, ifosfamide, etoposide, actinomycin D, and cyclophosphamide [12]. Molecular targeted therapy including insulin-like growth factor 1 receptor antibody has shown some promise in ESFT [1]. Postoperative radiotherapy must be added in the case of inadequate surgical margins. Even with aggressive treatment, the prognosis and the survival rate of this tumor are dismal, with the median survival being 15 months [13].
