Primary Renal Fibrosarcoma with Massive Tumor Size and Inferior Vena Cava Extension Mimicking Renal Cell Carcinoma: A Rare Case Report.

Primary renal fibrosarcoma is an exceedingly rare malignant mesenchymal tumor that accounts for only 1-3% of adult renal malignancies, often mistaken for sarcomatoid renal cell carcinoma (RCC) or leiomyosarcoma due to overlapping morphology. Thus, accurate disease diagnosis is crucial for its management. We report a case of a 49-year-old female, a chronic smoker, who presented with right flank pain and progressive abdominal swelling. Clinical examination revealed a firm mass in the right abdomen, and contrast-enhanced computed tomography demonstrated a large exophytic right renal mass with areas of necrosis, infiltration of the psoas muscle, and extension of tumor thrombus into the inferior vena cava (IVC). The patient underwent right radical nephrectomy with IVC thrombectomy, and a 20 × 15 cm irregular renal mass with 2 cm IVC thrombus was excised intraoperatively. Histopathology revealed a high-grade spindle cell neoplasm with interlacing fascicles and herringbone patterns, brisk mitoses (10-12 mitotic figures per 10HPF), and 20% necrosis. Immunohistochemistry showed diffuse vimentin positivity with negative staining for epithelial, myogenic, neural, and renal lineage markers, confirming the diagnosis of high-grade primary renal fibrosarcoma. This case is notable for being the largest renal fibrosarcoma reported to date, with rare IVC extension, features typically associated with advanced RCC rather than fibrosarcoma. Despite aggressive pathology, no metastases were identified, and the patient remained recurrence-free at 6 months postoperatively, with chemotherapy reserved for recurrence or metastasis. This report emphasizes the diagnostic challenges, surgical complexity, and clinical significance of primary renal fibrosarcoma and highlights the importance of including it in the differential diagnosis of large renal masses with vascular involvement.