Sarcomatoid differentiation in renal cell carcinoma (sarcomatoid renal cell carcinoma – sRCC) represents a dedifferentiated high grade neoplasm of the kidney that contains carcinomatous and sarcomatous component. sRCC is an uncommon tumor, with the incidence of 5-8% of renal carcinoma. Sarcomatoid differentiation is a result of divergent differentiation of malignant epithelial cells. During this process tumor cells lose their epithelial features, and acquire mesenchymal characteristics, which provides them higher APSTRAKTI 79 MATERIA MEDICA • Vol. 34 • Issue 1, suplement 1 • april 2018. capacity for migration and metastasis.This phenotype can be encountered in all RCC subtypes, including clear cell, papillary, chromophobe RCC, and carcinomaof collecting duct. Sarcomatoid component may resemble fibrosarcoma or undifferentiated pleomorphic sarcoma. Less often heterologous differentiation can be found, with histologic patterns resembling rhabdomyosarcoma, chondrosarcoma, or osteosarcoma. Rhabdoid phenotype is an aggressive form of divergent differentiation which has been described in clear cell RCC, papillary RCC, as well as in chromophobe RCC (CRRCC). CRRCC comprise about 6% of RCC, and has better prognosis compared to other RCC subtypes. Sarcomatoid differentiation is found in 8% of these tumors, and is an indicator of poor prognosis, as it is in other RCC types. Sarcomatoid component may represent terminally dedifferentiated clone which can arise from any RCC subtype, or can develop from a special clone. Although sRCC is most frequently found in high grade tumors, the occurrence of sarcomatoid differentiation in RCC, Fuhrman 1/2, detected in 30% of the cases, denies the claim that sarcomatoid differentiation is a continual process of classic RCC dedifferentiation, but a consequence of sarcomatoid stem cell activation within the tumor. In a case of sarcomatoid differentiation in renal carcinoma it is necessary to detect focuses of any morphological type of renal cell carcinoma, and to verify the immunohistochemical positivity of sarcomatoid component to epithelial and mesenchymal markers. Their epithelial origin is confirmed by the expression of cytokeratins, and EMA. PAX8, CAIX, and CD10 can be used to confirm the renal origin, which is of particular importance in finding of sarcomatoid cells in needle biopsy, as well as in metastatic RCC. CAIX, p53, and Bcl-2 can play a major role in transformation of RCC to highly malignant neoplasm with sarcomatoid differentiation. The expression of CAIX, p53, and Bcl-2 is indicator of poor prognosis and worse survival of the patients with sRCC. Significant micromorphological parameters that may influence the clinical course of the disease comprise percentage of sarcomatoid differentiation (>50%), presence of vascular invasion, extent of necrosis, and advanced TNM stage. Type and grade of sarcomatoid component, as well as RCC subtype do not influence the clinical outcome. The prognosis of patients with sRCC is worse compared to prognosis of other histologic forms of RCC. Sarcomatoid differentiation in RCC is a significant cause of mortality, with average survival of 4-9 months after diagnosis. The majority of unclassified RCCs contains variable percent of tumor cells with sarcomatoid morphology, without recognizable epithelial component. About 3% of RCC are exclusively sarcomatoid. These tumors are larger, more often invade adrenal glands and other adjacent organs, metastasize more frequently into regional lymph nodes and bones, and have significantly shorter survival (average is 4.3 months). Differential diagnosis of sRCC should exclude primary renal sarcoma, sarcomatoid urothelial carcinoma, and angiomyolipoma. Micromorphologically sRCC may resemble classic sarcoma, however it should be taken into account that primary renal sarcomas are extremely rare in adults, and comprise less than 1% of renal malignancies. The half of primary renal sarcomas are leiomyosarcomas, consisting of smooth muscle component, which is extremely rare in sRCC. In sarcomatoid urothelial carcinoma, the location of tumor is of significance, also the presence of carcinoma in situ, recurrence of urothelial carcinoma, and immunohistochemical positivity to CKhmw and p63. In angiomyolipoma, immunoreactivity to HMB45, as well as the presence of focuses with classic morphology, excludes sarcomatoid RCC. Conclusion: Sarcomatoid differentiation in renal cell carcinoma is a result of divergent differentiation of malignant epithelial cell, and can be found in all renal cell carcinoma subtypes. During this process tumor cells lose their epithelial features, and acquire mesenchymal characteristics, which provides them higher capacity for migration and metastasis. It is very important to recognize the presence of sarcomatoid differentiation in renal cell carcinoma, since it is an indicator of poor prognosis irrespective of RCC subtype.

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