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+An 80-year-old man who presented clinically with haematuria and left flank pain. Fist percussion in the left renal fossa was positive. Imaging techniques revealed a mass in the left renal fossa that blurred the renal silhouette. The patient underwent nephrectomy with proximal ureterectomy.
+In the nephrectomy specimen, the renal parenchyma was almost entirely replaced by a whitish heterogeneous lesion in which cystic cavities of necrotic appearance with friable and poorly demarcated intracavitary papillary formations and poorly demarcated yellowish areas combined randomly with others of necrotic and/or haemorrhagic appearance could be seen. When cut, a firm consistency was observed, mainly corresponding to the whitish-greyish areas, although in other locations a soft and even crunchy consistency was observed.
+Histological examination showed a neoplasm consisting of a proliferation of spindle and polygonal cells with marked atypia and nuclear pleomorphism, which adopted a solid growth pattern. More than 75% of it showed differentiation to osteo- and chondrosarcoma although squamous differentiation could also be demonstrated, mainly lining the inner surface of the previously described cystic formations. The high cellular atypia observed was made more evident by the presence of abundant multinucleated giant cells located within the solid spindle-shaped areas and around the areas of osteosarcomatous differentiation (14) (the latter are probably in fact atypical osteoclasts). The tumour contacted the renal capsule without going beyond it, invading vascular structures of the hilum and the adipose tissue of the pelvis. After rigorous sampling of the surgical specimen, few foci with a histological pattern of clear cell carcinoma could be seen. The immunohistochemical study of the neoplasm showed positive immunostaining for cytokeratins in both the clear cell and spindle cell components, the latter also being positive for vimentin (13). Based on the above, the diagnosis of renal sarcomatoid carcinoma with a predominance of areas with malignant heterologous differentiation (osteo- and chondrosarcoma) was made, referring to the presence of foci of carcinoma with a clear cell pattern and areas with squamous differentiation.
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