Merkel cell carcinoma metastasizing to the kidney mimicking primary neuroendocrine renal cancer

Primary large cell neuroendocrine carcinoma of the kidney

We report a case of a 56-year-old male with a primary large cell neuroendocrine renal carcinoma. Grossly, the left kidney was enlarged by a solid tumor that measured 145 x 125 x 100 mm. Histologically, the tumor consisted of large cells with a moderate to abundant amount of eosinophilic cytoplasm. The nuclei were irregular, some of them with finely or coarsely granular chromatin, others with vesicular chromatin and prominent nucleoli. The tumor cells showed multiple mitotic figures (up to 32 mitoses/10 HPF). In some areas, the tumor cells were arranged in solid sheets; however, the predominant pattern was solid-alveolar, trabecular and cribriform. Large areas of tumor necrosis were found. Immunohistochemically, the tumor cells were positive for synaptophysin, CD56 and CD57. Cytokeratin AE1/AE3, vimentin and CD10 were positive only focally. Chromogranin showed weak cytoplasmic positivity in rare tumor cells. Cytokeratin CAM5.2, cytokeratin 34betaE12, BerEP 4, EMA, TTF-1, cytokeratin 7, cytokeratin 20, calretinin, serotonin, somatostatin, gastrin, calcitonin, glukagon and insulin were negative. Primary large cell neuroendocrine carcinoma of the kidney is a rare tumor. To the best of our knowledge, only 3 cases of a tumor of this type have been reported to date.