Fat-poor renal angiomyolipoma (fpAML) and renal cell carcinoma (RCC) are difficult to differentiate and misdiagnosis can lead to unnecessary nephrectomy. We experienced a case showing a “collapsed shape” which reflected a fpAML tissue type. A renal tumor was incidentally discovered in a 42-year-old female during an abdominal ultrasound. RCC was suspected according to CT and MRI imaging results, and a partial nephrectomy was performed. However, the pathologic diagnosis was fpAML. Upon reevaluation of preoperative images, morphological change to the tumor due to contact with surrounding tissues: the collapsed shape was observed and could be identified by CT, which is the gold standard test for differentiating renal tumors. In cases where the collapsed shape is observed in a renal tumor, fpAML should be considered.
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