Intestinal type of mucinous borderline tumor arising from mixed epithelial and stromal tumor of kidney.

We report a mucinous borderline tumor arising from a mixed epithelial and stromal tumor of left kidney (MESTK). The patient was an 82-year-old woman who presented with gross hematuria and recurrent urinary tract infection for years. The patient had a cytoscopy with a retrograde pyelogram, which indicated a dilated left kidney with a central mass lesion. Subsequently, she underwent a radical left nephrectomy. Cross-sections of left kidney showed a 4.5 x 3.5 x 1.5 cm ill-defined cystic lesion with mucinous and solid areas. Histologic sections of the lesion showed numerous variable-sized dilated cysts with fibrous, fatty, vascular, and smooth muscle stroma. The cysts were lined by a various types of epithelium, including single layer of flat, cuboidal and mucinous epithelium, urothelium, intestinal epithelium, and endocervical epithelium. In areas, the mucinous epithelium showed complex proliferation with stratification, papillae formation, and nuclear atypia, resembling that of an ovarian mucinous borderline tumor, a colorectal tubular adenoma, or a low-grade appendiceal mucinous carcinoma. Immunohistochemically, the mucinous borderline tumor showed a colorectal phenotype, being strongly positive for CK20, CDX-2, and MUC2. There was no invasive mucinous tumor identified. We believe that this case represents the first reported example of mucinous borderline tumor arising from a MESTK.