Nested Subtype of Urothelial Carcinoma in the Upper Urinary Tract: A Series of 15 Cases.

Nested subtype of urothelial carcinoma (NVUC) of the upper urinary tract is rare. While this subtype has an aggressive behavior in the urinary bladder, little is known about this subtype in the renal pelvis and ureter. In addition, due to NVUC's bland morphology and resemblance to von Brunn nests proliferation, which can be florid in the renal pelvis and ureter, establishing a diagnosis of NVUC in these sites can be difficult, especially on small biopsies. Nineteen cases of NVUC of the renal pelvis and the ureter were identified. One was in-house and 18 cases were sent to the genitourinary pathology consult service. Four cases were excluded due to difficulty in obtaining the slides from other institutions. In all, 8/15 (53.33%) cases showed large nested morphology, 4/15 (26.66%) cases showed mixed large and small nested morphology, and only 3/15 (20%) cases showed predominantly small nested morphology. In all cases, the invasive component was pure nested subtype, and tumors were all unifocal. In 8/15 cases, there was the presence of an overlying low-grade papillary urothelial carcinoma. An associated desmoplastic stromal reaction was none/minimal in 12/15 (80%) cases, with a prominent reaction in the remaining 3/15 (20%) cases. Pathologic stages for the resections were pT1 (n=1), pT2 (n=3), pT3 (n=8), and pT4 (n=1). There was only 1 case with a lymph node metastasis at the time of resection who had pT4 disease. Follow-up was available in 10/15 cases, 7/10 cases showed no recurrences or metastases, and the remaining 3/10 cases later developed urothelial carcinoma of the bladder. In 5/15 cases, follow-up could not be obtained. Similar to NVUC in the urinary bladder, NVUC in the upper tract typically lacks an associated desmoplastic reaction and has by definition bland cytology. In contrast to NVUC of the urinary bladder, NVUC in the upper urothelial tract is: (1) more frequently associated with an overlying papillary urothelial carcinoma; (2) more commonly has a large nested pattern; and (3) appears to behave less aggressively.