Mechanical Implantation of Urothelium Into Periureteral Soft Tissue: A Series of 4 Cases Mimicking High-stage Urothelial Carcinoma.

In the upper genitourinary tract of the ureter and renal pelvis, any invasion of urothelial carcinoma (UC) beyond the thin muscularis is considered pT3 disease, which may prompt adjuvant chemotherapy. Ureteroscopy is commonly used in the workup of these lesions to obtain tissue biopsies and cytology specimens, or to place stents for the relief of obstructive symptoms. The current study describes 4 cases in which the presence of small nests of urothelium in periureteral and peripelvic soft tissue was noted after ureteroscopic instrumentation, mimicking high-stage UC. All 4 cases had a history of ureteral instrumentation with biopsy of the suspected UC. The diagnostic biopsies showed low-grade noninvasive papillary UC (n=2), papillary urothelial neoplasm of uncertain malignant potential (n=1), and benign inverted urothelial papilloma (n=1). In the definitive resection specimens, small nests of bland urothelium were present in the periureteral or peripelvic soft tissue, raising the possibility of pT3 UC. We interpreted these displaced small nests to be the result of mechanical implantation occurring during the ureteroscopic procedure on the basis of the finding of associated linear fibrosis, inflammation, hemorrhage, and foreign body giant cells. In the setting of low-grade or benign urothelial neoplasms in the upper urinary tract, caution is warranted when cytologically bland nests of the urothelium are identified beyond the muscularis that could represent mechanically induced implants as opposed to invasive UC extending into peripelvic or periureteral soft tissue.