Incidence and Pitfalls of Adipose Tissue Encountered in Urinary Bladder Biopsy/Transurethral Resection Specimens

Despite the College of American Pathologists' recommendation against diagnosing "fat invasion" in urinary bladder biopsies and transurethral resection of bladder tumor specimens (TURBT), some pathologists still consider this scenario as pathologic stage T3. However, a formal evaluation of fat in biopsies/TURBT has not been performed. Material obtained from TURBT is considered as clinical staging (cT) and that obtained from cystectomy is true pathologic staging (pT). Herein, we analyze adipose tissue incidence/distribution, cancer involving fat, staging ramifications, and clinical outcomes in a large series of biopsies/TURBT. Among 366 biopsies/TURBT specimens, data on adipose tissue presence, location, and quantity were analyzed. An initial analysis of 200 consecutive biopsies/TURBT specimens (including benign/cancer), adipose tissue was identified in 37% of 200 specimens (22% biopsies, 78% TURBT), primarily in the lamina propria (57%) or both lamina propria/muscularis propria (32%). A subsequent analysis of 183 invasive cancer (cT1/cT2) biopsies/TURBT revealed adipose tissue in 40% of specimens, predominantly within both the lamina propria and muscularis propria. Among all cT1/cT2 specimens, 26% (23/88) had cancer involving fat. Clinical follow-up on these putative "cT3" specimens revealed 10 patients who underwent radical cystectomy of which only 1 of 10 remained pT3/pT4 (although 8 patients had neoadjuvant chemotherapy). Adipose tissue is commonly found in biopsies/TURBT, predominantly localized in the lamina propria and sometimes extending into the muscularis propria. Importantly, the presence of tumor "invading" fat on biopsies/TURBT does not necessarily indicate pT3 disease. This underscores the need for standardized reporting practices, emphasizing the importance of reserving pathologic staging for cystectomy specimens.