Smoothelin immunohistochemistry is a useful adjunct for assessing muscularis propria invasion in bladder carcinoma

Aims: To prospectively evaluate the utility of smoothelin immunohistochemical expression for the evaluation of muscularis propria (MP) in diagnostic transurethral resection of bladder tumour (TURBT) specimens and cystectomies. Methods and results: Smoothelin immunohistochemistry was performed on a total of 26 TURBT and cystectomy specimens. All but two cases (24/26) demonstrated strong (3+) or moderate (2+) immunoreactivity of the MP with smoothelin. Muscularis mucosae (MM) never displayed strong (3+) reactivity, and in only one case did the MM have moderate (2+) reactivity; in this case the MP had strong (3+) reactivity. MM intensity mirrored the intensity of reactivity of blood vessels in all cases (26/26). Using moderate or strong immunoreactivity as a cut-off, smoothelin had a sensitivity of 92% for detecting MP and a specificity of 97% for distinguishing between MP and MM. In all unequivocal MP-invasive and lamina proporia-invasive cases by haematoxylin and eosin (H&E), smoothelin immunohistochemistry confirmed the original light microscopic diagnosis. In four cases in which there was equivocal MP involvement by H&E, smoothelin helped establish MP invasion. Conclusions: Smoothelin immunohistochemistry has diagnostic utility in the evaluation of MP invasion in urothelial carcinoma. Smoothelin could be used as an adjunct to traditional H&E-stained light microscopy and may help reduce the number of equivocal diagnoses.