Reactivity of carbonic anhydrase IX (CA IX) across the spectrum of renal cell carcinomas with sarcomatoid differentiation.

Carbonic anhydrase IX (CA IX) is traditionally considered to be an immunomarker of clear cell renal cell carcinoma (RCC). However, CA IX expression has also been documented in other RCCs subtypes. Discrimination between clear cell RCC and non-clear cell RCC is crucial for further patient management. The aim of this study was to assess CA IX immunoreactivity across the spectrum of RCC with sarcomatoid differentiation. The expression of CA IX was evaluated in 32 cases of RCCs with sarcomatoid differentiation (12 clear cell RCC, 7 papillary RCC and 13 chromophobe RCC). Seven urothelial carcinomas (UC) with sarcomatoid differentiation (originally from renal pelvis) and 23 soft tissue tumors were also included as a control cohort. The sensitivity for CA IX in sarcomatoid component of clear cell RCC was 91.7 % (moderate/strong CA IX staining in >60 % of sarcomatoid component). However, the CA IX specificity was rather low (45 %), as a significant proportion of sarcomatoid components in different renal cell carcinoma subtypes also stained with CA IX (2/7 papillary RCC, 9/14 chromophobe RCC). When urothelial carcinoma and soft tissue tumors were included in the evaluation, the specificity of CA IX staining for sarcomatoid clear cell RCC reached 60 %. In conclusion, CA IX shows decent sensitivity for sarcomatoid clear cell RCC, but with low specificity, hence limiting its diagnostic utility as a reliable marker of in tumors with predominant sarcomatoid component.