Mucinous tubular and spindle cell carcinoma, a rare, recently described distinctive subtype of renal cell carcinoma, may have some morphologic similarities to the more common papillary renal cell carcinoma, particularly the basophilic (type 1) tumors with prominent solid growth pattern. Tumor circumscription, compact tubular architecture, focal papillations, mucin production and foam cells (features seen in both papillary renal cell carcinoma and mucinous tubular and spindle cell carcinoma), as well as spindle cell morphology, have resulted in some cases sent to us in consultation with a question of possible sarcomatoid papillary renal cell carcinoma. In this study, tissue microarrays with triplicate samples each from 27 mucinous tubular and spindle cell carcinomas and 20 papillary renal cell carcinomas were created to simulate experience in renal biopsy specimens. From immunohistochemistry (IHC) data, published in the contemporary literature, a panel consisting of α-methylacyl-CoA racemase (AMACR), cytokeratin 7 (CK7), epithelial membrane antigen (EMA), renal cell carcinoma marker (RCC Ma), CD10, high molecular weight cytokeratin (HMWK), and c-kit was designed to test its utility in differential diagnosis. The immunoreactivity in mucinous tubular and spindle cell carcinoma was AMACR 93%, CK7 81%, EMA 95%, RCC Ma 7%, CD10 15%, HMWK 15%, and c-kit 5% and in papillary renal cell carcinoma was AMACR 95%, CK7 65%, EMA 88%, RCC Ma 25%, CD10 80%, HWMK 15%, and c-kit 18%. This largest study to date on IHC of mucinous tubular and spindle cell carcinoma dispels the specificity of AMACR for papillary renal cell carcinoma among the RCC subtypes. The histogenesis of mucinous tubular and spindle cell carcinoma from the distal nephron continues to be debatable, as our study showed the expression of the proximal convoluted tubule-related marker AMACR among these tumors. Thus, in tumors with predominant compact tubular growth and focal papillary architectures, careful attention to the presence of a low-grade spindle cell population may be helpful in the distinction of mucinous tubular and spindle cell carcinoma, as the key immunohistochemical stains for papillary renal cell carcinoma are also expressed in this subtype of renal cell carcinoma.
