Objective: Mucinous tubular and spindle cell carcinomas (MTSCC's) are recently described rare type of renal cell carcinoma (RCC). MTSCC's are characterized by small, elongated tubules lined by cuboidal cells and/or cords of spindled cells separated by pale mucinous stroma. They have morphological similarities to papillary RCC (papRCC). We evaluated the importance of the immunohistochemical features in the differential diagnosis of MTSCC and papRCC.

Material and Method: We re-evaluated 9 cases of MTSCC diagnosed between 2004 and 2010 and compared 10 cases of papRCC. All tumors were stained with alpha-methylacyl-CoA racemase (AMACR), cytokeratin 7 (CK7), CK19, renal cell carcinoma marker (RCC Ma), CD10 and kidney specific cadherin (KspCad).

Results: A total of 6/9 cases were considered classical. Two of 9 MTSCC's were classified as “mucin-poor”. Foamy macrophages were identified in 4 cases. The immunoreactivity in MTSCC was AMACR 100%, CK7 100%, CK19 100%, RCC Ma 50%, CD10 11%, and KspCad 38% while the values for papRCC were AMACR 100%, CK7 90%, CK19 100%, RCC Ma 100%, CD10 80%, and KspCad 0%.

Conclusion: MTSCCs may include little mucin and show a marked predominance of either of its principal morphological components. They may mimic other forms of RCC. Pathologists should be aware of the histological spectrum of MTSCCs to ensure an accurate diagnosis. Careful attention to the presence of a spindle cell population may be helpful in the differential diagnosis in tumors with predominant compact tubular growth. Immunohistochemical stains for papRCC are also expressed in MTSCC, but strong CD10 expression may not favor MTSCC.