Objectives: In our multicenter study evaluating metastatic papillary renal cell carcinoma (PRCC), 29% of tumors diagnosed as PRCC in collaborative institutes were finally diagnosed as other RCCs under central review. In those tumors, mucinous tubular and spindle cell carcinoma (MTSCC) was the leading histology, followed by unclassified RCC (ucRCC). We focused on those patients with MTSCC or ucRCC. Methods: We reviewed the processes for the pathological diagnoses of nine tumors and reviewed their clinical features. Results: All of the MTSCCs and ucRCCs were positive for AMACR, which is frequently positive in PRCC. Mucin was demonstrated in 80% of the MTSCCs, and its presence is important for their diagnoses. One MTSCC was diagnosed as a mucin-poor variant. The presence of spindle cells with low-grade nuclei was suggestive of MTSCC, but the diagnosis of high-grade MTSCC was difficult. Four tumors were diagnosed as ucRCC by histological and immunohistochemical findings. Three of the four tumors were suspicious of ucRCC in the initial review due to atypical findings as PRCC. Sunitinib and interferon-α were effective for one MTSCC patient who survived for >5 years. Two MTSCC patients who were Memorial Sloan-Kettering Cancer Center poor risk had unfavorable prognoses. One patient with mucin-poor MTSCC had an indolent clinical course. Two of four ucRCC patients showed durable stable disease with targeted agents (TAs) and survived >3 years. Conclusion: Some MTSCC metastases progressed very slowly and poor-risk tumors progressed rapidly. Systemic therapies including TAs showed some efficacies. Some patients who have metastatic ucRCC with microscopic papillary architecture can benefit from TAs.
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