Mucinous tubular and spindle cell carcinoma and its prognostic paradox: A population-based study.

OBJECTIVES To compare the clinicopathologic features, treatment patterns, and survival outcomes of mucinous tubular and spindle cell carcinoma (MTSCC) with those of clear cell renal cell carcinoma (ccRCC) and papillary renal cell carcinoma (PRCC). SUBJECTS AND METHODS This retrospective cohort study used SEER data from 1983 to 2022, including 461 MTSCC, 133,229 ccRCC, and 29,442 PRCC cases. Demographic, clinical, and treatment variables were analyzed using chi-square, ANOVA/Kruskal-Wallis tests, and Kaplan-Meier methods. Cox proportional hazards regression models were applied to estimate hazard ratios (HRs) for overall (OS) and disease-specific survival (DSS), adjusting for age, sex, race, stage, grade, treatment, and metastasis status. RESULTS MTSCC patients were more often female (54.9%), Black (20.6%), and aged ≥70 years (50.5%) compared to ccRCC and PRCC (P < 0.001). MTSCC had a lower incidence of distant metastasis than ccRCC (8.7% vs. 9.5%), but higher than PRCC (4.2%). Although most MTSCC patients presented with early-stage disease and underwent surgery (87.9%), they had the shortest mean survival (47.9 months) and the highest proportion of deaths within 100 months (83.5%). Kaplan-Meier analysis showed higher early mortality for MTSCC, with survival curves converging after 75 to 100 months. In adjusted models, MTSCC was associated with a nonsignificant increase in mortality compared to ccRCC (OS HR: 1.36, P = 0.422; DSS HR: 1.13, P = 0.832), while PRCC had a significantly higher DSS risk (HR: 1.24, P = 0.001). Poor survival in MTSCC was associated with older age, high-grade tumors, distant metastases, and absence of surgery. CONCLUSION MTSCC shows distinct demographic and clinical features and a paradoxically shorter survival despite early-stage presentation. Early mortality may contribute to its poorer outcomes, indicating that MTSCC is not uniformly indolent. Closer surveillance and individualized risk assessment are warranted in selected patients.