[Aggressive mucinous tubular and spindle cell carcinoma of the kidney: a clinicopathological and genetic analysis of four cases].

To understand the clinicopathological and molecular genetic characteristics of aggressive renal mucinous tubular and spindle cell carcinoma (MTSCC). The clinical features, histology, immunophenotype, molecular characteristics and prognosis of 4 cases of metastatic/recurrent renal MTSCC that were submitted to the Peking University Third Hospital (2 cases), Institute of Urology, Peking University (one case) and Zhejiang Provincial People's Hospital (one case) from 2015 to 2020 were retrospectively reviewed and analyzed. Among the four patients, two were male and two were female. The average age was 58 years, ranging from 28 to 77 years. Three patients underwent radical nephrectomy, while one underwent partial nephrectomy. The tumor size was 2-8 cm (mean, 5.6 cm). There were two cases classified as pT3a, one case as pT1b and one case as pT1a. Histologically, the tumors were mainly composed of tubules and spindle cell cords. For nuclear grade, three cases were G3 and one case was G2. Extracellular mucus was present in all four cases. Sarcomatoid features and tumor necrosis were observed in one and three cases, respectively. Immunohistochemistry showed that PAX8 (4/4), AMACR (4/4), CK7 (4/4), CKpan (3/3), vimentin (3/3) and CK8/18 (2/2) were positive in the tumor cells, but CAⅨ (1/4) or CD10 (2/3) were focally positive or negative. Fluorescence in situ hybridization showed that no trisomy of chromosomes 7 and l7 (2/2). Targeted next generation sequencing were performed in all four cases and showed that 3 cases had mutations in Hippo pathway involving MET (2/4), NF2 (1/4) and NTRK1 (1/4) genes. The other potentially pathogenic mutations involved KDM6A, SETD2 and PALB2. The follow-up period was 13 to 99 months. The time between diagnosis and metastasis/recurrence ranged from 6 to 58 months. Two patients died after lung metastasis occurred, one had multi-organ and multi-site lymph node metastases, and one achieved disease-free survival after resection of metastatic/recurrent foci. Renal MTSCC is a rare and distinct entity. The presence of high nuclear grade and pathological stage, high-grade morphology, lymphovascular invasion, and tumor necrosis suggests potential aggressive behaviors. It is thus recommended to report these histological features and conduct active follow-up and surveillance after surgery. The frequent mutations in MET, NF2 and NTRK1 suggest that dysregulation of Hippo pathway may be related to the development and progression of renal MTSCC.