Reclassification of rhabdoid tumor and pediatric undifferentiated/unclassified sarcoma with complete loss of SMARCB1/INI1 protein expression: three subtypes of rhabdoid tumor according to their histological features.

Rhabdoid tumor is characterized by rhabdoid cells and shows complete loss of SMARCB1/INI1 protein expression. In existing classifications, the diagnostic synonyms vary depending on the anatomic site: rhabdoid tumors in the central nervous system or extra-central nervous system are, respectively, classified as atypical teratoid/rhabdoid tumor or malignant rhabdoid tumor. In this study, we analyzed the histological, immunohistochemical, microRNA, and clinicopathological statuses of tumors initially diagnosed as malignant rhabdoid tumor (n=33), atypical teratoid/rhabdoid tumor (n=11), and pediatric undifferentiated/unclassified sarcoma (n=8) with complete loss of SMARCB1/INI1 expression, and considered the possibility of their histological reclassification. Our analysis indicated that the tumors could be histologically reclassified into three groups: conventional-type tumors resembling malignant rhabdoid tumor, atypical teratoid/rhabdoid-type tumors resembling atypical teratoid/rhabdoid tumor, and small cell-type tumors resembling malignant lymphoma. The reclassified conventional type was composed of 27 malignant rhabdoid tumors and 9 atypical teratoid/rhabdoid tumors (36 cases). The atypical teratoid/rhabdoid type consisted of six malignant rhabdoid tumors, two atypical teratoid/rhabdoid tumors, and two undifferentiated/unclassified sarcomas (10 cases). The six cases of small cell type were made up of six undifferentiated/unclassified sarcomas. All of the available tumor specimens were positive for vimentin and epithelial marker (EMA, CAM5.2, or AE1/AE3). MicroRNA profiles were not significantly different between the conventional- and small cell-type tumors (Pearson's correlation coefficient: 0.888300 or 0.891388). There was no significant difference in overall survival between atypical teratoid/rhabdoid tumor and malignant rhabdoid tumor (P=0.16). In addition, there were no significant differences in survival between any of the reclassified combinations. In conclusion, we could classify eight tumors initially diagnosed as undifferentiated/unclassified sarcomas into two cases of atypical teratoid/rhabdoid type and six cases of small cell type. We suggest that reclassification of malignant rhabdoid tumors into three groups according to their histologic features rather than the traditional classification by sites of origin would be favorable for their histopathological diagnosis.