Malignant rhabdoid tumor: A phenotype? An entity?--A controversy revisited.

The term malignant rhabdoid tumor (MRT) has been used to describe a heterogeneous group of neoplasms, having in common distinct so-called "rhabdoid" cytologic features. The recent discovery of a candidate tumor suppressor gene for MRT, INI1 on chromosome (Ch)22q11.2, has re-established this neoplasm as a distinct entity. Malignant rhabdoid tumor may arise either de novo from nonneoplastic cells or through tumor progression from other types of neoplasms. These latter tumors, in which other nonrhabdoid tumor components are identified, may be termed composite MRT. In order to avoid misdiagnosing MRT as other types of neoplasia, one must keep in mind three distinct clinicopathologic features--young age of onset, variable histologic and immunohistochemical patterns, and an aggressive infiltrative character. In difficult cases, cytogenetics, fluorescence in situ hybridization (FISH), and molecular genetic analysis may assist in diagnosing MRT.