genetic analysis. Examination of the cerebrospinal fluid is important (CSF), as one-third of patients will have intracranial dissemination with involvement of the CSF. Large tumor cells, eccentricity of the nuclei, and prominent nucleoli are consistent findings. Usually only a minority of AT/RT biopsies have rhabdoid cells, making diagnosis more difficult. Increasingly it is recommended that a genetic analysis be performed on the brain tumor, especially to find if a deletion in the INI1/hSNF5 gene is involved (appears to account for over 80% of the cases). The correct diagnosis of the tumor is critical to any protocol. Studies have shown that 8% to over 50% of AT/RT tumors are diagnosed incorrectly. Classification AT/RT may be related to malignant rhabdoid tumor (MRT), which occurs outside the CNS, usually in the kidney. The finding that AT/RT and MRT both have deletions of the INI1 gene indicates that rhabdoid tumors of the kidney and brain are at least closely related. AT/RT and MRT also have similar histology and similar clinical and demographic features. Moreover, 10–15% of MRT patients have simultaneous or subsequent brain tumors, many of which are secondary or primary MRT. Differential diagnosis The critical step in treatment planning is to determine the correct histology of the tumor. Misidentification of the tumor histology can lead to errors in treatment and prognosis. Atypical teratoid/rhaboid tumor closely resembles medulloblastoma, primitive neuroectodermal tumor, choroid plexus carcinoma, and some kinds of germ cell tumor. Because rhabdoid characteristics are not the only component of AT/RT, some sections of an AT/RT may resemble other tumors. These characteristics may be present only in focal areas or may be less pronounced. Consideration of AT/RT when a medulloblastoma or PNET is suspected is important, particularly in a child under the age of one. Cytogenetic studies can assist in differentiating MB/PNETs from AT/RTs. Some kinds of germ cell tumors secrete tumor markers AFP or bHCG; AT/RTs do not. Appearance on radiologic exam AT/RTs can occur at any sites within the CNS; however, about 60% are located in the posterior fossa or cerebellar area. The ASCO study showed 52% posterior fossa; 39% sPNET; 5% pineal; 2% spinal, and 2% multifocal. The tumors' appearance on CT and MRI are not specific, tending towards large size, calcifications, necrosis (tissue death), and hemorrhage (bleeding). Radiological studies alone cannot identify AT/RT; a pathologist almost always has to evaluate a brain tissue sample. The increased cellularity of the tumor