Department of Pathology, University of Oklahoma Health Sciences Center

March 2004, Case 403-2. Quiz set! Click here to see.

A large cerebellar mass

Shibo Li, M.D., Ph.D. ¹, Kar-Ming Fung, M.D., Ph.D.² Last updated May 1, 2004.

¹ Department of Pediatrics and ² Department of Pathology, University of Oklahoma Health Science Center, Oklahoma City, Oklahoma

Clinical information: Click thumbnails to see pictures.

The patient was admitted to our hospital because of a large cerebellar mass that was diagnosed in an outside institution. A surgery was performed. The followings are the representative photos of this mass.

 Click thumbnails to see pictures.

Pathology of the case:

Age of the patient:

• The finding that help to determine the age of the patient is the external granular layer (arrow) in Panel F. This layer is well illustrated in Panel G and H. There are four layers during this stage of cerebellar development: the external granular layer, the molecular layer, the Purkinje cell layer, and the internal granular layer.
• During formation of the cerebellum, there is an early migration of a population of precursor cells from the neuroepithelium to form a second proliferative zone on the outer surface of the cerebellar cortex. This layer is known as the external granular layer and give rise to at least four different classes of neurons: stellate cells, granular cells, Golgi cells, and basket cells.
• The external granular layer first appears in the 9^th week of gestaion and covers the whole surface by the 14^th week. The maximum thickness is reached at week 24^th week of gestation. The external granular layer is still present at the time of birth and is about 5-6 layers thick. In most babies, they are reduced to a single layer by 9 month of age and disappear entirely by 24 months of age. It is important not to mistaken the external granular layer as tumor cells or inflammatory cells.
• The cells of the external granular layer migrate inward to form the internal granular layer. The Purkinje cells, however, is originated from the ventricular germinal zone. During the 20^th to 32^th week of development, the cerebellar cortex is transformed from a 4 to 5 cells structure by the transient appearance of a conspicuous hypocellular zone between the Purkinje cell layer and the internal granular layer that is known as the lamina dessican. This layer is not present in this specimen.
• In this case, the external layer is present is about 4-6 cells thick and the lamina dessican is not seen. It is most likely obtained from a new born or slightly older than a new born. In reality, the age of this patient at the time of presentation was three month old.

Intraoperative consultation:

• Panel A is the cytologic preparation. The tissue is composed of pleomorphic cells with moderate to large amount of amphophilic to eosinophic cytoplasm. There is substantial variation in nuclear size of about 4 times and nucleoli are common an prominent. The cell lack the elongated cytoplasmic processes that are commonly seen in glial tumors [click here to see picture]. When the cytologic featrues are interpreted in conjunction with the clinical information, the most consistent diagnosis is atypical teratoid/ rhabdoid tumor (AT/RT).
• Panel B is the frozen section. The tumor is composed of medium sized to large tumor cells with pleomorphism and nucoleli. Some of the tumor cells have large amount of cytoplasm and eccentrically located nuclei (arrow) and these are the rhabdoid cells.
• Comment: It is almost a rule that the cytologic and nuclear morphology is often not as well preserved in frozen sections in comparison with cytologic (squash) preparations. In our opinion, cytologic preparations are often  very useful to separate AT/RT, ependymoma, and medulloblastoma in pediatric patients [click here to see pictures for comparison]. The frozen section, however, provides invaluable information on architecture.

Permenant sections, immunohistochemistry:

• Hematoxylin-eosin stain: The tumor is highly cellular as illustrated in the medium magnification in Panel C. There is a rich vascular supply. Small necrotic foci are present and are not shown here. On higher magnifications as illustrated in Panel D and E, there is substantial nuclear pleomorphism and variation in cell size. Mitotic figures are common. A subpopulation of the cells have substantial amount of eosinophilic cytoplasm and often prominent nucleoli. Some of them have eccentrically located nuclei as demonstrated in the cytologic prearation and some of these cells are multinucleated (arrow in Panel E). Retrospectively, the rhabdoid features are best visualized in the cytologic preparation. These features are highly consistent with an AT/RT.

• Immunohistochemistry: The tumor cells are uniformly and strongly positive for vimentin as illustrated in Panel I. A cytoplasmic membrane type of positive staining in many tumor cells is illustrated in Panel J. Many tumor cells are also positive for glial fibrillary acidic protein (GFAP), cytokeratin (clone AE1/AE3) and S-100 protein as illustrated in Panel K, L, and M respectively. The tumor cells are negative for desmin.

Cytogenetics:

• A piece of fresh tumor tissue was sent for cytogenetics studies at the time of frozen because of the high suspicion of AT/RT. Routine chromosomal analysis revealed that all twenty cells had apparently normal karyotype. No consistent structural or numerical changes were found.  Fluorecence in situ hybridization (FISH) analysis utilizing the BCR gene on 22q11.2 region was also performed, no deletion of 22q11.2 region was detected.  However, on interphase FISH of paraffin sections utilizing the same probe as illustrated in Panel N, only one hybridization signal of the BCR gene at chromosome 22q11.2 (green dot) can be detected but the control ABL gene has two dots (red dots). These findings indicate a heterozygous deletion of chromosome 22 involving at least 22q11. 2, which is commonly associated with AT/RT.

Comment:

• AT/RT often contain non-rhabdoid component. The proportion of rhabdoid cells can vary from a few to abundant. In cases with rarity of classic rhabdoid cells, a high index of suspicion and thorough search of the rhabdoid are important. Immunohistochemistry and cytogenetics can be very helpful in difficult cases.

Discussion: General    Pathology    Molecular pathology    AT/RT and other rhabdoid tumor    Differential diagnosis

General Information    

    Atypical teratoid rhabdoid tumor (ATRT) is a malignant primary childhood brain tumor and has grade IV histology out of the four tier system of the World Health Organization (WHO)¹. Malignant rhabdoid tumor was first used in 1981 by Haas et al. to described a highly aggressive childhood renal tumor with cytologic features suggestive of rhabdomyosarcoma ². Bonnin et al., described the association of rhabdoid tumors arising in the brain and kidney in children ³ in 1984. According to Weeks et al., about 13% of children with renal rhabdoid tumors developed brain tumors ⁴. The first rhabdoid tumor of the central nervous system was described by Biggs et al. in 1987 ⁵. In common to other malignant rhabdoid tumors arising in children and adults, they contain rhabdoid cells as its salient histologic feature. ATRT often contain non-rhabdoid components with medulloblastoma-like components and sometime malignant and neoplastic epithelial and mesenchymal components. Rorke et al. published the first comprehensive seires of 32 cases on rhabdoid tumors of the central nervous system and also used the term atypical teratoid/ rhabdoid tumor (AT/RT) to describe these tumors ⁶ as an attempt to emphasize the diverted histopathologic features in these tumors. Deletion and mutation of the INI1/hSNF5 gene in chromosome 22q11.2 has been shown to have strong correlation with AT/RT and other childhood rhabdoid tumor ⁷. The molecular pathology is nicely reviewed by Biegel (2004) ⁸. A cell line has been established by Yachnis et al. ⁹

    AT/RT is an uncommon tumor comprise about 2% or less of all childhood brain tumors and is less frequently seen than medulloblastoma. There is a slight male predominance and most of the tumors occur under the age of two and only rare examples has been reported in adults ¹⁰. Similar to other malignant tumor of the central nervous system, they rarely metastasize outside the central nervous system ¹¹. The posterior fossa and the cerebellar pontine angle are the most common place of occurrence with a combined incidence of 49% (38% in cerebellum, 11% in cerebellar pontine angle). AT/RT has a distinct tendency to invade into the surrounding tissue in the cerebellar pontine angle. The cerebrum harbors the other 28% of tumors and other uncommon sites including pineal, brainstem, supresellar, and spinal cord are primary locations of the rest of the tumor (13%)  ⁶. Disseminated tumor is seen in about one third of the cases at presentation. The prognosis is grave with a median overall survival of 8 to 10 months  ^{6, 12, 13}. Rare prolonged survivals have been reported ¹⁴. The radiographic features include non-specific features such as large tumor with non-homogeneous enhancement and necrosis ¹⁵.

Pathology    

    Rhabdoid cells are medium sized to large, oval to round cells. In the most classic example, the rhabdoid cell contains an eosinophilic, hyaline-like cytoplasmic globule resembling an inclusion body and occupies a large portion of the cytoplasm. This hyaline globule displays the nucleus to an eccentric location. The less classic examples contain a substantial amount of eosinophilic, often hyaline-like cytoplasm and eccentrically located nuclei. Although the morphologic features are suggestive of a rhabdomyosarcoma, cytoplasmic striations typical for rhabdomyoblastic differentiation should not be present. Marked pleomorphism is seen in most cases and nucleoli are usually prominent. Multinucleated giant cells are common. Mitotic figures and atypical mitosis are common. The degree of necrosis is variable but may not be extensive.

    Only about 13% of AT/RTs are composed exclusively of rhabdoid cells. Non-rhabdoid components are seen in the rest. About two third to three quarter of AT/RTs are associated with medulloblastoma or primitive neuroectodermal tumor(PNET) like components. Neoplastic epithelial components may occur as adenocarcinoma-like components, squamous cell with keratinization, or just simply epithelial cells arranged in nests. Neoplastic mesenchymal components can also be seen. The amount of rhabdoid cells can vary greatly from uncommon to substantial. AT/RT with substantial medulloblastoma/PNET like component may be mistaken as medulloblastoma or PNET ¹². Under the electron microscopy, rhabdoid cells contain bundles of tightly packed intermediate filaments arranged in whorls.

    Immunohistochemistry, the classic rhabdoid cells are strongly and uniformly positive for vimentin. In fact,  immunohistochemistry for vimentin is helpful in identifying rhabdoid cells. Other than vimentin, a long list of antigens is variably detected in rhabdoid tumors. Epithelial membrane antigen (EMA) is detected in the rhabdoid cells and epithelial components. About half of the cases are positive for smooth muscle actin. Expression of these two antigens are rather unusual for tumor of the central nervous system. S-100 protein is variably detectable among different cases.Intermediate filaments including neurofilament, glial fibrillary filaments, and cytokeratin are detectable in many cases. Synaptophysin is detectable particularly when a medulloblastom-like component is present. Placental alkaline phosphatase (PLAP) and beta-human chorionic gonadotrophin (bhCG) are not detectable but alpha fetal protein (aFP) can be demonstrated in some cases. ^{6, 12}

Molecular pathology:

    Deletion of chromosome 22q11.2 containing the INI1/hSNF5 gene and, less commonly, mutation of the INI1/hSNF5 gene is associated with over 90% of AT/RTs. Such deletion is also detected in other pediatric malignant rhabdoid tumors. When the medulloblastoma-like component is substantial, detection of 22q11.2 deletion by interphase fluorescence in situ hybridization (FISH) would be a helpful aid ¹⁶.

    The most common genetic aberration is monosomy or partial deletion ^{17, 18} that can be detected by interphase cytogenetics. Many rhabdoid tumors with balanced translocations involving chromosome 22q11.2 display homozygously deleted for INI1/hSNF5 ^{19, 20}. Intragenic deletion of the entire exon has also been detected  ^{20, 21}. Point mutations have also been detected in AT/RTs and other pediatric malignant rhabdoid tumors ⁶, ¹⁹. INI1/hSNF5 is a member of the SWI/SNF chromatin-remodeling complex that function in a reversible manner to remodel nucleosomes form a closed and native state to an open and active state ⁷, ²². INI1/hSNF5 probably functions as a tumor suppressor gene but the molecular pathway has not been fully revealed.

    Germ line mutations of INI1/hSNF5 gene have also been described ^{19, 21, 23}but the exact incidence of germline mutation is unclear. Malignant rhabdoid tumors arising in more than one anatomic sites are common in patients with germline mutations. Genetic aberrations of of INI1/hSNF5 have been currently described in choroids plexus carcinoma ²³, ²⁴.

AT/RT and other rhabdoid tumors:

    Tumors with malignant rhabdoid phenotype, irrespective of their location and organ being involved and the age of the patients, behave in a highly aggressive manner and carry a grave prognosis. Malignant rhabdoid tumors are most frequently seen in infants and children and are predominantly restricted to the kidney, the central nervous system (AT/RT) and soft tissue. In adults, malignant rhabdoid differentiation has been described in many different organs and often occurs as a component arising in a pre-existing neoplasm including carcinomas ^{25,  26, 27}, sarcomas ^{27, 28, 29}, melanocytic tumors  ^{30, 31}, or other neoplasms. Deletion or mutation of INI1/hSNF5 is usually not detected in adult malignant rhabdoid tumors.

    In contrast, pediatric malignant rhabdoid tumors are often associated with deletion or mutation of INI1/hSNF5 gene. Pediatric malignant rhabdoid tumors arise mainly in the kidney. The more common extrarenal sites are the soft tissue and central nervous system (i.e., AT/RT). Pediatric rhabdoid tumors arising in the kidney and soft tissue are often of pure rhabdoid phenotype and are distinct from composite type arising in adults. AT/RT shares the feature of adult malignant rhabdoid tumors in having non-rhabdoid component but also feature of pediatric malignant rhabdoid tumor in the high frequency of deletion or mutation of INI1/hSNF5 gene.

Differential diagnosis

    Although rhabdoid phenotype has been described in tumors of including meningioma ³² and glioblastoma ³³, the rhabdoid phenotype is not a frequently encountered histologic pattern in primary tumors of the central nervous system other than AT/RT. These reported cases occurred in adults. Classic feature of the “mother tumor” are usually found after careful search. The age of the patient is of great importance as only very few AT/RTs occur in adults. Before a diagnosis of AT/RT is made, the absence of a renal or extrrenal-extraneural rhabdoid tumor must be confirmed. The occurrence of two rhabdoid tumor can represent multifocal tumor or metastasis. As discussed earlier, patient with more than one malignant rhabdoid tumor in different locations may have germ line mutations.

    AT/RT with substantial proportion of medulloblastoma or PNET components can easily be misdiagnosed ¹². Rhabdoid cells can usually be found after careful search. A high index of suspicion and cytogenetic studies would be helpful in the diagnosis. AT/RT merits separation from medulloblastoma and PNETs for its grave prognosis.

    AT/RT with significant epithelial component can be mistaken as metastatic carcinoma. Again, age and clinical history is very helpful as metastatic carcinoma is extremely rare in infants. The pathologic features are also different.

    The more problematic situation is the separation of AT/RT and choroids plexus carcinoma. Both of these tumor occur in young infants and with cerebellar pontine angle a common site. To make this issue more complicated, deletion of chromosome 22q11.2 has also been described in some malignant childhood tumors that are reported as choroids plexus carcinoma ^{23, 24}. The separation between choroids plexus carcinoma and AT/RT with similar genetic aberrations is quite difficult. Considering that AT/RT allows epithelial differentiation, choroids plexus carcinoma component may well be a variant of AT/RT. Both choroids plexus carcinoma and AT/RT behave aggressively. Not until it is proved that they behave differently on treatment, their separation may not be of much practical value. As tumors with rhabdoid changes may behave aggressively, these “choroids plexus carcinoma’ may well behave like AT/RT.

    Germinoma and other germ cell tumors can be easily distinguished from AT/RT. Although germinoma have large cells with large nuclei and prominent nucleoli, it also contain significant amount of lymphocytes which is not usually seen in AT/RT. Immunohistochemistry and FISH can be very helpful in questionable case. Besides, germ cell tumors occur predominantly along the midline in the pineal or sellar/suprasellar regions which are uncommon sites for AT/RT.

Reference: 

1. Rorke LB, Biegel JA. Atypical teratoid/ rhabdoid tumor. In Kleihues P and Cavenee WK, World Health Organization Classification of Tumours, Pathology and Genetics, Tumours of the Nervous System. IARC Press Lyon, 2000, page 145-148.

2. Haas JE, Palmer NF, Weinberg AG, Beckwith JB. Ultrastructure of malignant rhabdoid tumor of the kidney. A distinctive renal tumor of children. Hum Pathol. 1981 12:646-57.

3. Bonnin JM, Rubinstein LJ, Palmer NF, Beckwith JB. The association of embryonal tumors originating in the kidney and in the brain. A report of seven cases. Cancer. 1984 54:2137-46.

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Cases of the Month  Evaluation  Coordinator: KarMing-Fung@ouhsc.edu