Case 11 -

Medulloblastoma, Large Cell/Anaplastic Type D. Ashley Hill, M.D. Mihaela Onciu M.D.

Case History: A 13-year-old girl who presented with headache. Examination showed cranial nerve XI palsy. Radiographic studies showed an enhancing posterior fossa mass and cerebrospinal fluid dissemination of tumor. Gross Examination: Several small pieces of purple-pink, friable tumor were received. A small piece was snap frozen in liquid nitrogen and stored at -80oC for biologic studies. Microscopic Examination: Sections showed a malignant embryonal neoplasm characterized by diffuse sheets of relatively cohesive cells with large, rounded, hyperchromatic nuclei and minimal eosinophilic cytoplasm. Occasional groups of cells with large prominent central nucleoli were seen. Rare cells showing "cell wrapping" phenomenon were also seen. There were numerous apoptotic cells. Frequent mitotic figures were seen. Diagnosis: Medulloblastoma with large cell/anaplastic features Discussion Medulloblastomas (MB) are malignant embryonal tumors of the cerebellum that have a predilection for children. The peak age for MB is 7 years; 70% of patients are less than 16 years of age at diagnosis. Approximately three-quarters of MB arise from the cerebellar vermis with the remainder involving the cerebellar hemispheres. Patients with MB typically present with either signs of cerebellar dysfunction or symptoms associated with blockage of cerebrospinal fluid (CSF) flow. Radiographically, the tumors are discrete, solid, contrast-enhancing masses. Poor clinical prognostic factors include the presence of CSF dissemination (present in 1/3 of patients at the time of diagnosis) and gross residual tumor following resection. While advances in therapy have led to overall improved survival, the secondary effects of high-dose chemotherapy and particularly radiation therapy on developing brains can lead to significant morbidity and decreased intellectual potential. As with other pediatric tumors, finding better ways to risk stratify these patients and individualize the treatment is an important goal. Although all MB are considered grade IV/IV tumors, it has become clear with recent studies that certain histologic subtypes may be predictive of outcome. The new World Health Organization (WHO) classification for MB recognizes four primary subtypes: classic, desmoplastic (nodular), medulloblastomas with extensive nodularity and large cell/anaplastic medulloblastomas (Table 4.1). The latter two categories represent favorable prognosis and poor prognosis tumors respectively. Classic Medulloblastoma Classic MB are characterized by diffuse sheets of loosely cohesive malignant small cells. On high power, the tumor cells have hyperchromatic round or angular nuclei and show some size and shape variability. Mitotic figures and apoptotic bodies may be frequent. MB may show neuroblastic or neuronal differentiation exhibited by the presence of Homer Wright rosettes or by linear columns of tumors cells vertically traversing the molecular layer. Rare MB contain cells with features of differentiating ganglion cells or astrocytic differentiation. Binucleation and a lack of uniform distribution or anatomic layering are features helpful in distinguishing tumor cells differentiating into ganglion cells from entrapped neurons. The significance of glial differentiation in MB is controversial. Desmoplastic (Nodular) Medulloblastoma Desmoplastic (also referred to as nodular) MB are tumors characterized by nodular foci referred to as "pale islands", hypercellular, more primitive internodular regions and increased connective tissue (reticulin-rich). The nodular foci consist of tumor cells showing neurocytic differentiation with bland, round nuclei and abundant cytoplasmic processes as indicated by the fine fibrillary background. The proliferative rate within the nodules is low. At the periphery of these nodules, the tumor cells are more condensed, show more primitive features and are mitotically active. Reticulin-rich areas in which primitive tumor cells grow in a linear arrangement between collagen fibers is another pattern. A comparison of the staining pattern for Ki-67 and synaptophysin will look like a photograph and its negative with the nodules showing strong staining for synaptophysin and a lack of Ki-67 and the internodular areas showing strong Ki-67 positivity and little synaptophysin. Mild to moderate degrees of nodularity do not appear to affect prognosis with the exception of those cases showing extensive nodularity (see below). Medulloblastoma with Extensive Nodularity and Advanced Neuronal Differentiation Giangaspero et al initially identified this subtype of MB as being associated with a good clinical outcome. These tumors consist almost exclusively of nodules with differentiating neurocytes embedded in abundant fibrillary material and minimal internodular tissue. Imaging studies can detect the nodularity of the tumor and is described as "grape-like". Patients with this variant are typically less than 3 years of age and have a good outcome. These tumors in the past were referred to as cerebellar neuroblastomas. Large Cell/Anaplastic Medulloblastoma Large cell/anaplastic MB (LC/A MB) are tumors characterized by increased mitotic activity, increased apoptosis (either as individual cells or "lakes" of dying cells), cytologic atypia and a poor prognosis. Tumors with "large cell" features have large round nuclei, vesicular chromatin and prominent nucleoli whereas "anaplastic" tumor cells are large, angular and pleomorphic with hyperchromatism and frequent molding. Both tumors also show a peculiar "cell wrapping" phenomenon in which a viable cell encircles another tumor cell or apoptotic body. Since both large cell and anaplastic features can be seen in combination, and both can arise within a classic or desmoplastic (nodular) MB, they are grouped together. A recent study by Eberhart and colleagues reviewed features of anaplasia within a large group of pediatric MB. By their definition, moderately anaplastic tumors contained unambiguously enlarged nuclei +/- cell wrapping and increased mitotic and apoptotic figures and severely anaplastic tumors contained markedly enlarged cells (at least twice the size of non-anaplastic cells) with cell wrapping and clusters of apoptotic cells. Tumors with moderately or severely anaplastic cells (representing 24% of all cases reviewed) showed a significantly worse event-free and overall survival. This effect on prognosis applies to both diffuse and focal anaplasia. The predictive effect of anaplasia on outcome appears to be independent of clinical stage. Other Medulloblastoma-Related Entities Tumors with extensive differentiation toward neuronal lines are designated cerebellar ganglioneuroblastomas. Rare MB will show skeletal muscle differentiation, medullomyoblastoma or true melanocytic differentiation, melanotic medulloblastoma. Medullomyoblastomas are the more common of these rare variants and are characterized by a biphasic appearance of classic MB intermixed with reasonably well differentiated rhabdomyoblasts. The primitive skeletal muscle component usually consists of large cells with long tails of abundant eosinophilic cytoplasm "strap cells" but occasional globular rhabdomyoblasts are seen. Immunohistochemical stains for synaptophysin and desmin will highlight the MB and skeletal muscle components of the tumor respectively. Some studies, however, suggest that patients with medullomyoblastoma may do worse than those with classic MB but the numbers are small. In melanotic MB, melanocytes can be clustered in tubular or papillary structures or can be dispersed singly among the small cells. Immunohistochemistry for melanocytic markers is confirmatory. The prognostic significance of divergent differentiation in a melanocytic MB is unclear. Differential Diagnosis For a discrete cerebellar lesion in a child, the primary clinical differential diagnosis is MB, ependymoma and juvenile pilocytic astrocytoma (JPA). Radiographic studies will typically be able to differentiate between the often cystic, contrast enhancing JPA from both the solid ependymoma and MB. Histologically, differentiating between MB and ependymoma is usually straightforward but ependymomas can occasionally be highly cellular and an occasional MB shows perivascular pseudorosettes. Ependymomas generally have a more abundant fibrillary background and show variation in cellularity with paucicellular areas. In difficult cases, immunohistochemical stains can be applied. Perivascular pseudorosettes in an ependymoma will be GFAP positive whereas in MB they will be synaptophysin positive. One of the more problematic differential diagnostic entities, primarily for the large cell/anaplastic category is the atypical teratoid/rhabdoid tumor (ATRT) or rhabdoid tumor. ATRT's are rare tumors with a predilection for very young children, almost always less than 2 years of age. Histologically, they have a widely variable appearance (thus the teratoid appelation) that often includes small cell areas resembling MB. In its classic form, the ATRT contains cells with large vesicular nuclei, prominent nucleoli and paranuclear eosinophilic globules of intermediate filaments (rhabdoid inclusions). Both tumors can have similarly notable mitotic activity, necrosis and apoptosis. In general, ATRT cells are larger, paler, have more cytoplasm, and ATRTs are more disorganized and histologically variable than MB. Adequate sampling to identify the diagnostic cytoplasmic inclusions and a panel of immunohistochemical stains is imperative in suspect cases (see below). Special Studies – Immunohistochemistry/FISH Most MB contain individual cells that are positive for synaptophysin. Staining will be more intense in those tumors with neuronal differentiation such as Homer Wright rosettes or nodular desmoplastic tumors. GFAP positivity may also occur in MB. Most GFAP-positive cells within a MB will be reactive astrocytes that are recognizable as stellate, and evenly spaced with a preferential concentration around vessels. Occasionally, tumor cells will show glial differentiation based on immunoreactivity for GFAP or ultrastructural features. The pattern of GFAP staining is usually patchy in comparison to that of an ependymoma or a high grade infiltrating astrocytoma if those diagnoses are a consideration. Immunohistochemical stains useful in differentiating ATRT from MB include vimentin, cytokeratin, epithelial membrane antigen, smooth muscle actin and synaptophysin. The first four stains will highlight the tumor cells of the ATRT. ATRT cells are typically negative for synaptophysin. The most common specific cytogenetic abnormality in MB is an isochromosome 17q which can be found in approximately 50% of cases. This abnormality can be detected by fluorescent in situ hybridization (FISH). MYC (c-myc) and MYCN amplification can be seen in as many as 10% of cases and has been reported to have a higher frequency in the large cell MB subtype. Preliminary studies suggest that MYC amplification portends a worse prognosis. FISH is also useful in detecting the INI1 deletion on chromosome 22q11.2 characteristic of ATRT. Clinical Prognostic Factors The presence of CSF dissemination (Table 4.2) or radiographically measured residual disease > 1.5 cm3 in localized tumors (M0) represent poor prognostic factors. Patients aged less than 3 years also tend to have an inferior survival which may be attributable to lower therapeutic dose of radiation given in this age group. CSF analysis and spinal MR used in conjunction appear to be more sensitive for detecting metastatic disease than either one alone. In addition, lumbar CSF examination appears to be more sensitive than shunt specimen examination for detection of tumor cells. Table 11.1 WHO Classification of Medulloblastoma – Summary of histologicfeatures Classic medulloblastoma Diffuse sheets of malignant small cells Round or angular hyperchromatic nuclei with shape and size variability May show neuronal differentiation in the form of Homer Wright rosettes Contain synaptophysin-positive cells Desmoplastic (nodular) medulloblastoma Nodular, hypocellular foci termed "pale islands" cells with neurocytic differentiation reticulin-poor low Ki-67 high synaptophysin staining Hypercellular internodular regions containing more primitive hyperchromatic cells of classic MB reticulin-rich high Ki-67 low synaptophysin staining Linear arrangement of tumor cells between collagen fibers is another pattern seen in some tumors Medulloblastoma with extensive nodularity and advanced neuronal differentiation Consists almost exclusively of nodules of differentiating neurocytes in fibrillary background Minimal internodular tissue Previously termed "cerebellar neuroblastomas" Good prognosis subtype Large cell/anaplastic medulloblastoma Large cell features - large round nuclei with prominent nucleoli Anaplastic features - large, angular, pleomorphic, hyperchromatic nuclei with molding Increased mitotic activity Abundant apoptosis, individual cells and groups of cells "lakes" Cell wrapping phenomenon Table 11.2 Chang Classification of Metastasis Staging for medulloblastoma M stage   M0 No gross subarachnoid or hematogenous metastasis M1 Microscopic tumor cells found in CSF M2 Gross nodular seeding in cerebellum, cerebral subarachnoid space or 3rd or 4th ventricles M3 Gross nodular seeding in spinal subarachnoid space M4 Extraneuraxial metastasis Adapted from Chang CH, Houseplan EM, Herbert C: An operative staging system for a megavoltage radiotherapeutic technique for cerebellar medulloblastomas. Radiology 93:1351-9, 1969 Recommended Reading Kleihues P, Cavenee WL (Eds.) World Health Organization Classification of Tumours. Pathology and genetics of tumours of the nervous system. Lyon: IARC Press; 2000. Giangaspero F, Perilongo G, Fondelli MP et al. Medulloblastoma with extensive nodularity: a variant with favorable prognosis. J Neurosurg 1999;91:971-7. Giangaspero F, Rigobello L, Badiali M et al. Large-cell medulloblastomas. A distinct variant with highly aggressive behavior. Am J Surg Pathol 1992;16:687-93. Brown HG, Kepner JL, Perlman EJ et al. "Large cell/anaplastic" medulloblastomas: a Pediatric Oncology Group Study. J Neuropathol Exp Neurol 2000;59:857-65. Eberhart CG, Kepner JL, Goldthwaite PT et al. Histopathologic grading of medulloblastomas: a Pediatric Oncology Group study. Cancer 2002;94:552-60. Eberhart CG, Burger PC. Anaplasia and grading in medulloblastomas. Brain Pathol 2003;13:376-85. McManamy CS, Lamont JM, Taylor RE et al. Morphophenotypic variation predicts clinical behavior in childhood non-desmoplastic medulloblastomas. J Neuropathol Exp Neurol 2003;62:627-32. Leonard JR, Cai DX, Rivet DJ et al. Large cell/anaplastic medulloblastomas and medullomyoblastomas: clinicopathological and genetic features. J Neurosurg 2001;95:82-8. Eberhart CG, Kratz JE, Schuster A et al. Comparative genomic hybridization detects an increased number of chromosomal alterations in large cell/anaplastic medulloblastomas. Brain Pathol 2002;12:36-44. Aldosari N, Bigner SH, Burger PC et al. MYCC and MYCN oncogene amplification in medulloblastoma. A fluorescence in situ hybridization study on paraffin sections from the Children's Oncology Group. Arch Pathol Lab Med 2002;126:540-4. Zeltzer PM, Boyett JM, Finlay JL et al. Metastasis stage, adjuvant treatment, and residual tumor are prognostic factors for medulloblastoma in children: conclusions from the Children's Cancer Group 921 randomized phase III study. 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A role for fluorescence in situ hybridization detection of chromosome 22q dosage in distinguishing atypical teratoid/rhabdoid tumors from medulloblastoma/central primitive neuroectodermal tumors. Hum Pathol 2001;32:156-62.