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An Unusual Cerebellopontine Angle Tumor.

A 66-year-old man presented to Loyola University Medical Center emergency room in April 1999 with a 10-day history of progressive nausea, vomiting, and orthostatic dizziness. The symptoms were unresponsive to routine treatment. A computed tomographic scan of the head (Figure 1) revealed a partially calcified mass in the right cerebellopontine angle. A cerebral angiogram was negative for aneurysm. The patient was admitted to the neurosurgery service. A subtotal resection of the tumor was performed.


The specimen received consisted of multiple irregular fragments of tan-pink soft tissue. Histologically, the tumor showed 2 separate morphologic patterns with collision of the 2 lesions. The larger tumor component was malignant and composed of cellular spindle cells, and the smaller one consisted of benign cuboidal cells (Figure 2). The spindle cells were arranged in fascicles and sheets, were pleomorphic with hyperchromatic nuclei, and had occasional mitotic figures (Figure 3). Areas of necrosis, myxoid degeneration, and focal calcification were observed. The smaller cuboidal cell neoplasm showed prominent perivascular pseudorosette formation and bland nuclei without atypia (Figure 4). A calcified portion of the mass was received separately.


The patient did not receive any radiation or chemotherapy owing to his medical problems of congestive heart failure, hypertension, and diabetes. He died of chronic heart failure and pneumonia 2 months after the surgery. No autopsy was performed.

What is your diagnosis?

Pathologic Diagnosis: Collision Tumor (Malignant Triton Tumor and Ependymoma)

The ependymoma stained positively with glial fibrillary acidic protein and S100. The triton tumor was focally positive with S100. Reactions for muscle actin (HHF35) and desmin were positive in the spindle cells. Few spindle cells were immunoreactive to myoglobin, consistent with the rhabdomyoblastic differentiation. Both spindle cells and ependymal cells were positive for vimentin. The Ki-67-MIB-1 proliferation index was high in the triton tumor (50% to 60%) and less than 5% in the ependymoma. Staining for p53 was also performed and showed strong positivity in spindle cell areas and mild focal positivity in the ependymoma.

Malignant peripheral nerve sheath tumors (MPNSTs) are highly malignant sarcomas arising either de novo or in transition from neurofibroma, usually in conjunction with neurofibromatosis type 1. Most MPNSTs are cellular, obviously malignant, and highly infiltrative. The degree of anaplasia, as well as the histologic pattern, varies considerably. Malignant triton tumor is a histologic variant of MPNST showing rhabdomyoblastic differentiation.[1,2] Immunohistochemical staining of MPNSTs shows variable reactivity for S100 protein. Reactivity for glial fibrillary acidic protein is uncommon.[3] In addition, the focal positivity for muscle actin, desmin, and myoglobin in the spindle component are present in areas with rhabdomyoblastic differentiation.

The main differential diagnoses include cellular schwannoma, spindle cell rhabdomyosarcoma, leiomyosarcoma, fibrosarcoma, and gliosarcoma. The immunostains are helpful in differentiating these tumors from malignant triton tumor An occasional "ancient" schwannoma may show nuclear pleomorphism but lacks mitosis and necrosis. Gliosarcoma is a mixed tumor containing both malignant glial and mesenchymal tissues intermixed, unlike in the collision tumor. Abnormalities of chromosomes 17 and 22 are associated with MPNST and malignant triton tumor.[4,5] Abnormal chromosomes 1 and 12 also appear to be significant.[2] In addition, MPNSTs have a relatively high frequency of p53 (tumor suppressor gene) mutations.[6,7]

The prognosis of MPNST is generally poor, but is related to multiple factors, such as history of neurofibromatosis type 1, prior irradiation, tumor location, surgical margin status, use of intraoperative electron irradiation or brachytherapy, dose of irradiation, and mitotic rate. For metastatic disease control, tumor size, stage of disease, histologic grade, histologic subtype, mitotic rate, and presence of necrosis[1] are significant factors. According to 1 study, the extent of tumor necrosis, mitotic rate, and the parameters related to silver-staining nucleolar organizer regions discriminate between less aggressive MPNSTs (disease-free for 4 years) and more aggressive MPNSTs (with recurrences or metastases during the first 4 years).[8] The MPNSTs with rhabdomyoblastic differentiation (malignant triton tumor) are reported to have a worse prognosis.[1,9] The negative surgical margin is the most significant prognostic factor for survival and local control of disease.[1] However, the anatomic location and infiltrative growth preclude a complete surgical resection.

In summary, to our knowledge this is the first reported case of a collision tumor (malignant triton tumor and ependymoma) at this anatomic location.


[1.] Wong WW, Hirose T, Scheithauer BW, et al. Malignant peripheral nerve sheath tumor: analysis of treatment outcome. Int J Radiat Oncol Biol Phys. 1998; 42:351-360.

[2.] McComb EN, McComb RD, DeBoer JM, et al. Cytogenetic analysis of a malignant triton tumor and a malignant peripheral nerve sheath tumor and a review of the literature. Cancer Genet Cytogenet. 1996;91:8-12.

[3.] Hirose T, Scheithauer BW, Sano T. Perineural malignant peripheral nerve sheath tumor. Am J Surg Pathol. 1998;22:1368-1378.

[4.] Jhanwar SC, Chen Q, Li FP, et al. Cytogenetic analysis of soft tissue sarcomas: recurrent chromosome abnormalities in malignant peripheral nerve sheath tumors (MPNST). Cancer Genet Cytogenet. 1994;78:138-144.

[5.] Decker HJH, Cannizzaro LA, Mendez MJ, et al. Chromosomes 17 and 11 involved in marker formation in neurofibrosarcoma in von Recklinghausen disease: a cytogenetic and in situ hybridization study. Hum Genet. 1990;85:337-342.

[6.] Kawai A, Noguchi M, Beppu Y, et al. Nuclear immunoreaction of p53 protein in soft tissue sarcomas: a possible prognostic factor. Cancer. 1994;73:2499-2505.

[7.] McCarron KF, Goldblum JR. Plexiform neurofibroma with and without associated malignant peripheral nerve sheath tumor: a clinicopathologic and immunohistochemical analysis of 54 cases. Mod Pathol. 1998;11:612-617.

[8.] Miracco C, Montesco MC, Santopietro R, et al. Proliferative activity, angiogenesis, and necrosis in peripheral nerve sheath tumors: a quantitative evaluation for prognosis. Mod Pathol. 1996;9:1108-1117.

[9.] Can Z, Saray A, Yilmaz S, et al. Malignant triton tumor of the maxilla: apatient report. Ann Plast Surg. 1999;42:96-99.

Accepted for publication December 21, 2000.

From the Department of Pathology, Loyola University Medical Center, Maywood, Ill.

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Article Details
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Author:Sun, Xiaoming; Thomas, Chinnamma
Publication:Archives of Pathology & Laboratory Medicine
Geographic Code:1USA
Date:Aug 1, 2001
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