Brain Pathology Case of the Month - November 2014

DIAGNOSIS

High grade malignant peripheral nerve sheath tumor with rhabdomyosarcomatous differentiation, also called "Malignant Triton tumor".

DISCUSSION

The differential diagnosis of a cerebellopontine angle tumor should always begin with a vestibular schwannoma which account for 80% of tumors at this location. Vestibular schwannomas, however, are rarely associated with hemorrhage. In contrast, intratumoral hemorrhage within a malignant peripheral nerve sheath tumor (MPNST) is not uncommon and in fact may be used a sign of malignant transformation of an otherwise benign nerve sheath tumor6. Meningiomas and epidermoids constitute the majority of the remaining tumors at 10% and 6%, respectively7.

Intracranial malignant Triton tumors are exceedingly rare, and only 5 other reports of cerebellopontine angle MTTs are reported in the literature1-4,9. The term "Triton" tumor is derived by an observational association with experiments of Triton salamanders in which the implantation of a nerve could result in growth of a limb containing skeletal muscle. Thus, a Triton tumor is a neoplasm containing neural and skeletal muscle elements10. A malignant Triton tumor (MTT) specifically refers to a malignant peripheral nerve sheath tumor with rhabdomyosarcomatous differentiation, a sarcoma subtype first described in by Masson 1932.

MTTs have a similar patient demographic to that of classic MPNST with a mean age in the thirties and without gender predilection4,5. A significant proportion of MTTs arise in the setting of neurofibromatosis 1, with reports ranging from 38 - 57%5,8. Previous radiation exposure has also been cited as a risk factor5,6.

Proper diagnosis is essential as MTTs typically follow a more aggressive clinical course than their MPNST counterparts. A recent series established that the overall 5-year survival for MTT is 14%, compared to MPNST of 34%5. With metastases not uncommon, with a cited rate of 31%, surgical excision followed by radiotherapy is the recommended therapeutic approach5.

REFERENCES

  1. Best PV. (1987) Malignant Triton tumour in the cerebellopontine angle. Report of a case. Acta Neuropathol 74(1):92-6.
  2. Comey CH, McLaughlin MR, Jho HD, Martinez AJ, Lunsford LD. (1998) Death from a malignant cerebellopontine angle Triton tumor despite stereotactic radiosurgery. Case report. J Neurosurg. 89(4):653-8.
  3. Han DH, et al. (1992) Malignant Triton tumor of the acoustic nerve, Case report. J Neurosurg 76:874-877
  4. Gong et al. (2012)A rare case of malignant Triton tumor in the cerebellopontine angle Diagnostic Pathology 7:43
  5. McConnell Y, Giacomantonio C. (2012) Malignant Triton Tumors-Complete Surgical Resection and Adjuvant Radiotherapy Associated With Improved Survival J. Surg. Oncol. 106:51-56
  6. Scheithauer BW: Malignant peripheral nerve sheath tumour (MPNST). In: Louis DN, editor. WHO Classification of Tumours of the Central Nervous System, 4th edition. Lyon, France: IARC 2007; pp. 160-162.
  7. Springborg JB, Poulsgaard L, Bertram J (2008) Nonvestibular Schwannoma Tumors in the Cerebellopontine Angle: A Structured Approach and Management Guidelines Skull Base 18(4): 217-227
  8. Stasik CJ, Tawfik O (2006) Malignant peripheral nerve sheath tumor with rhabdomyosarcomatous differentiation (malignant Triton tumor). Arch Pathol Lab Med 130:1878-1881.
  9. Sun X, Thomas C. (2001) Pathologic Quiz Case: An Unusual Cerebellopontine Angle Tumor. Arch Pathol Lab Med 125:1113-1114
  10. Woodruff JM, Chernik NL, Smith MC, Millett WB, Foote FW. (1973) Peripheral nerve tumors with rhabdomyosarcomatous differentiation (Malignant ''Triton'' tumors). Cancer 32:426-439.

Contributed by Jesse Lee Kresak, MD, Kyle M Fargen MD, Anthony T Yachnis, MD, Spiros Blackburn, MD

International Society of Neuropathology