Low-grade malignant triton tumor in the lumbar spine: a rare variant of malignant peripheral nerve sheath tumor with rhabdomyoblastic differentiation.

Malignant peripheral nerve sheath tumor (MPNST) is an uncommon type of sarcoma that arises from peripheral nerve sheaths and rarely involves the spinal roots. The origin of this tumor is thought to be Schwann cells or pluripotent cells of the neural crest. The subgroup of tumors in which malignant Schwann cells coexist with malignant rhabdomyoblasts is termed malignant triton tumor (MTT). MPNSTs can show different degrees of malignancy, but overall spinal MTTs are high-grade lesions. We report the exceptional instance of a spinal low-grade MTT in a 39-year-old man treated with total surgical removal followed by local radiation therapy. Histological low grade was based on the lack of necrosis, a low grade of atypia, a low mitotic rate and a Ki-67 labelling index <25%. After 18 months of follow-up the patient is alive with no evidence of disease. A thorough review of the literature yielded 57 well-documented spinal MPNSTs. Ten of them corresponded to MTTs, but none showed low-grade features. An analysis of the clinical, radiological and treatment data was performed to identify factors that might influence the outcome. Overall the 18-month survival rate was 45% but dropped to 0% in the subgroup of spinal MTTs. Besides, a size exceeding 2 cm, extra-spinal extension, association with neurofibromatosis and subtotal removal were all related to a worse outcome. In conclusion, spinal MTTs generally exhibit a more aggressive behavior than conventional MPNSTs. The occurrence of a spinal low-grade MTT with a better prognosis should also be recognized.