A Case of Malignant Peripheral Sheath Tumor Arising from Neurofibromatosis Type 1: DISCUSSION
MPNST is a malignant spindle cell tumor of the soft tissues that is thought to be derived from nerve sheath components such as Schwann cells or perineural fibroblasts. They may occur de novo, but are mostly associated with long-standing neurofibromas in patients with NF type 1. The transformation rate of such neurofibromas is estimated to be at about 4.6%. Tumors associated with NF type 1 are generally associated with a poor prognosis, in that they occur at a younger age, are more centrally located, are larger, and have more common metas- tases and recurrences, as well as being more poorly differentiated histologically.
The histogenesis of MPNST is still unclear, and at times the features show overlap with other soft tissue sarcomas. Clinically, MPNSTs occur in adults 20 to 50 years of age as huge masses, at times arising from preexisting neurofibromas, and are most commonly located on the proximal limbs, head and neck, and trunk, as well as the retroperitoneum, mediastinum, and viscera. The diagnosis of MPNST has always remained a challenge due to the lack of standardized diagnostic criteria, and its tendency to show overlap with over fibrous or neural crest- derived tumors. MRI can be very useful in identifying the extent of the tumor before surgery, as well as in detecting metastases or recurrences during follow-up. Electromyography may help localize small tumors of the plexus or roots.
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Histopathologically, the tumors are composed of spindle cells arranged in fascicular or whorling patterns with hemorrhagic or necrotic foci. There are hyperchromatic nuclei and widespread, multiple mitotic figures. The differential diagnosis of MPNST and benign schwannoma can be confusing; necrotic foci, atypical mitoses, and the lack of differentiated cells all indicate the former diagnosis. These three features determine the histologic grade of the tumor, which in our case was grade 3. Immuno- histochemistry can be of help in identifying nerve sheath differentiation. Markers such as S-100, vi- mentin, Leu, and myelin basic protein can be used. S-100 protein, which was positive in our case, is a calcium-binding protein present in Schwann cells, and stains 60% of all MPNSTs. Fukushima, et al. reported that the expression of Ki67 and p16 may be helpful in distinguishing between MPNST and benign peripheral nerve sheath tumor (BPNST). He states that the upregulation of Ki67 and the down- regulation of p16 are highly indicative of MPNST.
The treatment of choice for MPNST is wide surgical excision with adjuvant radiotherapy. The effect of chemotherapy is still questionable, but may be used in some cases. Our patient was treated with both surgery and radiotherapy, and is considering chemotherapy as a precautionary measure. Prognosis remains quite poor, with more aggressive disease in patients with tumors larger than 5 cm, with a histological grade of 2 or 3, with associated NF or metastases. Our patient was not only positive for all three factors, but showed regional lymph node metastasis as well; thus, the prognosis, though yet to be determined, seems quite poor.
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There have been 10 cases of MPNST reported in Korean dermatological literature, all of which are summarized in Table 1. Of these, 7 cases occurred in patients with underlying NF-1. The location of the lesions were quite variable, some occurring on the chest, others on the arms or legs. While most of the lesions were large in size, most cases showed no signs of metastasis, and were thus treated with excision. Metastases were found in only two cases, one of which was treated by excision and lym- phadenectomy. The other was given chemotherapy along with excision of the primary tumor. Despite the fact that wide surgical excision with adjuvant radiotherapy is the treatment of choice, our case is the only reported case of MPNST that has been treated with radiation therapy.
We herein report a case of MPNST arising in a patient with NF type 1, which, although rare, must be considered in the differential diagnosis of similar lesions.