Malignant peripheral nerve sheath tumor (MPNST) is a type of spindle cell sarcoma originating from the peripheral nerve, which usually results in the corresponding nerve sign on magnetic resonance imaging (MRI). Patients with MPNST may also have neurofibromatosis type 1.

A 78-year-old male was admitted to the hospital due to a tumor in his left knee. He had a previous history of superficial spreading melanoma on the left thigh. Color Doppler ultrasonography showed a hypoechoic mass in the subcutaneous soft tissues of the medial left knee with an abundant rich blood flow. Computed tomography scanning did not show obvious signs of bone destruction, but the skin adjacent to the tumor was slightly thickened. MRI examination revealed that the hypervascular lesion was well-circumscribed, lobulated, invaded the surrounding soft tissues and demonstrated heterogeneous enhancement but lacked an entering and exiting nerve sign. The MRI result indicated the invasiveness of the tumor. The patient underwent a left knee joint mass expanded resection and the first histopathological examination showed a MPNST with positive surgical margins. Therefore, the second extended resection was performed, and the patient had a good outcome in the short term.

MRI is a useful technique for revealing the biological characteristics of MPNST and provides clinical support for evaluation of the surgical area before operation.

Malignant peripheral nerve sheath tumor (MPNST) is a type of spindle cell sarcoma originating from the peripheral nerve that is secondary to neurofibromatosis, or heterogeneous differentiation components of the nerve sheath are observed under the microscope. Hence, MPNST was previously called neurogenic sarcoma and neurofibrosarcoma[1]. In some studies, patients who did not have a history of neurofibromatosis type 1 (NF-1) had a better prognosis than those who did have NF-1[2,3]. This case report describes an elderly man who had previously undergone resection of a melanoma on his left thigh, and several years later an MPNST occurred in his left knee joint without local or systemic metastasis. This report describes the diagnosis and management of this patient and provides a literature review.

Clinical features and all imaging results, especially MRI result, suggested the possibility of malignance of the mass of the left knee joint.

After careful evaluation of the patient’s symptoms, previous medical history and the MRI finding of peripheral tissue edema, the surgeon decided to perform an extended resection of the left knee joint mass. The first frozen section report showed a spindle cell tumor, which was approximately 10.5 cm × 5.0 cm ×2.7 cm in size with positive surgical margins. Hence, the scope of surgical resection was expanded again to the 2 cm of the remaining surrounding tissues, and then the second frozen section report showed negative surgical margins. The postoperative pathological report (Figure 4) suggested MPNST, which was positive for vimentin S100, CD99 (partial positivity) and Ki-67 (approximately 40% positivity in the hot spot area) and negative for HMB 45 and Melan-A.

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Figure 4 Postoperative pathological examination. A: Hematoxylin-eosin staining photomicrograph (original magnification × 200) of the mass showed tightly packed spindle cells; B: Immunofluorescence examination (original magnification × 200) demonstrated S100 positivity on immunohistochemical analysis.

The patient was in a stable condition without any related complications after operation. He was discharged from the hospital with recommendation of the adjuvant radiotherapy of the surgical area 2 wk after removal of the stitches. He received radiotherapy in another hospital and recovered well according to telephone follow-up at 3 mo after discharge.

MPNST is an uncommon sarcoma and usually demonstrates aggressive biological behavior. In general, appropriate surgery, radiation and chemotherapy are the conventional clinical treatments. However, large tumor size (especially > 5 cm), positive excision margins, high-level tumor grade, association with NF-1, local recurrence and metastases are predictive prognostic markers of inferior consequences of this disease[4]. A small minority of limb MPNSTs are found in the absence of NF-1. In Jordan, a recurrent MPNST occurred at the same site on the forearm of a 51-year-old man who had no preexisting NF-1 but had a positive family history of cancer[5]. Similarly, an elderly man, without a previous diagnosis of NF-1 or positive family history, presented with a giant MPNST in the left axilla and minute lung metastasis before surgery[6]. Both patients died several months after surgery due to the invasive behavior and poor prognosis of MPNST.

Compared with these two cases, tumor size in our patient was smaller, and he received early surgical intervention before the occurrence of symptoms or metastasis. His positive history of melanoma may have been related to the occurrence of MPNST, which is a good reminder for clinicians. It is noteworthy that the MRI result revealed the invasiveness of the tumor and provided clinical evidence for an extended resection. Moreover, surgical resection is insufficient for the management of MPNSTs. Some researchers have proposed that neoadjuvant radiotherapy could improve disease control and total survival[1,7,8]. This may be the reason why this patient had a good postoperative recovery.

In terms of radiological features, it is generally accepted that all peripheral nerve sheath tumors originate from a single nerve branch[9-11], which leads to corresponding imaging signs. However, in this case, the lesion lacked an entering and exiting nerve sign or a thickened nerve nearby on any of the radiological images. In compliance with our findings, Van Herendael et al[11] suggested that the presence of a nerve sign was less frequent in malignant neurogenic tumors on MR images. There are also other MRI characteristics of MPNSTs, for example large size (> 5 cm), ill-delineated edges, perilesional edema and internal cystic degeneration/necrosis. Combined with the patient’s history, our finding of an abundant blood supply in the lesion was indicative of a malignant tumor. Moreover, the presence of unenhanced focal areas on sagittal contrast-enhanced T1WI in this case was probably caused by intralesional necrosis and hemorrhage[10,11]. Due to the rarity and unspecific imaging manifestations of MPNST, clinical manifestations combined with postoperative pathological results are important for diagnosis of the disease. Specific pathological features, including tightly packed spindle cells with neoplasm necrosis, invasive growth into surrounding tissues and alternating loose and dense cellular areas as well as S100 protein and Ki-67 positive staining[12] were found in our case.

This report describes a patient with a solitary MPNST and a history of melanoma in the lower extremity, without radiological evidence of genetic origin. Clinicians should be aware of the possibility of MPNST in patients with a history of other epithelioid tumors. MRI results could reveal the biological characteristics of the MPNST. Prompt intervention such as extended excision plus postoperative radiotherapy could be beneficial to the outcome of this rare tumor.

We thank our colleagues at the Department of General Surgery and Department of Pathology, Zhuhai Hospital Affiliated with Jinan University, Zhuhai People’s Hospital.