Surgery for Malignant Peripheral Nerve Sheath Tumors

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CHAPTER 245 Surgery for Malignant Peripheral Nerve Sheath Tumors

Malignant peripheral nerve sheath tumors (MPNSTs) are an uncommon variety of soft tissue sarcoma of ectomesenchymal origin. The World Health Organization (WHO) coined the term MPNST to replace previous heterogeneous and often confusing terminology, such as malignant schwannoma, malignant neurilemoma, neurogenic sarcoma, and neurofibrosarcoma. Although MPNST is now used to identify any malignant tumor arising from a peripheral nerve or its attendant sheath, it does not refer to tumors arising from the epineurium or the vasculature of peripheral nerves.1,2

MPNSTs arise from major or minor peripheral nerve branches3 or sheaths of peripheral nerve fibers4,5 and are derived from Schwann cells or pluripotent cells of neural crest origin.6

Arthur Purdy Stout (1885-1967) played a pivotal role in the development of our current understanding of the pathogenesis of peripheral nerve sheath tumors by identifying the Schwann cell as the major contributor to the formation of benign as well as malignant neoplasms of the nerve sheath.79 Although this remains essentially true, the cell of origin of the MPNST remains elusive and has not yet conclusively been identified. Some have suggested these tumors may have multiple cell line origins.

Epidemiology and Risk Factors

It is estimated that from 5% to 10% of the 6000 soft tissue sarcomas diagnosed in the United States per year are malignant nerve sheath tumors, with an incidence of 0.001% in the general population.10 These tumors occur with equal frequency in males and females, although some series have shown a female preponderance.3,4 There is no racial association. Most studies show that the peak incidence of MPNSTs is in the seventh decade of life in the general population but in the third or fourth decade in people with neurofibromatosis type 1 (NF1),11,12 although these tumors may occur at a much younger age in either population.13

Most MPNSTs occur in patients with NF1, with a cumulative lifetime risk of up to 10%. Nonetheless, a surprising number occur as solitary MPNSTs, unassociated with neurofibromatosis or other predilections such as irradiation. Individuals with NF1 and internal plexiform neurofibromas are 18 times more likely to develop MPNSTs than patients without internal plexiform neurofibromas.14 In the general population, plexiform neurofibromas can undergo malignant transformation to an MPNST with an estimated lifetime risk of 3% to 5%, whereas in NF1 patients, it can be as high as 15% to 20%. The dermal neurofibromas seen in NF1, although more numerous and a more troubling cosmetic problem, do not undergo malignant transformation. Only rarely do MPNSTs arise from malignant degeneration of a schwannoma, ganglioneuroma, or pheochromocytoma. Ten percent of these tumors occur in patients who have undergone radiation treatments for other diseases, and they occur on average 15 years after the treatments.15

The effect of radiation on peripheral nerves was described initially in animal experimental work by Bergstrom and Cavanagh.16,17 The incidence of radiation-induced MPNSTs reported in large series ranges from 5.5% to 11%.6,13,18,19 There are also several published reports relating to patients with and without NF1 and radiation treatments.13,20,21 Ducatman and associates13 described 12 patients with postirradiation MPNSTs, 7 of whom had NF1. Two of the 7 had received radiation therapy for optic gliomas 5 and 17 years previously. More recently, Loree and colleagues21 described two of four NF1 patients who developed MPNSTs after head and neck irradiation, whereas Baehring reported such after radiation for Wilms’ tumor and Hodgkin’s disease.6

Diagnosis

The diagnosis of these tumors remains problematic because it is based primarily on clinical suspicion. As with any patient, a history and physical examination are the place to begin the assessment for a peripheral nerve tumor. In the history, special note should be made of when the mass, if palpable, was noticed, and the onset of symptoms such as pain and motor or sensory deficit. Rapid increase in the size of a mass or rapid onset of symptoms should immediately alert the surgeon to the possibility of a malignancy (Fig. 245-1). A patient with a known history of NF1, neurofibromatosis type 2 (NF2), or schwannomatosis who presents with a tumor that shows recent rapid increase in size, a new or progressive neurologic deficit, or pain should alert the examiner to a suspected malignant degeneration. The examiner should question and record the location, quality, and radiation of pain. The location and extent of motor weakness, if present, and the location and extent of sensory deficit should be defined and recorded. A family history of peripheral nerve problems or any other genetic disorders should be closely questioned, and a history of previous radiation treatments should be discussed. Systemic diseases or any preexisting conditions that can contribute to peripheral nerve problems should also be questioned (i.e., diabetes mellitus, cancer). Any recent illnesses, even those as seemingly minor as flu, should be questioned and recorded. Because many prescription medications can cause peripheral neuropathies, a medication history should also be recorded.

During the physical examination, special attention should be given to examining for the presence of café au lait spots, axillary freckling, inguinal freckling, and Lisch nodules (pigmented iris hamartomas), which can indicate the presence of a genetic disease such as neurofibromatosis. Any spinal scoliosis that may indicate the possibility of intraforaminal tumors distorting the spinal column should also be noted. All four extremities should undergo a complete motor examination with standard motor strength grading as well as a sensory examination. Reflexes should be tested, and the patient should be examined for the presence of Tinel’s sign at the area of the mass or suspected tumor. The distribution and extent of Tinel’s sign should also be noted because it will offer clues as to which nerve is involved. If a mass is palpable, the size, quality, and mobility should be noted. Traditional teaching relates that a nerve tumor is mobile from side to side but not along the length of the nerve proximally and distally. The likelihood of malignancy is increased with increasing tumor size, a consistency that is hard to palpation, and a mass that is fixed to the surrounding soft tissue.

Imaging

The “gold standard” for imaging of peripheral nerve tumors has become magnetic resonance imaging (MRI). If a solitary palpable mass is encountered clinically in a patient and a peripheral nerve tumor is suspected, an MRI of the involved extremity, plain and with contrast, is indicated. If there are multiple tumors palpable, or if an indication of neurofibromatosis or schwannomatosis is noted on examination, the imaging should be more comprehensive, including a full spine series to define any spinal or foraminal masses. Contrast images should always be ordered to evaluate the enhancing quality of the mass. Information on the enhancing qualities of the mass, combined with its appearance on T1- and T2-weighted images, can give valuable clues to the histopathology that may be encountered.

On any contrast-enhanced image that is being performed on a peripheral nerve tumor, fat suppression sequences should be used to better define the nerve in question. Newer techniques using magnetic resonance neurography (MRN) have the potential to offer enhanced visualization and definition of peripheral nerve mass lesions and produce higher-resolution images of the nerves with greater separation from the surrounding soft tissue. Proper MRN requires special phased-array surface coils and radiologists familiar with the short tau inversion recovery (STIR) sequences necessary to obtain the desired images.

MRI can contribute significant information about the suspected pathology. This information is extremely useful preoperative information. Unfortunately, whether a tumor is benign or malignant cannot be discerned definitively from the image alone (Fig. 245-2). Areas of hemorrhage or necrosis, heterogeneous enhancement, and cystic areas may suggest a malignancy but are no means definitive and can even be seen in benign tumors.

Careful assessment of the images should include surrounding blood vessels and nearby vital structures and whether any infiltration of these surrounding structures is present.

Positron emission tomography (PET) with the glucose analog 18FDG is a dynamic imaging technique that permits the visualization and quantification of glucose metabolism in cells and reflects the increase in metabolism in malignant tumors.22,23 A retrospective study of 18 NF1 patients demonstrated that 18FDG-PET is a potentially useful, noninvasive method for detecting malignant change in plexiform neurofibromas.22 However, the distinction between low-grade MPNSTs and benign plexiform neurofibromas was not clear in all the cases. The new tracer 18F-thymidine, which detects DNA turnover, might be helpful in distinguishing low-grade MPNSTs from active, benign plexiform neurofibromas in future PET-based studies.24

Nerve conduction and electromyography (EMG) are generally not required in the preoperative evaluation of MPNSTs because they are not diagnostic, nor do they influence management. They may, however, be useful in documenting baseline neurological function.

Treatment

When the diagnosis of MPNST is suspected, surgery is the mainstay of treatment of these tumors.2426 Resectability depends largely on location and ranges from 20% in paraspinal MPNSTs to 95% in tumors of the extremity.2,6,13,18,19,27,28 The ultimate aim of surgery is complete removal of the lesion with tumor-free margins.13,29

There are many different approaches to these tumors, some of which are controversial. Total excision, subtotal debulking, and biopsy may all be considered. Because the surgical resection to clean margins may require sacrifice of vital nerve and soft tissue structures, some surgeons advocate fine-needle biopsy first, before definitive surgery. Other surgeons believe that a fine-needle biopsy may miss malignant cell rests and lead to subsequent misdiagnosis or allow for spread of tumorigenic cells and therefore avoid this approach. A modification of this approach uses an open biopsy of multiple (at least four) quadrants of the tumor. Still others advocate a staged approach whereby first a gross total resection of the tumor is performed with care taken not to violate tumor capsule. A full pathologic examination of the entire tumor tissue ensues, and if malignancy is proved histologically on permanent sections, the patient is returned to surgery, in a timely fashion, for definitive oncologic surgery. The margins of the resection site and surrounding tissue are then explored, with sampled tissue sent for frozen section to be examined pathologically. As this surgery progresses, the definitive operation to clean margins to at least 2 cm in all directions is performed.

This last approach, in our opinion, allows for a thorough pathologic examination of the initial tumor tissue so that nothing is overlooked, a situation that may not be possible if only a small biopsy sample is taken. Furthermore, it allows the surgeon to discuss with the patient, in the interval between the two operations, more accurately what can be expected and the greater likelihood of neurological deficit after surgery as diseased nerve and soft tissue are removed.

Unfortunately, for neurogenic sarcomas that involve brachial or pelvic plexus or the proximal portion of the arm, a wide resection to clean margins is not accomplished without paralysis or even limb loss necessitated by vascular supply sacrifice. Thus, wide local resection appears to work better for neurogenic sarcomas involving the more distal portions of the limb. For more proximal lesions, amputation of the limb may be required, but many patients, given the cosmetic deformity and overall poor prognosis of these tumors, often refuse such recommendations (Fig. 245-3).

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