Schwannomas represent approximately 5 % of all benign soft tissue neoplasms [47]. They can occur at all ages, though they are uncommon in children. While most literature data indicate that schwannomas are most prevalent between the ages of 20 and 50, others have observed a bimodal age distribution with peak incidences in the third and seventh decades [62]. Men and women are equally affected. Schwannomas associated with neurocutaneous syndromes tend to occur in younger patients [24].

Schwannomas most commonly involve the major nerve trunks of the head, neck, and limbs. The flexor surfaces of the extremities are more frequently affected, especially near the elbow, wrist, and knee [24, 62, 76]. Consequently, the most common topographical locations of schwannomas include the spinal roots and cervical plexus and the vagus, peroneal, and ulnar nerves. Deeply situated schwannomas are predominantly found in the posterior mediastinum and the retroperitoneum [24, 62]. Relatively few schwannomas appear on the trunk, which contrasts with the distribution of neurofibromas in neurofibromatosis type 1 (NF1). Other sites of involvement include the scalp and the hands; the feet are usually spared. Less frequently, schwannomas have been recorded in the tongue, palate, and larynx.

Small schwannomas are usually asymptomatic. Pain, paresthesias, muscle atrophy, or other symptoms can occur when the tumor reaches sufficient size to compress the involved nerve or adjacent structures. On physical examination, they can be moved from side to side, but not along the long axis of the nerve [38].

Neurofibromas represent approximately 5 % of benign soft tissue neoplasms [47]. There is no obvious predilection for any racial or ethnic group to develop neurofibromas, and these lesions have been described throughout the world [38].

Three types of neurofibromas are classically described: localized, diffuse, and plexiform. Localized neurofibromas are the most common type, representing approximately 90 % of these lesions. The vast majority of localized neurofibromas are solitary and not associated with NF1 [24]. They develop mostly between the ages of 20 and 30. Like schwannomas, they affect both sexes equally [62]. Diffuse neurofibromas are an uncommon but distinctive type that primarily affects children and young adults. The majority of diffuse neurofibromas are isolated lesions not associated with NF1. Plexiform neurofibromas are essentially pathognomonic of NF1. Development of these lesions usually occurs in early childhood. Plexiform neurofibromas have a potential for malignant degeneration and are recognized as precursors for malignant PNSTs in NF1 patients [3].

Most localized, solitary neurofibromas are superficial lesions of cutis or subcutis (Fig. 17.7) [62]. Neurofibromas in NF1 are relatively frequent on the trunk, which contrasts with the distribution of peripheral nerve schwannomas, which are more frequent in the extremities [76].

Diffuse neurofibromas demonstrate a plaque-like elevation of the skin with thickening of the entire subcutis.

Plexiform neurofibromas may involve any or all of the following sites: the cranial and spinal nerve roots and ganglia; the major nerves of the neck, trunk, and limbs, including the sympathetic system and its ganglia; and the subcutaneous branches of the major nerves and the visceral sympathetic plexuses [76]. Below the diaphragm, plexiform neurofibromas are typically located in the retroperitoneal space and paraspinal region. Typical retroperitoneal localizations are (bilateral) parapsoatic and presacral neurofibromas. Less commonly, retroperitoneal neurofibromas are found near the celiac axis and near the origin of the superior mesenteric artery [6]. Involvement of the myenteric plexuses and mesentery of the gastrointestinal tract by plexiform neurofibromas is rare. Neurofibromas in this location cause pain, intestinal bleeding, and obstruction. The regional nerves are enlarged, and nodules of varying size are found, especially in Auerbach’s plexus. The most common site is the jejunum, followed by the stomach [35].

Typically, localized neurofibromas (Table 17.3) are benign, slowly growing, and variably encapsulated neoplasms [14, 38, 62]. Neurofibromas are intimately associated with the parent nerve, growing in a longitudinal fusiform manner with the nerve entering and exiting from the lesion. Surgical resection requires sacrificing the parent nerve because the neurofibroma cannot be separated from the nerve fibers [22].

As opposed to schwannomas, most neurofibromas are solid tumors macroscopically: areas of cystic degeneration, hypocellularity, and xanthomatous material are uncommon [76]. Neural axons traversing the tumor often follow a tortuous course. As in schwannomas, the capacity of neurofibromas for melanogenesis, although rare, has been well documented [38].

Plexiform neurofibroma relates to a diffuse enlargement and distortion of a major nerve trunk. Disorganized growth of nerves is noticed within and occasionally also outside the nerve trunk [62]. Multiple masses are found along the course of nerve bundles or trunks, with irregular cylindrical enlargement of the affected nerves [86]. Fusiform or spherical neurofibromas along their course can produce a moniliform effect or sometimes give rise to massive growths of a similar character [76].

In many neurofibromas, the peripheral zone contains rather myxoid tissue with high water content, while the central area contains more dense collagenous tissue [86]. This concept is important to understand because it is presumed to be the histologic correlate of the so-called target appearance of neurofibromas on US and MR imaging studies.

Solitary neurofibromas present as relatively non-tender masses in the skin or subcutaneous tissue. They are somewhat mobile and moderately firm but compressible. Isolated neurofibromas in the skin usually do not exhibit an associated local change in pigmentation of the skin, although hyper- or hypopigmentation is possible. If solitary neurofibromas occur along the course of a peripheral nerve, the clinical symptoms are usually related to a space-occupying lesion but can be similar to those described for schwannoma [38]. The clinical presentation of NF-associated neurofibroma will be discussed below (see Sect. 17.4).