Benign nerve sheath tumors
The main representatives of benign nerve sheath neoplasms are schwannomas and neurofibromas. On MRI (in proton density-weighted imaging [PD] and fat suppression [PDFS] sequences), they exhibit a number of distinctive features:
- Schwannoma is characterized by a fascicular structure. Its key feature is eccentric growth. The tumor displaces the nerve trunk, lying as if to the side of it. Long-standing lesions may develop into a so-called «ancient schwannoma» (ancient schwannoma) with non-enhancing cystic cavities and areas of hemorrhage with fluid levels.
- Neurofibroma is often visualized as an intramuscular mass. The target sign (target sign) is extremely typical for it: on axial PDFS slices, the periphery of the tumor shows a high hyperintense signal, while the center remains dark (low signal). The presence of this sign serves as a reliable marker of a benign process.
Malignant tumors (MPNST)
Malignant peripheral nerve sheath tumors have an aggressive clinical course and are frequently associated with neurofibromatosis type 1 (NF1). In NF1, tumors develop at a younger age, reach large sizes, and have an extremely poor prognosis: the five-year survival rate is only 21% (compared to 42% in the general population). PET is actively used to detect areas of malignant transformation in large plexiform neurofibromas.
Clinical red flags of malignant transformation include sudden rapid growth of the lesion, a change in its consistency, as well as the onset of neurological deficits and persistent pain syndrome.
MRI signs of malignancy:
- Large size, lobulated structure, and marked heterogeneity of the lesion.
- Loss of the benign target sign.
- Peripheral rather than central contrast enhancement.
- Presence of necrotic zones (non-enhancing) and areas of hemorrhage (hyperintense signal on T1-weighted images).
- Thickening and abnormal contrast enhancement in the distal or proximal parts of the affected nerve, as well as the formation of separate satellite tumor nodules.
Pseudotumors: ganglions and neuromas
Tumor-like lesions (pseudotumors) are just as common as true neoplasms and have a completely different nature:
- Intraneural ganglion (Intra-Neural Ganglion). This is a lobulated, fluid-filled cystic lesion that most commonly affects the common peroneal nerve in the knee region. The pathogenesis is related to joint degeneration: synovial fluid penetrates through a capsular defect into the epineurium of the articular branch, dissects along the course of the nerve, and compresses the fascicles. On MRI, the ganglion shows fluid-like signal characteristics. It may be accompanied by signs of muscle denervation. Treatment often requires not only nerve decompression but also excision of the affected joint to prevent recurrence.
- Traumatic neuroma (Traumatic Neuroma). It forms after nerve transection (e.g., following amputation). Unsuccessful attempts at axonal regeneration involving macrophages and Schwann cells lead to a bulbous (bulbous) thickening at the end of the stump. Clinically, it manifests as sharp pain upon percussion (Tinel's sign).
Morton's neuroma and other lesions
There are other, more specific forms of tumors and pseudotumors:
- Morton's neuroma. This is a pseudotumor representing an area of perineural fibrosis in the interdigital spaces of the foot. It is most commonly localized between the heads of the 3rd and 4th (less commonly 2nd and 3rd) metatarsal bones. It usually occurs in middle-aged women. On ultrasound, it appears as a hypoechoic zone that shifts upon compression of the forefoot. On MRI (T1-weighted images), the lesion exhibits low signal intensity, reflecting its fibrous nature, and contrasts well with the surrounding adipose tissue.
- Lipofibromatous hamartoma of the nerve. Formerly known as fibrolipomatous hamartoma. According to the WHO classification, it belongs to adipocytic tumors. It most frequently affects the median nerve and may be accompanied by macrodactyly. On MRI (axial T1-weighted images), it has a pathognomonic appearance: thickened nerve fascicles are surrounded by an elongated array of adipose tissue.
- Myxoma. A tumor of uncertain differentiation. Histologically, it is an accumulation of uniform spindle cells embedded in an abundant gelatinous (myxoid) stroma.
Briefly: answers to topical questions
What histological structural types are distinguished in schwannoma?
Microscopically, schwannomas exhibit two main tissue components.
- Antoni A type — areas of densely packed spindle cells with elongated hyperchromatic nuclei, forming palisading structures and pathognomonic Verocay bodies (Verocay bodies).
- Antoni B type — areas with a loose, myxomatous, and less cellular structure, with a haphazard cellular arrangement against the background of an edematous, microcysically altered stroma.
The ratio of these zones can vary widely within the same tumor and between different nodules.
What is the surgical strategy for treating an intraneural ganglion?
The surgical strategy for treating an intraneural ganglion (Intra-Neural Ganglion) consists of decompressing the affected nerve and eliminating the source of pathology. Since the pathogenesis is related to the escape of synovial fluid from a degenerated joint through a capsular defect into the nerve trunk, excision of the affected joint, in addition to nerve decompression itself, may be required to prevent recurrence.
What are the diagnostic criteria for neurofibromatosis type 1?
Diagnostic criteria for neurofibromatosis type 1 are divided into major and minor.
- Major criteria (Major Criteria) — café-au-lait spots (Café au lait spots), freckling (Freckling) in the inguinal or axillary regions, one plexiform or two any neurofibromas, optic pathway glioma (Visual pathway glioma), two or more Lisch nodules (Lisch nodules) on the iris, specific bone lesions, and an affected first-degree relative.
- Minor criteria (Minor Criteria) — short stature (Small stature), macrocephaly, scoliosis, pectus excavatus (Pectus excavatus), hamartomatous lesions (Hamartomatous lesions), and neuropsychological abnormalities.
The diagnosis is established in the presence of at least two major criteria.
SNET: Schwannoma — Eccentric, Neurofibroma — Target sign.
Class and exam questions
What is the target sign on MRI and what is it characteristic of?
This is an MRI sign (on PDFS images) where the periphery of the lesion shows a high signal and the center shows a low signal. It indicates the benign nature of the nerve sheath tumor and is most typical for neurofibroma.
What is the pathogenetic cause of intraneural ganglion formation?
The pathology arises due to a capsular defect of a degenerated joint. Synovial fluid penetrates into the epineurium of the articular nerve branch and dissects along its trunk, compressing the nerve fascicles.
How to identify an amputation neuroma clinically and on imaging?
Clinically, it manifests as pronounced pain upon percussion (positive Tinel's test). On MRI and ultrasound, a traumatic neuroma appears as a bulbous thickening at the end of the transected nerve.
What clinical and MRI signs indicate a malignant peripheral nerve sheath tumor (MPNST)?
Malignancy is indicated by rapid growth, the onset of pain, and neurological deficits. On MRI, this manifests as heterogeneity, loss of the target sign, the presence of necrosis, hemorrhage (high signal on T1), and peripheral contrast enhancement.
What else to study on this topic
- The role of PET imaging in neurofibromatosis type 1
- Differential diagnosis of extraneural and intraneural ganglions
- Features of surgical treatment for an intraneural ganglion
- MRI findings and pathogenesis of «ancient schwannoma»
- Histological classification of tumors of uncertain differentiation (myxoma)
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