Discussion

Choristoma is a term used to describe abnormally located normal tissue; neuromuscular choristoma (NMC) is a rare peripheral nerve lesion presenting with well-differentiated smooth-muscle cells admixed with nerve-fascicles [1, 2]. Patients with NMC can develop close-ranged desmoid-type fibromatosis (DTF) spontaneously or after invasive biopsy or resection [3, 4]. DTF is an infiltrative myofibroblast neoplasm lacking metastatic potential that can compress or invade nervous structures [5].

In our patient, contrast-enhanced CT (CECT) revealed two nodules in the left thigh and a sciatic foramen mass related to the sciatic nerve associated with mild atrophy of the thigh muscles (Fig. 1). Thigh nodules displayed inhomogeneous density and contrast enhancement (Fig. 1B, C, D) on CECT and mild uptake on [FDG18]-PET-CT (SUVmax 4.6) (Fig. 2). Contrarily, the sciatic foramen mass showed nor enhancement on CECT (Fig. 1A, D) neither remarkable uptake on [FDG18] PET-CT (Fig. 2).

Fig. 1
figure 1

Axial non-contrast and contrast-enhanced CT images through the pelvis and the proximal thighs at the equilibrium phase demonstrate a sciatic foramen mass (asterisk) related to the sciatic nerve without contrast enhancement (A, D) and two nodules in the left thigh (white arrows), the larger occupying the posterior muscle compartment, the smaller located between adductor magnus and semimembranosus muscles. Both nodules show inhomogeneous density and contrast enhancement (B, C, D)

Full size image
Fig. 2
figure 2

[FDG18] PET-CT through the pelvis and the proximal thighs demonstrating poor and mild uptake of [FDG18] PET-CT of the sciatic foramen mass (asterisk) and of the thigh nodules (white arrows) (SUVmax 4.6)

Full size image

On MRI, thigh lesions showed an infiltrative pattern, low signal on T1-weighted images, and mild hyperintensity on T2-weighted FSE and T2*-weighted GRE images, along with an hypointense interspersed collagenic component (Figs. 3 and 4). Imaging features and multifocality are highly suggestive of fibromatosis. The sciatic foramen mass was a well-circumscribed fusiform enlargement of the sciatic nerve with an MR signal similar to muscle on all the sequences (Figs. 3 and 4).

Fig. 3
figure 3

MRI through the pelvis and proximal thighs demonstrates a fusiform and striated enlargement of the sciatic nerve (asterisk) with signal similar to muscle on all the sequences (A, B, C, D), without loss of signal intensity between the in-phase and out-of-phase images (E, F). The thigh nodules (white arrows) show a low signal on T1-weighted images (A, B) and heterogeneous mild hyperintensity along with hypointense components on T2-weighted images (C, D)

Full size image
Fig. 4
figure 4

MRI through the thighs. Sagittal and axial reformatted images of 3D T2-weighted spin echo (A, B, D), demonstrate hypointensity of the thigh lesions (white arrows). Axial T2*-weighted gradient-echo (C) reveals an interspersed hyperintense component within the upper lesion

Full size image

Based on imaging findings, the diagnosis of thigh fibromatosis associated with NMC of the sciatic nerve was proposed.

A biopsy of the sciatic lesion demonstrated mature skeletal muscle fibers within the nerve (Fig. 5A). The resection of the largest thigh mass revealed elongated fascicles of bland uniform spindle cells embedded in an abundant collagenic matrix (Fig. 5B). Thus, the diagnosis of DTF associated with NMC was confirmed.

Fig. 5
figure 5

Histopathological photomicrographs (hematoxylin and eosin staining) of the sciatic nerve showing mature skeletal muscle fibers (asterisk) (A) and of the thigh lesion characterized by elongated fascicles of bland uniform spindle cells (arrows) embedded in an abundant collagen matrix (B)

Full size image

The NMC usually affects patients in early childhood with typical soft tissue mass with associated nerve-territory muscular atrophy, bone dysplasia, and limb length asymmetry [4]. The sciatic nerve and the brachial plexus are most frequently affected, more rarely cranial nerves or small peripheral nerves [6].

MRI is the most useful technique to diagnose this entity. NMC characteristics include well-circumscribed fusiform enlargement of the involved nerve, signal similar to skeletal muscle on all the sequences, absent or mild heterogeneous enhancement after contrast administration; usually, no cystic spaces, no intralesional fat component > 50%, and no nerve territory overgrowth are observed. Fibrous tissue can be recognized as longitudinal bands of low T1 and T2 signals. Additionally, minimal or no FDG uptake on PET/CT is described for NMC [7].

Imaging features of NMC may appear similar to that of fibrolipomatous hamartoma (FLH). Unlike NMC, FLH is characterized by an intralesional fat component > 50% and nerve territory overgrowth, and it is not associated with DTF [2]. The differential diagnosis must also include neurofibroma and lymphoma. Neurofibroma can be easily diagnosed due to the very high intrinsic T2 signal and focal enlargement of the soft tissues, ruling out NMC. Lymphomatous involvement of the nerve shows increased T2 signal compared to the muscle and usually demonstrates post-contrast enhancement [7].

The pathogenesis of the association between NMC and DTF is unclear. It has been hypothesized that NMC arises from the proliferation of a pluripotent precursor, which primes the contiguous myofibroblasts to develop DTF [3]. Another theory suggests that DTF development may be related to skeletal muscle degeneration induced by the lack of trophic stimuli due to NMC-related neural components’ alterations [8]

On imaging, DTF appears as an infiltrative mass-like lesion according to the different intralesional compositions. The variable distribution of myxoid matrix, cellular stroma, and fibrous tissue/collagen bands results in low-to-intermediate T1 signal and low-to-high T2 signal, variable gadolinium enhancement, and a mild-to-moderate FDG-uptake on PET-CT [9].

Since surgery may promote DTF development, the standard treatment for NMC associated with DTF usually consists in loco-regional or systemic chemotherapy and radiotherapy [3].

In conclusion, the radiological features and association between NMC and DTF should be known by radiologists in order to search for and recognize NMC proximally to a DTF-like mass.