Background
Musculoskeletal fibromatoses are fibrous tumours that result from an abnormal proliferation of fibroblasts [1]. Fibromatoses are generally divided into superficial (palmar/plantar) and deep forms (desmoid) and are listed on the WHO classification of soft tissue tumours under the category “fibroblastic/myofibroblastic tumours” [2].
Extra-abdominal desmoid-type fibromatosis typically manifests as a non-encapsulated and poorly circumscribed solitary soft-tissue mass, with spiculated and infiltrative margins, that can extend along or cross fascial boundaries [3,4]. On microscopy, they consist of poorly defined fascicles of uniform spindle cells and fibroblasts, interlaced in a dense collagen stroma [3,4]. These tumours grow slowly and do not metastasise, but their tendency for local recurrence has earned them the designation of “aggressive” fibromatosis [1,3,5,6]. Multicentric disease is possible [7].
Imaging Perspective
Imaging findings of extra-abdominal desmoid-type fibromatosis are non-specific, and biopsy is needed in virtually all cases. Still, imaging plays a role in mapping the extension of the disease and determining the relationship with surrounding structures [4,5]. Occasionally, some diagnostic clues can be present.
Ultrasound is often the first imaging modality, but the findings are variable and poorly described. Fibromatosis can manifest as an ill-defined, hypoechoic soft-tissue mass with hypervascularity on Doppler examination [4].
On CT, the lesions can show variable attenuation on non-contrast studies and marked post-contrast enhancement but, due to its infiltrative growth, the margins are often indistinct on this modality. Bone pressure erosions, cortical scalloping without invasion of the medullary canal and dystrophic or psammomatous calcifications can be seen in some cases [6–8].
MRI is the core imaging technique and generally shows an intermuscular-seated mass with infiltrative characteristics, sometimes with a peripheral rim of fat (split-fat sign) or extension along fascial planes (fascial tail sign) [4,6]. Signal intensity on MRI varies according to the extent of collagen and cellularity of the lesion, but these lesions frequently present intermediate signal intensity on both T1 and T2-weighted sequences [4]. On post-contrast imaging, the tumours generally demonstrate moderate to marked enhancement [7]. Non-specific internal low signal intensity bands can occasionally be seen on T2 or post-contrast T1-weighted images and can support the diagnosis [4]. Despite not having been extensively reported in the literature, these lesions have been shown to enhance progressively, with no delayed washout on dynamic contrast-enhanced imaging (DCE) [9], resulting in type II time-intensity curves which are typically associated with benign conditions. Diffusion-weighted imaging (DWI) characteristics have also not been extensively reported, but some studies show that desmoid tumours mean ADC is significantly higher than that of malignant soft tissue tumours, suggesting that DWI could be useful to differentiate desmoid tumours from malignant soft tissue tumours [10,11].
Differential diagnosis
In this case, there were bilateral solid lesions of the masticator spaces that were considered more likely to have a soft-tissue than bone origin due to their location, bilaterality and pattern of bone involvement, the latter suggesting a slowly growing process that was attributed to pressure erosion from the soft-tissue masses. Nevertheless, the diagnosis of a non-malignant fibroblastic/myofibroblastic tumour was difficult to establish because of the infiltrative behaviour of the lesions and the lack of clear growth along the fascial planes.
Based on the imaging appearance, the main differential diagnosis was a soft-tissue sarcoma, but this could be considered less likely given the fact that there was a one-year history of a slowly increasing left preauricular swelling and that on imaging, the lesions were bilateral. As described, DCE and DWI MRI imaging could have been helpful in favouring benign aetiology.
Mandibular Langerhans cell histiocytosis could be a possible diagnosis as these lesions can be clinically asymptomatic, typically occur in paediatric patients, may be multifocal, and commonly present with lytic bone and soft-tissue components. However, there is usually a more aggressive pattern of mandibular bone involvement, with a permeative involvement of the alveolar bone, which can progress to show the “floating tooth” sign.
Osteomyelitis or other primary bone tumours, such as Ewing sarcoma, were considered unlikely due to the temporal evolution of the clinical symptoms, lack of signs of infection, bilaterality, and pattern of bone involvement.
Lymphoma could be a diagnostic consideration, particularly when contemplating the encasement of the left facial artery, but the overall infiltrative nature of the lesions and the lack of concomitant significative lymph node enlargement would make it less likely.
Soft-tissue sarcoidosis, despite being rare, should be included in the differential diagnosis as it can have odd imaging appearances and be responsible for infiltrating soft-tissue masses.
Outcome
Surgery is considered the mainstay of treatment, but minimally-aggressive treatment strategies are generally preferred in children. The therapeutic strategy should be defined by a multidisciplinary team and the “wait-and-see” strategy or the use of low-dose chemotherapy when systemic treatment is required are frequently favoured to avoid repeated resections or destructive surgery [1].
All patient data have been completely anonymised throughout the entire manuscript and related files.
