Fibromatosis, also known as desmoid tumors , is a rare type of neoplasm; its estimated annual incidence is 900 patients in the USA . Despite its low metastatic potential, local invasion can lead to deformities and mutilating surgeries. They result in significant psychological and economic effects on the lives of young adults [5, 6].
Most common sites include the abdominal wall and cavity, chest wall, scapular area, and the limbs, rarely affecting the axilla . Less frequently, intra-abdominal forms are associated with familial adenomatous polyposis or Gardner’s syndrome, usually affecting the mesentery. It is a condition always to be within the differential diagnosis of soft tissue tumors on imaging [1, 2].
Risk factors include the first year after pregnancy and the use of oral contraceptives . Pathophysiology relies on the excess of intracellular beta-catenin, a molecule fundamental to intercellular adhesion of mesenchymal cells .
Differential diagnosis include nodular fasciitis, fibroma of the tendon sheath, tumors of the neural sheath, fibroadenomas, dermatofibroma, and leiomyoma .
Imaging has a major impact on therapeutic and surgical planning. Aspects such as the infiltrative nature of the tumor, margins for resection, and recurrence rates are determinant in morbidity and in the impact on quality of life [5, 6].
US is a non-expensive and highly accessible initial method. Findings include a regular or irregular heterogeneous solid mass, circumscribed or with poorly defined margins . Echogenicity is variable and closely related to histological heterogeneity. Color Doppler vascularization is also variable, more commonly absent .
On computerized tomography (CT), fibromatosis appears as a mass with soft tissue density, with different degrees of attenuation according to histological composition, with well-defined or ill-defined margins. A higher proportion of collagen increases tomographic density. This is particularly useful on clinical follow-up, since the increase in density reflects a good response to treatment .
Necrosis and calcification are not expected. Bone involvement occurs in 5 to 30% of cases, with a greater role for computed tomography in these situations .
MRI currently represents the imaging modality of choice in the evaluation of soft tissue injuries of the extremities and trunk .
MRI findings include heterogeneous iso- to hyperintense masses on T1- and T2-weighted sequences. Signal is highly variable, reflecting the proportion of cellular and fibrotic components of the tumor. The hypointense areas on T2 represent high cellularity and dense collagen, and the hyperintense areas in T2 represent low cellularity [1, 9].
Classical signals are described, such as the band sign, which consists of the presence of hypointense lines or bands in T1 and T2, without enhancement; the tail signal, which like the ultrasound, reflects the infiltrative nature of the tumor, forming elongated extensions; the split fat sign, represented by a tenuous annular enhancement of the adipose planes adjacent to it, and, finally, the flame sign, which consists of linear enhancements extending across the fascial planes .
The main therapeutic modality is excision. Desmoid tumors are associated with high levels of recurrence, especially when surgical margins are compromised . Other more modern options include radiotherapy, anti-estrogenic drugs, non-steroidal anti-inflammatory drugs, cytotoxic chemotherapy, and tyrosine kinase inhibitors .
Macroscopically, it is a whitish, shiny tumor, containing thick trabeculae resembling scar tissue, some arranged in a swirl, in addition to strands of fibrous tissue infiltrating the adjacent musculoadipous planes .
On microscopic analysis, fibromatosis presents as long and extensive fascicles of uniform spindle cells, with pale cytoplasm and wavy nuclei, included in a vast collagen stroma. Elongated vessels with thin walls and areas of micro-hemorrhage are common. Cell atypia and other signs of malignancy are not expected . Immunohistochemistry is important for diagnosis, being positive for betacatenin and smooth muscle actin .