Teaching Case

A 31-year-old female patient presented with a painless, slow-growing mass in the right abdominal wall. A palpable fixed and firm tumour in the right middle third of the lateral abdominal wall was noticed. The patient had no history of trauma, surgery or childbearing. Laboratory testing was unremarkable.

Ultrasound (US) was the method of choice for initial screening, revealing a fairly well-defined, heterogeneous hypoechoic mass localized in the right rectus abdominis muscle (Fig. 1). Detectable flow by Doppler US was considered indeterminate.
Additional evaluation with computed tomography (CT) depicted a well-circumscribed, mild enhancement soft-tissue mass originating from the right rectus abdominis muscle. There was no evidence of calcifications or necrosis within the mass. No signs of invasion of the adjacent intraabdominal organs were found (Fig. 2).
Subsequently, an US-guided core needle biopsy was performed and the results were compatible with desmoid-type fibromatosis.
After preoperative workup, the patient was scheduled for surgery. While waiting for the procedure, a painful augmentation of the mass motivated a reevaluation CT (2 months later) which confirmed an important tumour growth but still without intraabdominal cavity invasion (Fig. 3).
The patient underwent a successfully complete resection of the tumour with negative microscopic margins.

Desmoid-type fibromatosis (DF), also known as aggressive fibromatosis, is a locally aggressive tumour, with no potential for metastasis [1]. This fibroblastic neoplasm is rare and represents less than 3% of all soft-tissue tumours [1].
DF can be classified according to its location as extra-abdominal, intra-abdominal, or abdominal wall [1]. Abdominal wall DF arises from musculoaponeurotic structures of the abdominal wall, most often from the rectus or internal oblique muscles and their fascial coverings [3].

Endocrine factors are highly implicated in this form of DF, which explains why young women during or after pregnancy are the most commonly affected group [1]. They may also occur secondary to trauma, following surgery, or related to hereditary syndrome familial adenomatous polyposis [1].
Typically, they present as a solitary slow growing, firm and painless mass [3].

On imaging, DF tends to be a fairly well-circumscribed solid mass and its appearance varies according to the amount and distribution of its histologic components (spindle cells, myxoid matrix, collagenous stroma) [1, 2].
Ultrasound (US) is a useful imaging technique in the initial screening of a palpable abdominal wall mass. On US, it appears as an oval soft-tissue mass with variable echogenicity [3]. DF can be associated with the fascial tail sign, which reflects thin linear extension along fascial planes, and the staghorn sign, indicating intramuscular finger-like extensions of the tumour [1, 2]. Vascularity is variable at colour Doppler US [1].
On CT, abdominal wall DF has variable attenuation, similar to or slightly higher than skeletal muscle [1, 2]. In larger masses, we may find a more heterogeneous appearance. Necrosis or calcification is very rare [1]. The majority of these tumours demonstrate mild-to-moderate enhancement [1].
On MRI, DF tends to have a heterogeneous pattern with high T2-weighted signal intensity early in its evolution, but becomes lower in T2 signal intensity as it evolves (collagen deposition increases) [1, 2]. Non-enhancing linear bands (band sign) are usually described, likely corresponding to the dense collagenous stroma [1].

Despite the characteristic imaging findings, definitive diagnosis must be established with histopathologic analysis [3]. Nevertheless, CT and MRI are the best imaging modalities for assessing resectability, surgical planning and follow-up [3]. Important imaging findings with surgical implications include the longitudinal extent and depth of the tumour, involvement of internal organs, and proximity to the costochondral junction or the lower ribs [1].

Surgery is the treatment of choice for progressive or symptomatic tumours, while a conservative approach is acceptable for tumours that are not causing notable impairment [3].
After surgical resection, the local recurrence of abdominal wall DF is about 15%-30% [1].