Teaching Case

A 5-year-old male presented to an external centre with a palpable mass on the right side of the abdominal wall. He was referred to our institution upon detection of a mass on the anterior abdominal wall on ultrasonography (US).

Abdominal US examination revealed a heterogeneous hypoechoic solid mass, 24x34x36 mm in size, within the right rectus abdominis muscle (Figure 1). The lesion was well defined and had arterial and venous flow on Doppler US examination (Figure 2). Subsequently, on abdominal magnetic resonance imaging (MRI), the mass was heterogeneously hyperintense on T2-weighted and isointense on T1-weighted sequences compared to muscle (Figure 3, 4). On T1-weighted contrast enhanced sequences it significantly enhanced, except centrally, where it was also hypointense on T2-weighted sequences (Figure 5). It showed restricted diffusion (Figure 6, 7). In the differential diagnosis, soft tissue tumours such as rhabdomyosarcoma were considered. The diagnosis of desmoid tumour (aggressive fibromatosis) was established upon histopathological analysis via trucut biopsy. Total excision of the mass was performed (Figure 8, 9).

Desmoid tumours are relatively rare locally aggressive fibroblastic soft tissue tumours with an annual incidence of 2-4/1000000 cases among adult patients. In the aetiology, adenomatous polyposis coli (APC) gene mutation affecting the beta-catenin pathway, which is effective in cell proliferation, is implicated. In familial adenomatous polyposis and Gardner syndrome, APC mutation is seen and coexistence with desmoid tumour is frequent [2].
The mean age for the paediatric patient is 5-8 years of age [1]. Patients often present with a painless growing mass. Desmoid tumours are most commonly detected in the extremities. Other body parts affected with a decreasing frequency are head and neck, abdominal wall, and abdomen. Intra-abdominal desmoid tumours are the most common group reported to be associated with APC gene mutation and have higher mortality rates than those originating from other regions [3]. The possible association of prior trauma has been reported [3].
The main purpose of radiological evaluation is to determine the exact size and extension of the tumour relative to important anatomical structures. Magnetic resonance imaging (MRI) is the modality of choice due to its superior soft tissue resolution. On imaging the lesions are usually solid masses with smooth and well-defined margins. The tumours located on the abdominal wall are firstly screened with ultrasonography (US). On US, the appearance of the tumour is variable: hypo-, iso-, hyperechoic with either homogeneous or heterogeneous internal echogenicity [4]. Doppler US reveals flows within the tumours [4]. On MRI; low signal intensity on all sequences are characteristic but not specific for desmoid tumours [5]. As in our case, Lee et al [6] reported T1 isointensity, heterogeneous T2 hyperintensity compared to adjacent muscle, and significant enhancement following intravenous (IV) gadolinium administration in 29 cases with the diagnosis of aggressive fibromatosis. At CT, the lesions have variable attenuation compared to muscle and may either enhance or not enhance following IV contrast material administration [7]. Due to inconclusive imaging findings and radiation exposure, particularly in paediatric patients, CT should be avoided in the preoperative assessment of desmoid tumours.
Traditional treatment is gross total excision of the lesion. Recurrence after surgery is 26-76% and is closely related to the negative surgical margin [4].
Desmoid tumours are uncommon solid masses in the paediatric age group with no specific radiological findings. However, they should be kept in mind in the differential diagnosis of soft tissue tumours of paediatric patients.