Figure 1: Axial T1-weighted in-phase image demonstrates that the soft tissue is isointense to muscle.

Figure 2: Axial STIR image demonstrates heterogeneous T2 hyperintensity in the soft tissue mass in
the left rectus muscle.

Figure 3: Sagittal T2-weighted TSE image demonstrates a large lobulated soft tissue mass (7.9 x 6.1 x 8
cm) in the left rectus muscle. The inferior epigastric artery appears to be coursing superficial to this mass, with small feeding branches. Changes are present in the subcutaneous fat superficial to this mass indicative of prior abdominal wall incision site (arrow). The mass does not demonstrate invasion into the internal organs.

Figure 4: Axial VIBE image demonstrates diffuse enhancement of the soft tissue mass.

Desmoid tumor. (See companion case Endometrioma)

Soft tissue tumors of the abdominal wall that are known for their aggressive biological behavior include soft tissue sarcomas (STS), desmoid tumors, and dermatofibrosarcoma protuberans (DFSP). Desmoid tumors, also termed aggressive fibromatosis, are rare (0.03% of all neoplasms) mesenchymal tumors characterized by proliferation of fibroblastic cells and abundant collagen fibers that arise from fascial or musculoaponeurotic structures. Although these tumors can occur at any age, desmoid tumors occur most often in postpartum women, with peak prevalence in patients between 20 and 30 years of age, and in patients with scars due to abdominal surgery. Desmoid tumors may occur as sporadically or in association with Gardner Syndrome (29% incidence) or familial adenomatous polyposis (3.2-32% incidence). Although the pathogenesis of desmoid tumors remains unknown, various factors are strongly associated with their development including previous trauma—often surgical, hormonal factors, or genetic disorders. The preponderance of cases affliciting young women during or after pregnancy, tumor development following exposure to oral contraceptives, and tumor regression during menopause and after tamoxifen treatment are all evidence for the suspected endocrine/hormonal basis for desmoid tumors.

Desmoid tumors are classified as extra-abdominal, intra-abdominal, or located within the abdominal wall. Abdominal wall desmoid tumors arise most commonly from the aponeurosis of the rectus abdominus muscle with out without intraabdominal extension. Intra-abdominal desmoids arise from the mesentery, retroperitoneum, or pelvic wall. The most common site for mesenteric desmoid tumors is at the base of the small bowel mesentery. Extra-abdominal tumors typically occur in the shoulder, chest wall, thigh, inguinal region, and back. Clinical symptoms are masked by the slow growth of the tumor and depend on the site to tumor involvement. Desmoid tumors can cause symptoms by infiltration, compression, displacement of intestinal, genitourinary, vascular, or neuronal tissue (e.g. intestinal obstruction, hydronephrosis).

Grossly, desmoid tumors vary in size from 1 to 15 cm in greater diameter. In general, they are unicentric, infiltrative lesions with poorly defined borders. However, discrete, well-circumscribed tumors also occur. These masses are relatively avascular, firm, and unencapsulated with coarse white trabeculae that simulate scar tissue. On histological analysis, desmoid tumors are benign fibrous neoplasms consisting of elongated spindle-shaped cells separated by dense bands of collagen.

Radiological investigations of desmoid tumors, typically CT or MRI, should be undertaken to define the extent of the tumor, the relationship to local structures, and complications such as hydronephrosis and small bowel obstruction, especially prior to surgical resection. Desmoid tumors have a similar attenuation to muscle on contrast-enhanced CT images. However, CT cannot distinguish a desmoid tumor from similar soft tissue tumors, making histological diagnosis necessary. MRI enables better tissue characterization of desmoid tumors by demonstrating intratumoral areas of low signal intensity on all pulse sequences. The low signal intensity is due to the presence of abundant collagen within the lesion. Desmoid tumors appear as low-signal intensity masses in a background of high-signal intensity fat on T1-weighted MR images. These tumors have variable signal intensity on T2-weighted MR images. In mature desmoids, areas of abundant fibrosis results in low signal intensity on T2-weighted images. Longstanding tumors are low in signal intensity on T1 and T2-weighted MR images and enhance only minimally after intravenous gadolinium chelate. In the acute phase, tumors may have regions of high signal intensity on T2-weighted images that also show heterogeneous increased enhancement. Tumor recurrence is a frequent finding after surgery and is easily detected using MRI.

Despite their benign histologic appearance and negligible metastatic potential, the propensity of desmoid tumors for local infiltration is significant in terms of deformity, morbidity and mortality resulting from pressure effects and potential obstruction of vital structures and organs. Desmoid tumors are treated with surgical resection. Since these tumors have a high rate of recurrence, adjunct radiation therapy is also recommended. Radiation therapy may be used as a treatment of recurrent disease or as primary therapy in cases with extremely complicated surgical resections. Pharmacologic therapy with antiestrogens and prostaglandin inhibitors may also be used. In cases of recurrent extra-abdominal desmoid tumors in which surgery is contraindicated or in cases of recurrence, a chemotherapeutic regimen of doxorubicin, dacarbazine, and carboplatin may be effective.

References:

1. Semelka, Richard. Abdominal-Pelvic MRI. New York: Wiley-Liss Inc, 2002. pp. 659-660, 939, 941.
2. Trovato MJ and RA Schwartz. Desmoid Tumor. (2003). eMedicine (04/19/04).
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5. Vandevenne JE, De Schepper AM, et al. New Concepts in Understanding Evolution of Desmoid Tumors: MR Imaging of 30 Lesions. European Radiology. 1997; 7: 1013-1019.
6. Casillas J, Sais GJ, Greve JL, et al. Imaging of Intra- and Extraabdominal Desmoid Tumors. RadioGraphics. 1991; 11: 959-968.