Aggressive Fibromatosis of the Quadratus Lumborum in A 28-Year-Old-Female: A Case Report

Background: Desmoid tumors (aggressive fibromatosis/desmoid-like tumors) are rare neoplasms that consist of proliferations of clonal fibroblastic proliferation that are aggressive and locally invasive. While the exact cause of desmoid tumors is unknown, they are associated with hereditary conditions such as Gardner’s syndrome and familial adenomatous polyposis both of which arise due to inactivating mutations of the APC gene. On the other hand, fibromatosis that arise sporadically generally has an activating mutation of the CTNNB1 gene which encodes beta-catenin. A history of trauma such as injuries and previous surgeries has also been associated with the development of desmoid tumors. Desmoid tumors are rare with an estimated incidence of 2-4 per million people per year and make up 0.03% of all neoplasms. Early diagnosis and treatment of desmoid tumors are crucial in minimizing morbidity and mortality. Fibromatosis primarily arise in the extra-abdominal setting which comprises approximately 58% of all cases. Among this, most arise in the shoulder or pelvic girdle region. Here, we report an uncommon site of origin for desmoid tumors.

Case Presentation: A 28-year-old female with no previous history of trauma presented with lower back pain with radiculopathy radiating to her right hip. An MRI of the lumbar spine revealed a mass in the right iliac fossa and an abdominal and pelvic MRI showed a large circumscribed posterolateral right intra-abdominal wall mass extending up from the right iliac fossa with low signal bands on T1W and T2W imaging. There were intermediate signal areas elsewhere with islands of enhancement. Subsequent abdominal and pelvis CT showed a right posterior pelvic mass measuring 19x13x10 cm concerning for a neoplasm with fibrotic components. The mass originated from the right quadratus lumborum muscle. It involved the posterior distal transversus abdominis and internal oblique abdominal muscle anteriorly. Distally, it involved the iliacus muscle. Differential diagnoses were abdominal wall aggressive fibromatosis/desmoid tumor, solitary fibroid tumor, sarcoma, unusual GIST tumor, neuroectodermal tumor versus nerve sheath tumor, metastasis, lymphoma. A biopsy was performed and results were consistent with aggressive fibromatosis.

Conclusion: Desmoid tumors are rare tumors with a locally aggressive and variable course and high risk of local recurrence. The diagnostic workup often includes imaging with MRI and CT and which allows for determining the origin and involvement of adjacent structures to guide possible interventions. Diagnosis can only be confirmed through biopsy which will reveal a monoclonal proliferation of fibroblasts. Treatment ranges from observation, radiotherapy, surgery, and various medications. Because this case presents a desmoid tumor originating from an uncommon site of origin in the quadratus lumborum region, we hope to provide a better clinical picture for the screening and diagnosis of similar aggressive fibromatosis.