Intra-abdominal desmoid fibromatosis mimicking tumour recurrence after the operation: A case series.

Introduction: Desmoid fibromatosis is a multifactorial disorder classified as a category of intermediate, locally aggressive behaviour, which might be associated with CTNNB1 or APC mutations, trauma, surgery, or pregnancy.

Case reports: We present two cases of postoperative intra-abdominal desmoid fibromatosis. The first case occurred 14 months after the resection of a retroperitoneal gastrointestinal stromal tumour. The second case was located in the mesentery, as evidenced on an 18-month followup after a laparoscopy-assisted anterior resection for adenocarcinoma at the rectosigmoid junction. Under the clinical diagnosis of recurrence, tissue excisions were conducted. Microscopically, the tissue was composed of bland spindle cells without cytological atypia, admixed with collagen bundles. Both tumours exhibited nuclear expression of β-catenin on immunohistochemical staining, which is a desirable criterion for desmoid fibromatosis.

Discussion: Although positron emission tomography aids the diagnosis of recurrence, the radiological features of desmoid fibromatosis in computed tomography or magnetic resonance images are nonspecific and preoperative diagnosis of desmoid fibromatosis is difficult. The histological diagnosis of desmoid fibromatosis is difficult, especially when the specimen is small. The histological differential diagnosis of desmoid fibromatosis includes other myofibroblastic or fibroblastic tumours or lesions. Additional studies, such as β-catenin immunohistochemistry or CTNNB1 mutation analysis, can enable accurate diagnosis of desmoid fibromatosis. A correct diagnosis is essential, because the current therapeutic strategy is a "waitand- watch" approach, which is significantly different from those of the other locally aggressive, intermediate soft tissue neoplasms. We have summarised the clinicopathological, histological and immunohistochemical features of the post-operative desmoid fibromatosis.