Fibromatosis of the Sigmoid Colon With CTNNB1 (β-Catenin) Gene Mutation, Arising at the Site of Ileocolic Anastomosis for Resection of Gastrointestinal Stromal Tumor.

We describe a case of intra-abdominal fibromatosis, which occurred in a 44-year-old woman who had a previous history of gastrointestinal stromal tumor (GIST) of the sigmoid mesocolon, which was treated with imatinib and resection. A mass was detected at the site of ileocolic anastomosis of the previous small bowel resection and sigmoid colectomy, nearly 3 years later. Clinically, this was suspected to represent recurrent GIST and was excised, but histology and mutational analysis showed desmoid-type fibromatosis with a mutation in codon 41 of exon 3 of the CTNNB1 (β-catenin) gene. The occurrence of fibromatosis at the site of excision of GIST is very rare, but its recognition is important as the treatment of the two neoplasms differs significantly. As imaging cannot reliably distinguish between these 2 entities, histological diagnosis is crucial for correct clinical management.