β‐Catenin (CTNNB1) mutations and clinicopathological features of mesenteric desmoid‐type fibromatosis

Aims and methods: Desmoid‐type fibromatosis (desmoid) is a fibroblastic tumour that shows locally aggressive growth. Mesenteric desmoid is a rare lesion that shares morphological and biological features with fibromatoses occurring in the abdominal wall or in extraabdominal sites, but differs in terms of gross appearance and clinical presentation. We report on a series of 56 cases of mesenteric desmoids from our consultation files and compare them with cases of non‐mesenteric desmoids and retroperitoneal fibrosis. Results: Primary diagnosis of desmoid‐type fibromatosis was correct in 42%, and gastrointestinal stromal tumour was a common misdiagnosis. Nuclear expression of β‐catenin was detected in 91.6% of all desmoids. Mutational analysis of exon 3 of the β‐catenin gene ( CTNNB1 ) revealed that mesenteric desmoids carried mutations significantly more often (51/56, 91.1%) than non‐mesenteric tumours (20/28; 71.4%; P = 0.027). p.T41A occurred significantly more frequently in mesenteric fibromatoses (80.4%) than in abdominal wall and extra‐abdominal fibromatoses (46.4%; P = 0.002). Two novel mutations (p.S45C and p.D32G) were found. In retroperitoneal fibrosis, mutations and nuclear β‐catenin expression were absent. β‐Catenin‐negative desmoids either carried a CTNNB1 mutation or were associated with Gardner syndrome. Conclusions: Our study provides evidence that some clinical and genetic features of mesenteric desmoids differ from those of non‐mesenteric fibromatosis, and corroborates the usefulness of mutational analysis, especially in diagnosing β‐catenin‐negative mesenteric desmoids.