Yield of Colonoscopy in Identification of Newly Diagnosed Desmoid-Type Fibromatosis with Underlying Familial Adenomatous Polyposis

BACKGROUND: Desmoid-type fibromatosis can arise in patients with familial adenomatous polyposis (FAP), therefore patients with desmoids often undergo colonoscopy to rule out FAP. Because finding FAP is uncommon, we sought to define subsets of desmoid patients in whom colonoscopy frequently identified FAP.

METHODS: Patients with desmoid-type fibromatosis were identified from surgery and pathology databases at a single institution, and information on colonoscopy and FAP diagnosis was collected retrospectively. CTNNB1 mutation status was defined by Sanger sequencing and digital polymerase chain reaction of archived specimens.

RESULTS: Among 626 patients with desmoids, 26 were diagnosed with FAP. In 20 patients, FAP diagnosis predated the desmoid diagnosis. Among patients without prior FAP diagnosis, 161 underwent colonoscopy, which identified only six cases of FAP (diagnostic yield 3.7%). Yields were substantially higher among patients with four characteristics: age < 40 years (11% yield), intra-abdominal or retroperitoneal tumors (5.4%), multifocal disease (29%), and family history (8%) (all p < 0.001). All cases of FAP were detected in patients younger than 40 years of age and with at least one of the other three characteristics. CTNNB1 mutation status was available in 82 patients with known FAP status. None of the 61 patients with CTNNB1 mutations were diagnosed with FAP, while 7 of the 21 patients with no CTNNB1 mutation detected (24%) were FAP patients.

CONCLUSIONS: Patients with desmoid-type fibromatosis and undiagnosed FAP generally have multiple risk factors, which may be used to selectively recommend colonoscopic screening. Routine CTNNB1 sequencing may also rule out FAP and allow for deferral of colonoscopy.