Prevalence of extracolonic malignancies in patients with familial adenomatous polyposis.

103 Background: Familial adenomatous polyposis (FAP) is an autosomal-dominant inherited colorectal cancer syndrome resulting from mutations in the adenomatous polyposis coli ( APC ) gene. Patients with FAP have nearly 100% lifetime risk of colorectal cancer, typically managed with prophylactic colectomies. As patient survival has improved, the incidence of extracolonic tumors, including thyroid malignancies and desmoid tumors, has increased. However, limited research exists on the prevalence of extracolonic malignancies in FAP patients. Methods: We prospectively enrolled 219 patients with FAP in the Hereditary Gastrointestinal Cancers Cohort at the MD Anderson Cancer Center between December 2015 and July 2024. Inclusion criteria included either a clinical or molecular diagnosis of FAP. Data on demographics, cancer history (including cancers diagnosed both prior to and during the study period), BMI, tobacco use, medications, endoscopy results, thyroid ultrasound findings, and abdominal/pelvic CT results were collected. Screening for thyroid and desmoid tumors was provider-dependent, with additional screenings as deemed appropriate. Descriptive and inferential statistics were used to identify associations between demographic and clinical factors and the prevalence of extracolonic malignancies. Results: Among 219 patients, 20.5% developed desmoid tumors, 18.7% developed colorectal cancer, and 6.8% developed well-differentiated thyroid cancer (6.4% papillary thyroid cancer, 0.4% follicular thyroid cancer). Prophylactic colorectal surgery was performed in 151 patients (68.9%). Per chart review, benign thyroid disease and hypothyroidism were present in 21% and 11.4% of patients, respectively. Female sex was significantly associated with hypothyroidism (p=0.007) and thyroid cancer (p=0.009). BMI > 30 was also linked to increased thyroid cancer risk (p=0.046). Desmoid tumors were found in 20.5% of patients, most commonly in the abdominal wall. Female sex, BMI > 30, and history of colorectal surgery were significantly associated with desmoid tumor development (p=0.004, 0.0064, 0.0038). Conclusions: FAP patients exhibit a higher prevalence of benign thyroid disease, thyroid cancer, and desmoid tumors compared to the general population. Given these findings, adherence to existing surveillance guidelines, such as those outlined by the National Comprehensive Cancer Network (NCCN), is essential. Our findings further support the importance of these established surveillance strategies in the management of FAP patients.