To analyze the clinicopathological characteristics, and the genotype-phenotype relationships of classical familial adenomatous polyposis(FAP) based on the Chinese population.

Sixty-nine patients diagnosed with classical FAP in Fudan University Shanghai Cancer Center from January 2006 to December 2022 were included and their clinicopathological characteristics, genotype-phenotype relationships were analyzed after genetic test.

Among these 69 patients, forty patients (58%) had definite family histories, medianage of canceration is 34 (28,42) years old. Compared with the patients (49/69, 71%) with APC germline mutations, patients without APC germline mutations might show older ages (39.2±13.3 vs. 34.1±9.9, t=-1.723, P=0.089) and lower rate in extra-colonic lesions (30% vs. 51%, χ²=2.536, P=0.111). As for the extra-colonic lesions detected in 31 patients, gastroduodenal polyps (20 cases) were the most common lesions and mainly located in the fundus and body of stomach; subsequently, desmoids (13 cases) were often found among child-bearing women during 2 years after surgery and mainly located in the abdominal cavity and wall. Additionally, thyroid cancer (3 cases), hysteromyoma (2 cases), and adrenal tumors (1 case) were also found. Furthermore, p. E1309Dfs, the most common variant, was associated with gastric and duodenal polyps, desmoid, adrenal adenoma; patient harboring p.Y935* might be related with the manifestations of gastroduodenal polyps, desmoid, and thyroid cancer.

There are more patients with de novo classical FAP in the Chinese population, and patients without APC mutations may have a lower rate of extra-colonic lesions. Relationships of genotype and phenotype are helpful for the treatment of FAP.