Associations between small intestine cancer and other primary cancers: An international population-based study

Cancer of the small intestine is a rare neoplasm, and its etiology remains poorly understood. Analysis of other primary cancers in individuals with small intestine cancer may help elucidate the causes of this neoplasm and the underlying mechanisms. We included 10,946 cases of first primary small intestine cancer from 13 cancer registries in a pooled analysis. The observed numbers of 44 types of second primary cancer were compared to the expected numbers derived from the age-, gender- and calendar period-specific cancer incidence rates in each registry. We also calculated the standardized incidence ratios (SIR) for small intestine cancer as a second primary after other cancers. There was a 68% overall increase in the risk of a new primary cancer after small intestine carcinoma (SIR = 1.68, 95% confidence interval [CI] = 1.47-1.71), that remained constant over time. The overall SIR was 1.18 (95% CI = 1.05-1.32) after carcinoid, 1.29 (1.01-1.63) after sarcoma, and 1.27 (0.78-1.94) after lymphoma. Significant (p < 0.05) increases were observed for cancers of the oropharynx, colon, rectum, ampulla of Vater, pancreas, corpus uteri, ovary, prostate, kidney, thyroid gland, skin and soft tissue sarcomas. Small intestine cancer as a second primary was increased significantly after all these cancers, except after oropharyngeal and kidney cancers. Although some of the excess may be attributable to overdiagnosis, it is plausible that most additional cases of second primary cancers were clinically relevant and were due to common genetic (e.g., defects in mismatch or other DNA repair pathways) and environmental (e.g., dietary) factors.