Abstract
Background. Head and neck cancers account for a relatively large number of second primary malignancies （SPM）. It is important to search for SPM in the head and neck area and the upper gastrointestinal tract. In addition, there are few reports in Japan on the risk of SPM of oral cancer.
Methods. A retrospective study was conducted on 155 oral cancer patients treated at our hospital.
Results. There were 47 cases that developed SPM, in which 11 developed synchronously and 39 developed metachronously: three cases developed SPM both synchronously and metachronously. The risk factors for SPM were smoking history of 25 packs/year or more for synchronous SPM and 37 packs/year or more and age （67 years or older） for metachronous SPM, and fewer than three cases of young or non-smoking metachronous SPM of the upper gastrointestinal tract.
Conclusion. Duplicate cancer search is less likely to focus on the upper gastrointestinal tract in younger or non-smoking patients.
