Second respiratory and upper digestive tract cancers after oral cancer.

Between 1958 and 1975, 377 patients with squamous cell carcinoma of the floor of the mouth were treated. 123 new cancers (not recurrences) developed in 101 (27%) of these patients after treatment. 82 (67%) were squamous cell carcinomas of the respiratory and upper digestive tracts, one-third of these being in the mouth. These tumors developed at a constant rate of 3.6% per year and led to excess mortality of 5.2% per year. Men with oral cancer have a relative risk of 13.4 for new tumours in the upper respiratory and upper digestive tracts compared with the general population; in females the relative risk for all sites is 82.6. All patients with mouth cancer should thus be examined regularly because of this high risk of new cancers.