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Multiple primary cancer risk in patients with squamous cell carcinoma of the oral cavity

During 6375 patient‐years of observation, 136 new cancers developed in 117 of 1429 patients with squamous cell carcinoma of the oral cavity. The risk of multiple cancers was excessive in the upper digestive or respiratory tract in patients with squamous cell carcinoma of the oral cavity, accounting for 87 (64%) of the 136 later cancers. In addition, multiplicity of cancers was site‐specific, being common in the oral cavity–esophagus–lung axis and uncommon in sites such as the maxillary sinus and epipharynx. In addition, the observed‐to‐expected ratio was different in each site of the oral cavity–esophagus–lung axis, i.e., 57.5 for the oral cavity and pharynx, 12.0 for the esophagus, 7.3 for the larynx, and 2.0 for lung cancers.