Second primary tumours after a squamous cell carcinoma of the oral cavity or oropharynx using the cumulative incidence method.

The aim of this study was to define the incidence of second primary tumours (SPTs) after treatment of a first primary oral or oropharyngeal squamous cell carcinoma (SCC) and to define patient groups with an increased or decreased risk of developing SPT with adjustment for competing risks. Cancer registry data from 917 consecutive patients with primary oral or oropharyngeal SCC were reviewed. Outcomes considered were the incidence and location of the SPT. Cumulative incidence was assessed instead of cumulative risks for SPTs. 149 patients (16%) developed a metachronous SPT (median follow-up time 2.6 years). The 5-year and 10-year cumulative incidence was 13% (S.E. 1.2) and 21% (S.E. 1.7), respectively. Most SPT developed in the upper aerodigestive tract (n=65) and lungs (n=35). No statistically significant risk factors were identified when considering patient and index tumour characteristics. The advantage of this study was the large and homogeneous patient population and the correction for competing risks, resulting in a lower but more accurate estimation of the incidence of SPTs. Despite this lower, but still continuous risk, regular follow-up for over 10 years is indicated for all patients treated for cancer of the oral cavity and oropharynx.