Results of radiation therapy in carcinoma of the base of the tongue. The Curie Institute experience with about 166 cases. Between 1960 and 1980, 166 patients with squamous cell carcinoma of the base of the tongue were treated with primary irradiation at the Curie Institute (Paris, France). Distribution according to the TNM system 1978 International Union Against Cancer (UICC) was the following: 22 T1 lesions, 47 T2 lesions, 64 T3 lesions, and 33 T4 lesions. Regional nodes were not palpable in 50 cases, 35 had N1 nodes, 12 had N2 nodes, and 69 had N3 nodes. All patients received external beam radiation. The 2-year, 3-year, and 5-year overall survival rates for all patients were, respectively, 45%, 37%, and 27%. Local control was significantly related to the initial status of the primary, to the tumor regression at the end of the radiation therapy, and to the histologic differentiation. The 2-year local control was 96% for T1 lesions, 57% for T2 lesions, 45% for T3 lesions, and 23% for T4 lesions. Local control was 70% if the tumor regression was complete at the end of the treatment and 27% if the tumor regression was partial. No significant differences were found in primary local control with respect to degree of infiltration, age, and dose of radiation therapy over a dose of 60 Gy in 6 weeks. The 3-year regional control was 86% for N0, 78% for N1, and 60% for N2 and N3. Among the tumor characteristics analyzed, the most useful ones for predicting local control and survival were clinical tumor staging parameters and tumor radiation-induced regression. A new therapeutic approach based on the evaluation of the tumor regression at 50/55 Gy is under discussion. 