[en] Purpose: To examine the efficacy of fast neutron radiotherapy in the treatment of locally advanced adenoid cystic carcinoma (ACC) of the trachea and to compare outcomes with and without high-dose-rate (HDR) endobronchial brachytherapy boost. Methods and Materials: Between 1989 and 2005, a total of 20 patients with ACC of the trachea were treated with fast neutron radiotherapy at University of Washington. Of these 20 patients, 19 were treated with curative intent. Neutron doses ranged from 10.7 to 19.95 Gy (median, 19.2 Gy). Six of these patients received an endobronchial brachytherapy boost using an HDR ¹⁹²Ir source (3.5 Gy x 2 fractions). Median duration of follow-up was 46 months (range, 10-121 months). Results: The 5-year actuarial overall survival rate and median overall survival for the entire cohort were 89.4%, and 97 months, respectively. Overall survival was not statistically different among those patients receiving an endobronchial boost compared with those receiving neutron radiotherapy alone (100% vs. 68%, p = 0.36). The 5-year actuarial locoregional control rate for the entire cohort was 54.1%. The locoregional control rate was not statistically different among patients who received an endobronchial boost compared with those who received neutron radiotherapy alone (40% vs. 58%, p 0.94). There were no cases of Grade ≥3 acute toxicity. There were 2 cases of Grade 3/4 chronic toxicity. Conclusions: Fast neutron radiotherapy is an effective treatment for locally advanced adenoid cystic carcinoma of the trachea, with acceptable treatment-related toxicity