Adenoid cystic tumors are rare tumors of the head and neck region. Despite their slow growth, re-irradiation is often necessary due to the high metastatic risk. Patients are usually irradiated with photons or, as here at the Heidelberg University Hospital, with a combination of carbon ions and photons. So far, there is no data from Europe available for the sole irradiation with carbon ions. The present ACCO (Adenoid Cystic Carcinoma and Carbon ion Only irradiation) study, a prospective, open-label, phase II, single-arm, investigator-initiated study, will therefore investigate the sole radiotherapy of carbon ions in this tumor entity. Irradiation is applied - significantly shorter than the combination therapy - in about 4 weeks (22 fractions); patients are followed up for further 5 years after the start of therapy. Carbon ions alone are expected to increase local tumor control rates from 60% to 70% after 5 years (primary objective criterion of this study). In order to reject the null hypothesis with a power of 80% and a significance level of 5%, 175 patients are included (including a drop-out rate of 15%). Secondary objective criteria are progression-free survival, overall survival, acute and late toxicity, and quality of life.