The aim of the study was to improve clinical outcomes of oral mucosa cancer in case of surgical treatment failure.Materials and methods. A clinical case of combined radiation treatment of a patient with oral floor mucosa cancer St III T3N1M0 was presented. Pathogistological diagnosis # xxxxxx was: highly differentiated squamous cell carcinoma. A treatment plan was developed. Chemoradiotherapy at the first stage included 3 courses of intra-arterial chemotherapy (Selective (superselective) embolization (chemoembolization) of tumor vessels in the basin of the left and right facial and left lingual arteries, cisplatin 150 mg, combined with a daily infusion of 5-fluorouracil 4000 mg), with an interval 21 day. Chemoradiotherapy at the second stage included volume modulated arc therapy (VMAT) on the area of ​​the primary tumor (oral floor mucosa) and the regional metastasis pathways (total boost dose 60 Gy to the area of ​​regional lymph flow, otal boost dose 50 Gy to the oral cavity). Chemoradiotherapy at stage 3 included interstitial radiation therapy. A single boost dose (SBD) for the primary focus was 3 Gy/per fraction, 2 times a day, with an interval of 6 hours until total boost dose reached 21 Gy, 7 fractions.Results. After treatment, a complete regression of the tumor was noted. No pathological accumulation of radiopharmaceuticals, features of the tumor process was detected on PET/CT 48 months after treatment.Conclusions. Chemoradiotherapy combined with brachytherapy can be considered as an equal alternative to surgical treatment of patients with oral mucosa cancer.