A 73-year-old man is registered for regular check-ups in an oncological clinic after completion of the combined treatment for oral mucosa cancer stage II (radiation therapy and surgery). During one of the routine check-ups an area of exposed mandibular bone is detected. There are no inflammatory changes of surrounding mucosa. A sinus tract with weak granulation is observed. Mandibular X-ray shows sequestrum without clear margin between healthy and necrotic bone. What diagnosis is most likely?