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 in?ammatory changes of surrounding mucosa. A ?stula tract with soft granulation is detected. Mandibular X-ray shows a sequestrum without clear margin between healthy and necrotic bone. What is the most likely provisional diagnosis?