Based on the imaging findings the main differential diagnoses are, chronic osteomyelitis, MRONJ, or osteoradionecrosis. Findings are not typical for invasion from an oral cavity squamous cell carcinoma, nor of a metastasis or lymphoma. 

The key to making the diagnosis is recognizing evidence of prior neck dissection and resection of the left submandibular gland, and with that the assumption of prior radiation therapy, then osteoradionecrosis becomes the favored diagnosis. Confirmation can then be made with appropriate history and biopsy.