Imaging findings in a case of primary intraosseous carcinoma arising from a mandibular cyst

Primary intraosseous carcinoma not otherwise specified (PIOC NOS) is a rare tumor assumed to arise from the epithelium, such as odontogenic cysts or benign tumors. Its clinical and imaging diagnoses are often challenging, especially in the early stages, as it mimics jaw cysts and benign tumors, and no specific findings have been identified. This report presents the case of a 66-year-old male patient with mandibular PIOC, highlighting the imaging findings over time. Magnetic resonance imaging (MRI) before symptom onset showed a cystic lesion in the right mandible with a soft tissue component. Both the fluid component and soft tissue exhibited low apparent diffusion coefficient values (1.0 × 10^–3 mm²/s and 1.3 × 10^–3 mm²/s, respectively). Subsequent MRI approximately 5 months later during symptom onset showed a slight increase in the soft tissue component. Based on the clinical and imaging findings, ameloblastoma was suspected, prompting a biopsy for confirmation. However, the histopathological findings showed squamous cell carcinoma (SCC). MRI performed approximately 1 month later exhibited significant tumor growth and extension beyond the jawbone, consistent with a malignant tumor. Histopathological examination identified areas with a basal layer in a palisading arrangement, indicating a pre-existing odontogenic cyst, and showed a transition from epithelial dysplasia to SCC. In addition, carcinoma cell invasion and proliferation into the cyst were observed. Based on these findings, PIOC of the right mandible was determined to be the definitive diagnosis.