Intradiploic epidermoid cyst mimicking skull metastasis in a patient with primitive neuroectodermal tumor: correct diagnosis with 99mTc-MDP hybrid SPECT-CT.

A 14-year-old child was diagnosed with primitive neuroectodermal tumor (PNET) of pelvis. Baseline 99mTc-methtylene diphosphonate (MDP) bone scintigraphy done to know the extent of the disease, although of suboptimal quality, revealed involvement of right hemipelvis (A, arrowhead) (Fig. 1). Also, there was a focus of increased radiotracer uptake in the occipital bone (B, arrow). Since the patient had no history of trauma, in the clinical context the lesion was thought to be metastasis. The patient underwent chemotherapy and was again sent for bone scintigraphy, for response monitoring. The follow up planar bone scintigraphy done 6 weeks after completion of chemotherapy showed significant reduction in radiotracer uptake in the right hemipelvis (C, arrowhead), but the skull lesion still showed intense radiotracer uptake (D, arrow). To characterize the skull lesion, the patient underwent hybrid single photon emission tomography-computed