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RADIOLOGY: HEAD: Case# 33667: INTRACRANIAL NEOPLASM. Patient is a 50 yo gentleman with a history of a medulloblastoma. Patient was intrathecally administered 80 mCi of an I-131 labeled monoclonal Fab fragment directed against a tumor antigen. A pretreatment and a 1 month post-treatment PET study were performed. In each study, the patient was injected with 10 mCi of 18-FDG IV and after allowing 30 min. for localization, emission PET data was acquired. A focus of hypermetabolic activity in the cerebellum suggesting active tumor. One month post therapy markedly diminished metabolism of the tumor focus is noted. In this case 18-FDG-PET studies have been used to evaluate tumor therapy and differentiate tumor expansion from radiation necrosis. Typically, T1 gadolinium-enhanced MRI images often demonstrate enhancement around the relatively vascular expanding margins of tumors but a similar appearance can be seen with radiation necrosis. Additionally, PET can direct biopsies toward more metabolically active tumor sites thus reducing sampling error and improving pathological diagnosis. Serial studies can also detect transformation of low grade astrocytomas to higher grades. Careful registration of PET with anatomical studies such as CT/ MRI is vital to proper interpretation because; 1) variations in metabolism seen commonly within tumors can complicate edge definition between normal and abnormal tissue on PET images; and 2) metabolic activity of a tumor is graded relative to the surrounding normal tissue which is better depicted with MRI/CT. Metabolism is often graded on relative intensity using a scale of 1 to 5. 1 is grey (1). The lesions seen in the first study were graded as 5.

Peter Anderson
head, radiation, radiology