PET Imaging of Gliomas

Noninvasive imaging methods, including positron emission tomography (PET), have become essential for diagnosis and staging of gliomas, and monitoring of treatment response. The utility of these techniques have been found to be highly dependent on tumor grade. According to the World Health Organization (WHO) classification of tumors (Kleihues and Sobin 2000), gliomas are classified into 3 main histological types: astrocytoma, oligodendroglioma and glioblastoma. These histological types are further classified on the basis of anaplasia and degree of malignancy as: grade I, noninvasive glioma (pilocytic astrocytoma); grade II, less-invasive glioma (astrocytomas and oligodendrogliomas); grade III, invasive glioma (analplastic astrocytoma/oligodendrogliomas); and grade IV, highly invasive glioma (glioblastoma, or GBM). Low-grade gliomas (grade I and II) typically affect younger patients. Grade I glioma is the most common form of glioma in children and is less frequent in adults (Burkhard et al. 2003) while grade II gliomas are common in adults (mean age of onset is 40 years) (Hagerstrand et al. 2008). Median survival for low-grade glioma is varied but prognosis and treatment require regular follow ups. Low-grade gliomas grow slowly or stabilize spontaneously and with surgical resection, median survival can be 20 years or more (Burkhard et al. 2003). For high-grade gliomas, the mean age of onset is 40 years for grade III glioma and 61 years for GBM (Ohgaki and Kleihues 2005). GBM is the most malignant and most common glioma, accounting for 45% 50% of all adult gliomas. Median survival for grade III glioma is 2-3 years and for GBM is 1 year (Chen 2007). For optimal disease prognosis, treatment and follow up, one should be able to delineate the tumor lesion and most importantly, differentiate benign lesions from neoplastic lesions, low-grade from high grade tumors, and tumor progression from therapy induced necrosis. As will be discussed later, efforts are also being directed toward defining early imaging predictors of response to therapy. Conventional imaging with magnetic resonance imaging (MRI) provides excellent anatomical definition of brain tumors. MRI is highly sensitive in identifying lesions, mass effect, edema, hemorrhage, necrosis and signs of increased intracranial pressure (Chen 2007). Pathologic changes are characterized on MRI by increased water content (edema) and blood-brain barrier (BBB) disruption, visualized as contrast enhancement (Grosu et al. 2002). Most tumors (low-grade or high-grade) have prolonged T1 and T2 relaxation times and thus