Anatomic, physiologic and metabolic imaging in neuro-oncology.

Primary brain tumors arise from various cell types of the brain, including glial cells, neurons, neuroglial precursor cells, pinealocytes, pericytes of the vessels, cells of the hypophysis, lymphocytes and the meninges [1, 2]. The incidence of primary brain tumors varies between subtypes, with the most common primary brain tumors in adults being gliomas and meningiomas. Gliomas can be histologically classified into astrocytomas, oligodendrogliomas, mixed oligoastrocytomas, ependymal tumors and tumors of the choroid plexus. Tumor malignancy or grade is generally assessed according to the World Health Organization (WHO) criteria, taking into account the presence of nuclear changes, mitotic activity, endothelial proliferation and necrosis [1, 3]. The most fatal and common primary brain neoplasm is the glioblastoma multiforme (GBM), which corresponds to WHO grade IV. Despite aggressive multimodal treatment strategy (surgery, radiation and chemotherapy), median survival of patients with GBM is limited to less than 14 months. A complex series of molecular events occur during tumor growth resulting in dysregulation of the cell cycle, alterations in apoptosis and cell differentiation, neo-vascularization as well as tumor cell migration and invasion into the normal brain parenchyma. Genetic alterations also play an important role in the development of glioma, including a loss, mutation or hypermethylation of the tumor suppressor gene, such as p53 or other genes involved in the regulation of the cell cycle. During progression from low-grade to high-grade, step-wise accumulation of genetic alterations occurs. Growth of certain tumors seems to be related to the presence of viruses and familial diseases that accelerate the progression of molecular alterations, or exposure to environmental chemicals, pesticides, herbicides and fertilizers [4-6]. A better understanding of tumorgenesis is crucial for the development of specific molecular therapies that specifically target the tumor and reduce patient morbidity and mortality. Positron emission tomography (PET), computed tomography (CT) and magnetic resonance imaging (MRI) are generally used for non-invasive diagnosis and