Description
Non-malignant tumors of the brain and central nervous sys-tem occur at an incidence of 17.9 per 100,000 in the United States [ 25 , 26 ]. The most common primary non-malignant tumors are gliomas and meningiomas [ 25 , 26 ]. Gliomas are tumors intrinsic to the brain that arise from glial cells (sup-porting cells), and include astrocytomas, oligodendroglioma, and mixed tumors (oligoastrocytomas). Meningiomas arise outside of the brain from arachnoidal cells of the meninges (coverings of the brain). They represent the most frequently diagnosed brain tumors in adults, accounting for 28–35 % of cases [ 25 , 26 , 87 , 112 ]. Brain tumors can be distinguished based on behavior, by displaying a “non-malignant” or “malignant” phenotype. Histologically, meningiomas are classified into three subtypes: World Health Organization (WHO) grade I, II, or III. WHO grade I non-malignant (also known as “benign”) meningiomas represent the vast major-ity, encompassing 90 % of cases. WHO grade I meningiomas have a 10-year survival of 70–92 % and a recurrence rate of 7–20 % [ 23 , 51 , 65 , 96 , 112 ]. WHO grade II (atypical) menin-giomas represent 9 % of cases and have a 30–40 % recur-rence rate. WHO grade III (anaplastic) meningiomas represent 1 % of cases and have a recurrence rate of 50–80 % [ 96 ]. Astrocytomas also display a wide range of histological subtypes, classified as WHO grade I, II, III, or IV tumors. WHO grade I pilocytic astrocytomas occur predominantly in children. WHO grade II diffuse astrocytomas are the most common low-grade intrinsic brain tumor in adults. WHO grade III (anaplastic) and WHO grade IV (glioblastoma) tumors are classified as malignant gliomas. This chapter focuses on adult non-malignant brain tumors with special emphasis on low-grade gliomas (WHO grade II) and benign meningiomas (WHO grade I). Malignant brain tumors are discussed elsewhere in this book.
Low-Grade Gliomas
Low-grade gliomas represent 15 % of all newly diagnosed primary intrinsic brain tumors in adults [ 26 ]. Their incidence has increased over the past two decades due to advances in diagnostic imaging and improved diagnostic accuracy [ 26 , 50 , 107 ]. WHO grade II gliomas include diffuse (infiltrative) astrocytoma representing 2.8 % of glioma cases, oligodendro-gliomas representing 1.3 % of cases, and mixed oligoastrocy-tomas representing 1 % of cases [ 26 , 77 ]. Between 1,500 and 1,800 new low-grade gliomas are diagnosed in the United States each year. Tumor cells infiltrate into brain parenchyma where they can later transform, acquiring more malignant behavior over time. Median time to tumor progression and recurrence is 5.5 years, and median time to malignant trans-formation is 10.1 years with maximal safe resection. An esti-mated 50 % of patents with diffuse low-grade gliomas will progress and show recurrence within 5 years of initial diagno-sis [ 32 , 79 , 85 , 105 ].
The median age at diagnosis for low-grade gliomas is 45 years, with patients ranging from 20 to 70 years of age [ 26 ]. Low-grade gliomas are more common among Caucasian men. Histologically, infiltrative astrocytomas display a mod-est increase in cellularity, disruption of the normal orderly pattern of glial cells, and elongated nuclei (Fig. 1.1 ). There is no endothelial proliferation or tissue necrosis as is commonly seen in malignant gliomas. Oligodendrogliomas occur pre-dominantly within the gray matter of the cerebral hemi-spheres, are well-circumscribed, calcified, and have a slight predominance for the frontal lobes. Histologically, oligoden-drogliomas are characterized by uniform cell density and round nuclei with perinuclear halos displaying a classic “fried egg” appearance (Fig. 1.1 ). In 1994, a co-deletion in the long arm of chromosome 1p36 and the short arm of chromosome 19q13 was shown to predict chemosensitivity and better prognosis in patients with low-grade gliomas [ 57 ].