These masses are centered on the midline and fill the fourth ventricle to a variable degree. The differentiation of these two types from one another may not be feasible on imaging when the lesion is sufficiently large as to almost completely fill the fourth ventricle. The superior portion of the fourth ventricle is typically free of tumor and forms, together with the overlying dilated cerebral aqueduct, a triangular or oval shape that has been called a “capped fourth ventricle” [8]. By virtue of their midline location, these tumors displace the brainstem anteriorly, but not laterally; thus, the entire length of the brainstem is visible on midsagittal MR images [2]. The obex (i.e., the point on the midline of the dorsal surface of the medulla at the most caudal aspect of the fourth ventricle floor) is seen to be infiltrated in the midfloor type but not in the roof type (Fig. 21.1). Conversely, there is a clear-cut cleavage plane between the posterior surface of the tumor and the vermis in the midfloor type. This morphological appearance on midsagittal images is extremely important in the differentiation from medulloblastomas, which originate from the vermis and project anteriorly in the fourth ventricle without any such separation. In the roof variant, the demarcation between tumor and vermis may not be as conspicuous owing to the tumor’s origin from the ependyma of the inferior medullary velum; however, the cleavage is usually visible at least in the superior portion of the fourth ventricle roof, above the fastigium (Fig. 21.1).

Plastic extension occurs either through the foramen of Magendie or the foramina of Luschka (Figs. 21.1 and 21.2). Extension through the Magendie causes the tumor to grow into the cisterna magna and, often, to herniate into the foramen magnum and cervical spinal canal, surrounding and compressing the medulla and spinal cord [8]. Extension into the lateral recess and Luschka foramen may occur either mono- or bilaterally and may involve the cerebellopontine angle; however, midline lesions typically do not extend further into the prepontine cistern [2].

These masses are centered off the midline, as they typically arise in one lateral recess of the fourth ventricle and displace the brainstem laterally, but not anteriorly. This morphological arrangement is the opposite to that of midline (either midfloor or roof) variants. As a consequence of this displacement, the entire length of the brainstem cannot be seen in one single midsagittal plane (Fig. 21.1). Lateral ependymomas often extend through the homolateral Luschka foramen into the cerebellopontine angle and may reach the prepontine cistern, where the mass engulfs the basilar artery (Fig. 21.3), a feature not seen in midline lesions. However, they typically do not extend across the fourth ventricle floor into the contralateral recess and Luschka foramen, unlike the midfloor type. Lateral tumors may also extend inferiorly into the cisterna magna through the Magendie foramen. However, the obex is consistently not infiltrated and can therefore be recognized as a discrete structure [2]. Engulfment of nerves and vessels in the cisternal spaces makes radical excision difficult if not impossible and is the main reason for the worse prognosis as compared with midline tumors.