MRI is the first radiologic diagnostic tool. The radiological evaluation of brain stem lesions is discussed in the chapter “Genetics”. The authors just want to underline basic points which are very important during decision-making on these lesions. The study has to demonstrate in detail the normal anatomical structures and the extent of the pathology. We should compare the volume of the lesion on T1- and T2-weighted images which can be informative for differentiating between the diffuse and the focal tumor. In diffuse tumors there isn’t any distinct boundary and the tumor can be demonstrated as a hypointense on T1-weighted and hyperintense on T2-weighed MR images expanding the entire medulla throughout to pons. In a large radiological study (without histopathological verification), Barkovich et al. [7] showed that the “brain stem hypertrophy” is a predictor for a bad outcome of brain stem gliomas (Fig. 34.8a, b). Also Albright et al. proposed MRI instead of biopsy or operation to confirm the diagnosis for the diffuse tumors [8]. Contrary to this, Behnke stated that MRI has little predictive value for histopathological diagnosis in intra-axial brain stem tumors [2].

Focal tumors are well circumscribed with a limited size (<2.5 cm). Tumor may be cystic or solid that may or may not enhance. Pilocytic tumors usually enhance homogeneously due to characteristic vascular proliferation of the tumor [9]. Abbott et al. reported that cysts that are enhancing are pathognomonic for a malignant glioma. As the tumor grows, it can reach to surface and can make the exophytic component [10]. Dorsal exophytic neoplasms are also not biologically similar to anterior and lateral exophytic tumor.

Cervicomedullary tumors respect the boundaries. Their axial growth is limited by the pyramidal decussation at cervicomedullary junction, at which point the tumor growth turns posteriorly towards the fourth ventricle. Small proportion of tumors have invasive potential in this location; therefore, they grow through the decussation and do not show posterior exophytic component. MR scan shows distortion of the medulla with enlarged upper cervical cord (Fig. 34.8c). Tumors can enhance gadolinium either homogeneously or nonhomogeneously.

A preoperative DTI study is also essential to have an idea of the location of corticospinal tracts (Fig. 34.9). We also agree that the histopathology of the medulla tumors cannot be reliably predicted by means of MRI alone [11].

The main objectives for surgery of the medulla tumors are to obtain tissue samples for pathological examination and to decrease the tumor volume. The extent of tumor resection should be as large as possible in exophytic tumors to relieve from preoperative clinical signs and symptoms. Radical tumor removal is important for the patient’s survival; unfortunately the surgical removal of the medullary tumors carries a high risk of surgically related complications since the tumor infiltration may preclude the functional preservation of the surrounding brain stem parenchyma. In experienced hands the risk of morbidity is acceptable and results for complete or subtotal resection are good. In the management of diffuse tumors, surgery has no role.

Analysis of the clinical history and physical and neurological evaluation must be done precisely. Short duration of symptoms and presence of multiple cranial nerve deficits are suggestive of a diffuse tumor which in most cases is a high-grade pathology with a bad outcome [2, 7, 12]. In such a condition, a radical excision is not indicated [4].

Detailed MRI workup in three planes is necessary to have an idea about the anatomical relationship of the lesion with normal structures.

All of the probable complications must be told with precise detail to the patient relatives before the operation. They should be informed about the serious complications of loss of swallowing and airway protection, resulting to the need for gastrostomy and tracheostomy.

A good preoperative evaluation of the patient by the anesthesiology team will help to avoid intraoperative and early postoperative complications.

Operations inside the medulla are delicate because of many important structures that are located within a very small volume of brain tissue. Morbidity can be decreased by using intraoperative neurophysiological methods which are described in detail in chapter “Treatment of medulloblastoma:​ chemotherapy.” Motor structures can be identified by electrically stimulating the lower part of the surface of the floor of the fourth ventricle and recording the EMG responses from muscles that are innervated by the respective motor systems. The hypoglossal nerve can be identified and recordings are made from genioglossal muscle. IX, X, and XI cranial nerves can be identified using similar methods [13]. SEP and MEP usage allows surgeons to be more aggressive on the tumor [13, 14].

Intraoperative magnetic resonance imaging (ioMR) has been shown to be a useful tool for maximizing the extent and safety of resection in glioma surgery [15]. The image quality provided by current high- and ultrahigh-field ioMR imaging systems is not more inferior to postoperative MR imaging studies. Therefore, an optimal evaluation of the surgical outcome can be obtained during the procedure, and further improvements can be performed before the procedure ends without increasing morbidity. Whether this further resection of LGGs without increasing surgical morbidity will translate into prolonged survival is not currently known.

Three-tesla ioMR imaging protocol takes less than 10 min including the transfer and the authors obtain very high-resolution T2-weighted MR images without the use of intravenous contrast [16]. Functional imaging sequences, such as DTI sequences, were effectively used pre- and postoperatively but were not successful intraoperatively because of magnetic inhomogeneity [15].