Evaluation of Radiosurgery Target Volume Definition for Tectal Gliomas with Incorporation of Magnetic Resonance Imaging (MRI): An Original Article Acute Onset Paraplegia Due to A

Tectal gliomas are rare, typically low grade tumours located in close vicinity of the cerebral aqueduct...


Introduction
Tectal gliomas are rare, typically low-grade tumours located in close vicinity of the cerebral aqueduct. These lesions in the tectal region of the mesencephalon are considered as a distinctive subgroup of brainstem gliomas with respect to their unique radiological, pathological, and clinical features. Focal, intrinsic lesions of the tectum are typically located at the periaqueductal region, and contrast enhancement is uncommon. Primary symptoms suffering from tectal gliomas occur due to obliteration of aqueduct of Sylvius resulting in noncommunicating hydrocephalus. Given the typically indolent nature, tectal gliomas lead to symptoms associated with late-onset hydrocephalus. Affected patients generally have a history of headache and may have other symptoms such as nausea, vomiting, gait and visual disturbances. Although benign behaviour is not uncommon for tectal gliomas, these tumours have been described as smallest lesions in the body leading to death [1][2][3]. Computed tomography (CT) may aid in detection of hydrocephalus, however, may not be capable of revealing the tectal lesion in about half of patients suffering from tectal glioma [3,4]. Majority of these tumours are visualized as periaqueductal, dorsally exophytic, well-circumscribed, non-enhancing, intrinsic focal lesions of the tectum extending from the quadrigeminal plate with T2-hyperintensity and T1-isointensity on Magnetic Resonance Imaging (MRI) [4][5][6][7][8]. Pineal cysts and germ cell tumours may be considered for differential diagnosis of tectal gliomas.
Given the indolent nature of these tumours, observation may be an option in selected patients. However, therapeutic options including surgical interventions, systemic agents, and radiation therapy (RT) have been utilized for management of tectal gliomas as upfront or salvage therapies [2,3,[9][10][11][12][13][14][15][16][17]. Contemporary RT techniques and radiosurgery in the forms of Stereotactic Radiosurgery (SRS), Stereotactic Body Radiotherapy (SBRT) and Hypo fractionated Stereotactic Radiation Therapy (HFSRT) may be utilized for focused irradiation of several central nervous system disorders as well as tumours located throughout the human body . Target volume definition is an indispensable part of radiosurgery given that smaller volumes are treated to higher doses per fraction. Treatment planning for radiosurgery has been traditionally based on CT-simulation images of the patient acquired in treatment position. In this study, we assess the incorporation of MRI for radiosurgery treatment planning of tectal gliomas.

Materials and Methods
Determination of radiosurgery target volume with and without incorporation of MRI has been assessed for linear accelerator (LINAC)-based radiosurgery of tectal glioma in this study. Ground truth target volume serving as the reference has been determined by board certified radiation oncologists after comprehensive evaluation, collaboration, and consensus. Informed consents have been acquired prior to treatment, and management of patients with radiosurgery has been decided by a multidisciplinary team of experts from neurosurgery, neuroradiology, and radiation oncology. Detailed evaluation has been performed regarding the lesion size, location, patient symptomatology and preferences. CT-simulation to be used for radiosurgery treatment planning has been performed at CT-simulator (GE Lightspeed RT, GE Healthcare, Chalfont St. Giles, UK) available at our institution. Planning CT images have been acquired and then transferred to the delineation workstation (SimMD, GE, UK) for contouring of treatment volumes and critical organs. Target volume determination for radio surgical management has been performed by use of CT simulation images only or fused CT and T1 gadolinium-enhanced MR images. Target determination with CT only and by incorporation of CT-MR fusion has been comparatively evaluated. Determination of ground truth target volume has been performed by board certified radiation oncologists after detailed assessment, collaboration, colleague peer review and consensus for actual treatment and comparison purposes.

Results
Definition of target volume by use of CT-only imaging and CT-MR fusion-based imaging has been comparatively assessed in this study. Ground truth target volume which has been determined by the board-certified team of radiation oncologists after detailed evaluation, collaboration, colleague peer review and consensus has been found to be identical to target determination by use of CT-MR fusion-based imaging.
Radiosurgery treatment planning has been accomplished by ERGO ++ (CMS, Elekta, UK) radiosurgery planning system. A single 360-degree arc, double 360-degree arcs, or five 180-degree arcs have been used in radiosurgery planning to achieve optimal target coverage and normal tissue sparing. Treatment delivery has been performed by Synergy (Elekta, UK) LINAC available at our institution. Determination of the target volume on planning CT and MR images has been optimized by selecting the appropriate windows and levels for radiosurgery planning. Precise delineation of target volume and critical structures has been provided by supplementing the axial planning CT images with sagittal and coronal images. Target volume coverage and normal tissue sparing has been optimized by use of Arc Modulation Optimization Algorithm (AMOA). Figure 1 shows axial planning CT image and Figure 2 shows the axial MR image of a patient with tectal glioma.

Discussion
Tectal glioma is considered as a benign subgroup of brainstem gliomas with an indolent disease course. Nevertheless, management of patients may be required for relief of symptoms arising from hydrocephalus. RT may be used as part of management for tectal gliomas. There have been significant improvements in the discipline of radiation oncology recently such as Image Guided Radiation Therapy (IGRT), Adaptive Radiation Therapy (ART), Intensity Modulated Radiation Therapy (IMRT), Breathing Adapted Radiation Therapy (BART), and stereotactic irradiation with SRS, HFSRT, and SBRT [41][42][43][44][45][46][47]. Radiosurgery has been used for treatment of tectal gliomas with encouraging outcomes [16,17]. Smaller lesions are well suited for radiosurgery given the higher delivered dose per fraction. Radiosurgery may be exploited for its improved precision in RT delivery with stereotactic localization and optimized normal tissue sparing due to steep dose gradients around the target volume. However, it is more sensitive to delineation errors since typically little or no margin around the target volume exists. In this context, definition of target volume for radiosurgery is a highly important part of successful patient management.
While determination of larger target volumes may result in untowards treatment related toxicity, smaller than actual target volumes may lead to geographical misses with resultant treatment failure. Multimodality imaging may improve target localization, and the use of fused CT and MR images may supplement each other for precision in delineation of target volume for radiosurgery. Utility of multimodality imaging for radiosurgery treatment planning for tectal gliomas has been poorly addressed in the literature. In this context, our study adds to the existing literature and supports the incorporation of MRI for improved radiosurgery treatment planning for tectal gliomas consistent with another studies [48][49][50][51][52][53][54][55][56][57][58][59].
In conclusion, incorporation of MRI into target definition process may improve radiosurgery treatment planning for tectal gliomas despite the need for further supporting evidence.