## 1 Introduction  
### Intracranial Radiotherapy  
Radiotherapy continues to advance to be able to target areas with higher conformality. The introduction of stereotactic radiotherapy (SRT), sometimes referred to as stereotactic radiosurgery, has meant that radiation can be directed with increasing precision. This results in better outcomes by enabling a higher dose to the area of pathology while reducing the radiation dose to healthy brain structures. Popular implementations include the Gamma Knife and Cyberknife platforms [1]. Intracranial radiotherapy requires planning the target area, which is typically done using MRI to identify the target region and eloquent brain structures that need to be avoided. This is usually performed with structural MRI sequences such as T1w, T2w, and FLAIR. This is achieved by manual segmentation of the pathology, with eloquent areas additionally segmented as organs at risk (OAR). Examples of OAR that are currently advised to be included are the brain stem, optic chiasm, cochlea, and others, with each OAR having an advised maximum radiation dose measured in Gray (Gy) to avoid significant late dysfunction and neurological toxicities [2]. Then, a CT with a frame is required for dose treatment planning. With the advent of intensity-modulated radiation therapy (IMRT), the dose to pathological structures can be conformed to in 3D with excellent conformality. IMRT splits the beams into many smaller beamlets, enabling radiation to be directed into complex shapes and have a non-uniform radiation intensity [3]. IMRT additionally utilises computerised inverse planning where the radiation dose to the target region and dose limits to healthy brain and OAR are specified and optimised in advance. Both Cyber and Gamma Knife SRT delivery platforms include treatment planning software that enables integrated SRT planning allow for a combination of frameless MRI and the planning CT [4]. When targeting primary intracranial malignancies, the tumour visible on imaging is referred to as the gross tumour volume (GTV); this is then expanded by a standard margin (dependent on the tumour type) to incorporate any peritumoral infiltration that is unseen on imaging and is referred to as the clinical target volume (CTV). Therefore, the expansion of the CTV is important to target these migrating cells to reduce the chance of recurrence. Then, the CTV is typically expanded further by a few mm to allow for patient set-up errors that can occur despite immobilisation and be used as the final planning target volume (PTV) [5]. An example of the differing target volumes is shown in Figure 1. These expansion steps are typically done isotropically unless there are barriers to tumour spread e.g., skull or dura when delineating the CTV. The European Organization for Research and Treatment of Cancer (EORTC) current guidance for SRT in glioblastoma treatment is to define the GTV as the T1w contrast-enhancing tumour, or resection cavity, and then, the CTV is defined as the GTV expanded isotropically by 2 cm. Then, 3-5 mm is additionally added to define the PTV [2]. The CTV is set at a 2 cm isotropic expansion, as roughly 80% or more recurrences are within 2 cm of the enhancing tumour core. Figure 1: Diagram displaying an example of the gross tumour volume (GTV) with the isotropically expanded clinical target volume (CTV) and the planned target volume (PTV) on a temporal lobe glioma. Addition of the brain stem as an organ at risk (OAR) on the right and how the PTV could be altered using inverse planning to reduce radiation dose to the brain stem. Radiotherapy to the brain can lead to many neurological complications due to the damage of healthy brain structures and is usually dependent on dose, fraction size, volume of irradiated tissue, and concurrent chemotherapy use [6]. Complications include vascular abnormalities such as radiation necrosis and ischemia but additionally include progressive neurocognitive decline. This can lead to many long-term side effects in addition to short-term complications [7].