## 3 Review Results
### Virtual Dissection of Eloquent WM Tracts  
The benefits of knowing the position of eloquent WM tracts are immediately clear, as if known, their radiation exposure can be minimised, and more of their function preserved. This concept has already been used in stereotactic neurosurgery to avoid important WM tracts [18]. This can be applied to radiotherapy to include eloquent tracts as OAR and minimise their radiation dose. However, the possible tracts passing near the area of pathology may not be obvious, and disease processes such as tumours may alter or obscure the normal anatomy. Therefore, the application of tractography requires a highly skilled operator and is potentially subjective. An example of altering a CTV by including a tract as an OAR is shown in Figure 4,  
A study used deterministic tractography in Gamma Knife radiosurgery to model suspected tracts at risk, such as the optic radiations and arcuate fasciculus, including them as OAR in target volume planning, showing that this is possible. However, one limitation is that the study does not make any comparison to a control group to show that the tracts dose is reduced [20]. Additionally, it remains unclear whether reducing radiation dose to this part of the standard clinical target volume leads to higher rates of local recurrence or not. Several examples in the literature focus on methods of minimising radiation to the pyramidal tract/corticospinal tract (CST), due to it being one of the most salient WM pathways and being easily identified with the more basic DTI processing. A study compared the predicted radiation does to the pyramidal tract with and without the use of including the tracts. The study found there was a significantly reduced dose to the pyramidal tract without dose reductions in the area of pathology [21]. The study used the cerebral peduncles, posterior limb of the internal capsule, and primary motor cortex selected on T1w MRI as the seed points for tractography. Another retrospective study that used a non-randomised control method with 52 subjects in total found that modelling of the corticospinal tracts reduced the risk or motor complications in patients treated with SRT, although not all non-control subjects had the CST dissected, and it was operator dependent [22]. \begin{table}
\begin{tabular}{p{128.0 pt} p{128.0 pt} p{128.0 pt} p{128.0 pt}} \hline
**Reference** & **Type of Study** & **Number of Subjects** & **Summary** & **Main Outcomes** \\ \hline [PERSON] et al., 2012 [31] & Prospective integration & 155 (all with AVM, 71 of which had tractography) & Routine integration of tractography in radiotherapy of AVMs at a single centre. Selected tracts user selected if suspected to be close to the pathology. & Tractography used in 71 out of 155 radiotherapy cases during the study period with 60\% of cases using tractography finding the critical tracts within 5 mm of the lesion. \\ \hline [PERSON] et al., 2017 [23] & Theoretical planning study & 16 (6 AVM, 8 CM, 2 Menig) & Integration of fMRI to select tractography seed point to model the corticospinal tracts and ’sensor pathway’. Functional regions and tracts included as OAR during planning for CyberKnife radiosurgery. Treatment plans with and without integration were compared. \\ \hline [PERSON] and [PERSON] 2019 [19] & Systematic review & NA & Systematic review of literature on DTI in intracranial radiotherapy. & Finds 13 articles evaluating DTI in radiotherapy that include AVM, many of which use prospective integration. \\ \hline \end{tabular}
\end{table} 
Table 1: Cont. A limitation of using structural anatomy for selecting seed points is that functional areas are not always anatomically consistent. A paper attempted to overcome this issue by using fMRI to define the seed region of the pyramidal tract. They compared radiotherapy plans with and without the inclusion of the CST as an OAR and found that their inclusion reduce the average radiation dose by 22.7% [23]. However, a limitation of this study is that fMRI and anatomically defined pyramidal tracts were not compared.