Utility of Multimodality Imaging Based Target Volume Definition for Radiosurgery of Trigeminal Neuralgia: An Original Article

Objective: Trigeminal neuralgia, also named “tic douloureux”, is the facial pain which is usually one-sided in the distribution of 5th cranial nerve. After Leksell used radiosurgery as a therapeutic option for trigeminal neuralgia, this treatment modality has been accepted in the treatment of refractory pain caused by trigeminal neuralgia. Studies investigating the role of radiosurgery have rapidly increased regarding the poor results in pain control with invasive procedures, high rates of relapse and remarkable morbidity. Non-invasiveness and effectivity are the most important advantages of radiosurgery. The aim of this study is to assess incorporation of multimodality imaging into target volume definition of trigeminal neuralgia radiosurgery. Materials and methods: Target definition with and without incorporation of Magnetic Resonance Imaging (MRI) was evaluated for linear accelerator-based radiosurgery of trigeminal neuralgia. Ground truth target volume to be used as the reference was defined by the board-certified treating physicians after comprehensive evaluation and consensus. Results: Comparative assessment of target volume determination using Computed Tomography (CT) only imaging and CT-MR fusion-based imaging revealed that target volumes generated by CT-MR fusion-based imaging were identical to ground truth target volumes determined by the treatment team consensus. Conclusion: MRI may serve as a valuable imaging modality improving target definition for linear accelerator-based radiosurgery of trigeminal neuralgia.


Introduction
Trigeminal neuralgia, also named "tic douloureux", is the facial pain which is usually one-sided in the distribution of 5th cranial nerve. The most commonly known cause of trigeminal neuralgia is the vascular compression of the trigeminal nerve [1,2]. The vascular compression is in superior cerebellar artery in 64% of the cases [3]. Vascular compression leads to continuous demyelination and remyelinisation cycles which causes myelinisation abnormalities of the nerve. High frequency ectopic impulses from this impaired myelinisation area may cause pain [4]. Medical treatment is the first step in trigeminal neuralgia that may lead to committing suicides.
However, additional therapies are required in the setting of temporary response or refractory pain despite medical treatment.
Carbamazepine, fenitoin, gabapentin, and baclofen may be used as medical treatment [5]. Invasive interventions are done when medical treatment is ineffective or when the side effects of medical treatment are unacceptable. Interventional procedures include percutaneous radiofrequency (RF) rhizotomy, percutaneous glycerol rhizotomy, percutaneous balloon micro compression and microvascular decompression (MVD). MVD is the most invasive procedure, however, repeating pain rates are lower with MVD [6].
Relapse is observed approximately in 30% of patients initially treated with MVD.
Studies investigating the role of radiosurgery have rapidly increased regarding the poor results in pain control with invasive procedures, high rates of relapse and remarkable morbidity.
In this context, radiosurgery has been used as a treatment modality for management of trigeminal neuralgia as well as several other intracranial and extracranial benign and malign indications .
The aim of this study is to assess incorporation of multimodality imaging into target volume definition of trigeminal neuralgia radiosurgery.

Target definition with and without incorporation of Magnetic
Resonance Imaging (MRI) was evaluated for linear acceleratorbased radiosurgery of trigeminal neuralgia in this study. Ground truth target volume to be used as the reference was defined by the board-certified treating physicians after comprehensive evaluation and consensus. Informed consents were provided before treatment, and management with radiosurgery was decided after thorough assessment including multidisciplinary collaboration of experts from neurosurgery, neuroradiology, and radiation oncology. A

Target volume determination based on CT-only imaging and
CT-MR fusion-based imaging was comparatively evaluated. Ground   were correlated with the length of the irradiated nerve segment [37]. In a study by Riesenberger et al. [38] 53 patients with trigeminal neuralgia were treated with single dose frame-based radiosurgery with a follow-up of at least 36 months. Median maximal dose was 80 Gy. 31 patients had significant pain relief after 48 months of follow-up in the study [38]. These results are similar to those achieved with MVD and rhizotomy. However, 36% of patients developed facial numbness after treatment. Kondziolka et al. [39] reported on 503 patients resistant to medical treatment who were treated with radiosurgery. 205 patients had undergone previous surgery and relapsed. Maximal dose was 80 Gy. 85% of patients had pain relief with or without medical treatment [39]. In this study, we assessed incorporation of multimodality imaging into target volume definition of trigeminal neuralgia radiosurgery. Indeed, usefulness of neuroimaging with MRI for definition of central nervous system radiotherapy and radiosurgery target volumes has been an important focus of research in the literature [40][41][42][43][44][45][46][47][48][49]. Our results appear to be compatible to these studies supporting the incorporation of MRI in radiosurgery treatment planning. Ground truth target volumes determined by comprehensive evaluation, consensus and colleague peer review were identical to target volumes defined based on CT-MR fusion-based imaging.

Conclusion
In conclusion, our study supports the utilization of MRI to improve target definition for linear accelerator-based radiosurgery of trigeminal neuralgia despite the need for further studies.