The Continuous Assessment of Cranial Motion in Thermoplastic Masks During CyberKnife Radiosurgery for Trigeminal Neuralgia.

Tewfik J Bichay,Alan Mayville

doi:10.7759/cureus.607

Abstract

Stereotactic radiosurgery (SRS) treatment is characterized by high doses per fraction and extremely steep dose gradients. This requires a great degree of accurate localization to the appropriate treatment position, and continuous immobilization during the treatment session. In the case of Trigeminal Neuralgia (TGN) treatment this is especially true as the very small target volume makes positional accuracy critical. In this study we carried out a quantitative analysis of patient motion during the full treatment fraction within a radiosurgery immobilization mask system. Patient cranial movement was assessed by using the image guidance stereo x-ray cameras on a CyberKnife (CK) M6 robotic radiosurgery system (Accuray, Sunnyvale, CA). A total of five patients received treatments for either right or left TGN. The duration of treatment varied from 24-64 minutes. Orthogonal images were taken every 15 seconds during the treatment to assess patient movement. Approximately 60 stereo images were taken per patient and a total of 560 images were analyzed in this study. The mean absolute movement in each of longitudinal, lateral or vertical directions was approximately 0.3 mm for the duration of the treatment; however, on occasion much greater movement was observed during a fraction. The maximum displacement was in the longitudinal direction and reached 2.4 mm compared to the initial setup. Images taken at the end of the treatment session showed that the patients typically return to a position closer to the original setup position than the maximum excursion that occurred. This data suggests that although this mask system appears stable during much of the treatment session; for some patients there may be momentary patient movements that take place. Frequent imaging and correction can help mitigate the effect of this movement. It is important to understand the limitations of non-invasive mask systems when used for very high precision treatment.

Highlights

Intracranial radiosurgery requires delivering high radiation dose to a very specific localized area while at the same time keeping the patient immobilized
In this study we determined whether continuous monitoring of patient position, at 15-sec intervals throughout the treatment fraction, would demonstrate any additional motion that would not be apparent by only imaging pre- and post-treatment
We have analyzed this data in a similar manner, evaluating the final patient position compared to the initial position

Summary

Introduction

Intracranial radiosurgery requires delivering high radiation dose to a very specific localized area while at the same time keeping the patient immobilized. Localization and immobilization were achieved with invasive devices such as the halo and Brown-Roberts-Wells (BRW) frames These frames can accurately localize the cranium to within 1 mm of the target area [1,2]. While some have argued that submillimeter SRS is not feasible with current technology [5] others have demonstrated that thermoplastic masks can maintain stability within 1 mm in any linear direction [2,3,4,6] These studies typically rely on x-ray imaging of the patient at the start of the treatment session with a repeat imaging session at the end of the treatment fraction, or may rely on repeated phantom imaging where no motion would be expected [7]. The intent was to determine the effectiveness of this mask-based system in immobilizing the cranium for the duration of critical radiosurgery

Methods

Results

Discussion

Conclusion