Intrafraction Verification Using ExacTrac 6D Imaging for Frameless Cranial Radiosurgery

Abstract

Background: Radiosurgery has been an integral component in the management of intracranial tumors. It involves the administration of a high dose to the tumor. An essential aspect of this kind of treatment delivery is accurate localization of the tumor target that is precisely reproducible during treatment. To achieve this, the methods of positioning and immobilization of the patient play a critical role. Conventionally, the patient has been immobilized with invasive head frame and the location of internal anatomy deduced by external coordinates. This method has some drawbacks. - It works on the assumption that the external coordinates represent the isocentre location correctly. - It depends on the spatial relationships between the frame and the skull. Slippage and deformity of the frame, although rare, can cause an error in positioning. - From the patient's perspective, the placement of the frame is a source of discomfort. It also puts them at risk for bleeding and infection. Frameless SRS has come into use in recent times. This is a noninvasive method. However, giving up the use of an invasive head frame leads to increased intrafraction motion. To overcome this, we require constant verification of the patient's positioning at every step of the treatment. Aim: The objective of this study is to compare the precision and accuracy between Brainlab ExacTrac imaging and CBCT and evaluate the intrafraction motion of patient undergoing intracranial SRT. Methods: For this study we included 10 patients with intracranial tumors who are eligible to undergo SRS. A frameless stereotactic head frame mask was prepared to immobilize the head during SRS. During treatment the patients were immobilized with the same frame and the 6D positional accuracy was checked for every single field before the treatment execution by taking verification image using 6D ExacTrac and Onboard Imagers. The 6D ExacTrac tolerance were set to rotation < 1.00 and translational shift < 0.7 mm. We make sure of intrafarction movement by taking frequent ExacTrac images during the treatment of every 30 s. After the treatment the registration between the planning CT image and CBCT image taken before treatment were cross verified 6 dimensionally using offline review. The difference in shift between ExacTrac and CBCT are evaluated by means of RMS. Results: The RMS of the difference in the translational dimensions between the ExacTrac System and CBCT was < 1.0 for online matching and < 1.10 for offline matching. Furthermore, the RMS of the difference in the rotational dimensions between the ExacTrac System and CBCT were < 0.81 for online matching and < 0.96 for offline matching. Conclusion: It is concluded that the discrepancies in residual set-up error between the ExacTrac 6D x-ray and the CBCT were minor. ExacTrac 6D imaging provides additional advantage of patient intrafraction movement control by means of imaging during treatment.