Six degrees of freedom CBCT‐based positioning for intracranial targets treated with frameless stereotactic radiosurgery

Abstract

Frameless radiosurgery is an attractive alternative to the framed procedure if it can be performed with comparable precision in a reasonable time frame. Here, we present a positioning approach for frameless radiosurgery based on in‐room volumetric imaging coupled with an advanced six‐degrees‐of‐freedom (6 DOF) image registration technique which avoids use of a bite block. Patient motion is restricted with a custom thermoplastic mask. Accurate positioning is achieved by registering a cone‐beam CT to the planning CT scan and applying all translational and rotational shifts using a custom couch mount. System accuracy was initially verified on an anthropomorphic phantom. Isocenters of delineated targets in the phantom were computed and aligned by our system with an average accuracy of 0.2 mm, 0.3 mm, and 0.4 mm in the lateral, vertical, and longitudinal directions, respectively. The accuracy in the rotational directions was 0.1°, 0.2°, and 0.1° in the pitch, roll, and yaw, respectively. An additional test was performed using the phantom in which known shifts were introduced. Misalignments up to 10 mm and 3° in all directions/rotations were introduced in our phantom and recovered to an ideal alignment within 0.2 mm, 0.3 mm, and 0.4 mm in the lateral, vertical, and longitudinal directions, respectively, and within 0.3° in any rotational axis. These values are less than couch motion precision. Our first 28 patients with 38 targets treated over 63 fractions are analyzed in the patient positioning phase of the study. Mean error in the shifts predicted by the system were less than 0.5 mm in any translational direction and less than 0.3° in any rotation, as assessed by a confirmation CBCT scan. We conclude that accurate and efficient frameless radiosurgery positioning is achievable without the need for a bite block by using our 6 DOF registration method. This system is inexpensive compared to a couch‐based 6 DOF system, improves patient comfort compared to systems that utilize a bite block, and is ideal for the treatment of pediatric patients with or without general anesthesia, as well as of patients with dental issues. From this study, it is clear that only adjusting for 4 DOF may, in some cases, lead to significant compromise in PTV coverage. Since performing the additional match with 6 DOF in our registration system only adds a relatively short amount of time to the overall process, we advocate making the precise match in all cases.PACS number: 87.55.tm; 87.55.Qr; 87.57.nj

Quantifying isocenter measurements to establish clinically meaningful thresholds

“…Now radiolucent cranial and body immobilization devices allow photon imaging to verify proper patient position. Therefore, this test has been modified from the use of film with the advent of on‐board imagers as the image acquisition device 21 , 22 , 23 , 24 …”

Are pitch and roll compensations required in all pathologies? A data analysis of 2945 fractions

“…Based on these publications, it is clear that adjusting only for 4DoF may, in some cases, lead to a significant reduction in PTV coverage. As stated by Dhabaan et al, 12 while it may not be required to make the pitch and roll adjustments in all cases, it would be difficult to establish, while the patient is on the treatment couch, whether a particular rotational adjustment would make a significant difference in PTV coverage. Since performing the additional match with 6DoF only adds a relatively short amount of time to the overall process, we suggest making the precise match in all cases.…”

“…[12][13][14] They reported a loss of dose coverage of up to 5% to the planning target volume (PTV) if all three rotational shifts were not applied. 13 In another study, 14 the effect of applying 4DoF instead of 6DoF shifts was examined when treating stereotactic body radiation therapy cases, such as spine radiosurgery.…”

“…Finally, authors underline how the kV-CBCT, simulation CT matching, should be, in principle, more accurate than simple two projections based on bony anatomy, in particular in case of solid tumours. In this context, Dhabaan et al 12 suggested a volumetric imaging system (e.g. CBCT) positioning in frameless stereotactic radiotherapy.…”

A planning target volume margin formula for hypofractionated intracranial stereotactic radiotherapy under cone beam CT image guidance with a six-degrees-of-freedom robotic couch and a mouthpiece-assisted mask system: a preliminary study

“…4 Reported maximum registration errors along any Cartesian co-ordinate axis were 0.5 mm for a phantom; 1 and 1.0 or 3.2 mm (mask dependent), 2 2.0 3 and 1.2 mm 4 for patients. The mean 6 standard deviation (SD) along any Cartesian co-ordinate axis was 0.07 6 0.17 mm for a phantom based on 12 plans and 5 repeated CBCT acquisitions, 1 0.2 6 0.4 mm for 10 patients with 6 fractions 3 and 0.4 6 0.3 mm for a phantom and 0.5 6 0.3 mm for patients including manual couch angle adjustments.…”