Residual setup errors in cranial stereotactic radiosurgery without six degree of freedom robotic couch: Frameless versus rigid immobilization systems

Abstract Purpose and Objectives This IRB‐approved study was to compare the residual inter‐fractional setup errors and intra‐fractional motion of patients treated with cranial stereotactic radiosurgery without a 6 degree of freedom (DoF) couch. We evaluated both frameless non‐invasive vacuum‐suction immobilization (Aktina PinPoint) and TALON rigid screw immobilization. Materials and Methods Twenty consecutive patients treated by Varian TrueBeam STX or Tomotherapy were selected for data collection. The dose and number of fractions received by each patient ranged from 18 Gy in 1 fraction (SRS) to 25 Gy in 5 fractions (SRT). Twelve patients were immobilized using PinPoint, a frameless suction system (Aktina Medical, New York) and eight patients were immobilized using the TALON rigid screw system. Customized head cushions were used for all patients. Six Atkina patients received pre‐ and post‐treatment cone‐beam CT (CBCT) to evaluate the intra‐fractional motion of the Aktina system. The intra‐fractional motion with the TALON rigid screw system has been reported to be negligible and was not repeated in this study. All patients received pre‐treatment CBCT or megavoltage CT (MVCT) to assess inter‐fractional setup accuracy. Shifts to the final treatment position were determined based on matching bony anatomy in the pre‐treatment setup CT and the planning CT. Setup CT and planning CT were registered retrospectively based on bony anatomy using image registration software to quantify rotational and translational errors. Results For the frameless Aktina system, mean and standard deviation of the intra‐fractional motion were −0.5 ± 0.7 mm (lateral), 0.1 ± 0.9 mm (vertical), −0.5 ± 0.6 mm (longitudinal), −0.04 ± 0.18°(pitch), −0.1 ± 0.23°(yaw), and −0.03 ± 0.17°(roll) indicating negligible intra‐fractional motion. Inter‐fractional rotation errors were −0.10 ± 0.25° (pitch), −0.08 ± 0.16° (yaw), and −0.20 ± 0.41° (roll) for TALON rigid screw immobilization versus 0.20 ± 0.69° (pitch), 0.34 ± 0.56° (yaw), 0.35 ± 0.82° (roll) for frameless vacuum‐suction immobilization showing that the rigid immobilization setup is more reproducible than the frameless immobilization. Without rotational correction by a 6 DoF couch, residual registration error exists and increases with distance from the image fusion center. In a 3D vector space, a tumor located 5 cm from the center of image fusion would require a 0.9 mm margin with the TALON system and a 2.1 mm margin with Aktina. Conclusions With image‐guided radiotherapy, translational setup errors can be corrected by image registration between pre‐treatment setup CT and planning CT. However, rotational errors cannot be accounted for without a 6 DoF couch. Our study showed that the frameless Aktina immobilization system provided negligible intra‐fractional motion. The inter‐fractional rotation setup error using Aktina was larger than rigid immobilization with the TALON system. To treat a single lesion far from the center of image registration or for multiple lesions in a single plan, additional margin may be needed to account for the uncorrectable rotational setup errors.

STX or Tomotherapy were selected for data collection. The dose and number of fractions received by each patient ranged from 18 Gy in 1 fraction (SRS) to 25 Gy in 5 fractions (SRT). Twelve patients were immobilized using PinPoint, a frameless suction system (Aktina Medical, New York) and eight patients were immobilized using the TALON rigid screw system. Customized head cushions were used for all patients. Six Atkina patients received pre-and post-treatment cone-beam CT (CBCT) to evaluate the intra-fractional motion of the Aktina system. The intra-fractional motion with the TALON rigid screw system has been reported to be negligible and was not repeated in this study. All patients received pre-treatment CBCT or megavoltage CT (MVCT) to assess inter-fractional setup accuracy. Shifts to the final treatment position were determined based on matching bony anatomy in the pretreatment setup CT and the planning CT. Setup CT and planning CT were registered retrospectively based on bony anatomy using image registration software to quantify rotational and translational errors.
Conclusions: With image-guided radiotherapy, translational setup errors can be corrected by image registration between pre-treatment setup CT and planning CT.
However, rotational errors cannot be accounted for without a 6 DoF couch. Our study showed that the frameless Aktina immobilization system provided negligible intra-fractional motion. The inter-fractional rotation setup error using Aktina was larger than rigid immobilization with the TALON system. To treat a single lesion far from the center of image registration or for multiple lesions in a single plan, additional margin may be needed to account for the uncorrectable rotational setup errors.
cranial stereotactic radiosurgery, immobilization, margin, Aktina, TALON, six DoF robotic couch 1 | INTRODUCTION Immobilization devices are commonly used in radiation therapy as they provide reproducible patient positioning. For intracranial lesions, treatment positioning, and immobilization that provide sub-millimeter accuracy is desired for single fraction stereotactic radiosurgery (SRS) or fractionated stereotactic radiotherapy (SRT). 1,2 The positioning accuracy includes inter-fraction setup reproducibility (localization) and intra-fraction motion (fixation). Inter-fraction error relates to the setup accuracy of the same posture and position between CT simulation and treatment delivery or subsequent deliveries (SRT). The intra-fraction error is the amount of patient motion over the course of a single treatment. There are many commercially available immobilization devices designed to reduce intra-fraction and inter-fraction motion in intracranial radiation treatment. Invasive devices include the Leksell, 3,4 Reichert-Mundinger, 5 and Brown-Roberts-Wells (BRW) 6 frames. Traditionally, those invasive head frames are attached to the patient during the entire planning and treatment course. It is believed the device can fix the cranium rigidly for perfectly reproducible patient geometry.
Thus no treatment margin beyond the treated target is needed. [2][3][4][5][6][7] Non-invasive options use one of or a combination of thermoplastic masks, bite blocks, cradles, and optical surface tracking imaging systems to localize and immobilize the brain. 8 While the invasive devices can often offer sub-millimeter positional accuracy, the invasive nature limits their acceptance by patients and deters fractionated treatments. In the non-invasive realm, vendors are constantly developing new techniques to improve the positioning accuracy of their devices.
In this study, we compared two commercially available devices used in our clinic: Aktina PinPoint™ (non-invasive) 8 and TALON Cranial SRS frame (invasive). 9 Both intra-fraction and inter-fraction setup errors were evaluated. Additionally, recommendations on margins required to compensate for residual set up errors were discussed.

| METHOD AND MATERIALS
Twenty consecutive patients treated by Varian TrueBeam STX or Tomotherapy were selected for data collection and analysis. The particular TALON is used throughout the individual patient's treatment course, SRS or SRT. Between CT simulation and treatment, it is detached from the screws in the patient's skull but the integrity of the ball joints and geometry remains untouched. Essentially, the TALON system is a removable stereotactic immobilization system using a one-time invasive placement of two titanium screws and a noninvasive daily application.

2.B | Aktina frameless immobilization
The Aktina PinPoint (Aktina Medical, Congers, New York) is a noninvasive frameless alternative designed for intracranial single fraction and multi-fraction SRS treatments [ Fig. 1(b)]. This device contains a patient-specific mouth piece which is constructed during CT simulation to fit the individual patient's dental anatomy. After the mold is made, vacuum suction between the mold and the upper hard palate establishes a firm seal. The mouth piece is attached to the couch via Aktina metal frame to prevent motion in any direction.
Together with a customized head cushion, the Aktina helps to position the patient's head in the same geometry and renders cranium fixation during the course of treatment. Because of its noninvasive nature, the usage of Aktina has potential advantage over any invasive device including TALON.

2.C | CT simulation
All 20 patients analyzed in this study were CT-simulated using the GE Discovery 590RT™ 16 slice large bore CT scanner. Patient was either immobilized with TALON (8 patients) or Aktina (12 patients) in conjunction with Accuform™ (CIVCO, Kalona, Iowa) head cushion.
The same scanning technique (120 KV, 200 mA, 512 × 512 image size with 1.25 mm slices and spacing) was used. Images were transferred to Eclipse (Varian, Palo Alto, CA) treatment planning computer. The CT data were registered in Eclipse using rigid registration with MRI for contouring. Typical organ and target contours included GTV, PTV, brain stem, lens, eyes, optic nerves, optic chiasm, and brain. Figure 2 shows the regions of interest on registered CT and MR images. Because this study was conducted before the 6 degrees of freedom robotic couch was installed, not all rotational (pitch, roll, and yaw) correction could be performed for the actual treatments.  Assuming points X and X′ are the same anatomy points in 2 image datasets (Fig. 4), the misalignment can be corrected by formula (1).

2.E | Image fusion
where P ¼

3.B | Inter-fraction set up error
The inter-fraction reproducibility of a device can be different from intra-fraction immobilization characteristics. In addition to the device design, user or therapist's skill and experience level can play an important factor. A good immobilization device should be able to F I G . 3. Auto registration between simulation planning CT (primary image) and pre-treatment CBCT (inside the box) using whole brain bony anatomy.
minimize user dependence and be independent of skill and experience level of the therapist. In our study, Aktina and TALON did show significantly different characteristics in inter-fraction reproducibility. While translation error can be corrected by couch shift, residual rotational error due to setup uncertainty cannot be corrected without a 6 DoF robotic couch. The uncorrected rotational error would potentially contribute to inaccuracy in radiation treatment delivery. For all three rotational dimensions, the differences were statistically significant with P-values of 0.007656, 0.000353, 0.026248 in pitch, yaw, and roll, respectively. For both devices, roll direction had more set up uncertainty than the other two directions. That seems to be logical as it is more difficult to prevent patient from rotating the head in the roll direction.

3.C | Setup margin
Varian, Elekta, and other conventional C-Arm linacs can correct the yaw by couch rotation and Accuray Tomotherapy machine can correct the roll by automatic gantry rotation adjustment. However, without a 6 degree robotic couch, not all residual rotational errors can be corrected. Additional GTV to PTV margin may be necessary to compensate the set up inaccuracy to avoid geographical miss. The data of our study agreed reasonably to a Gaussian distribution with a mean centered approximately at zero (Fig. 5). In order to have 95% confidence interval, the additional margin needed varies with the distance of the tumor target to the image fusion center. For a single lesion, the registration can be focused at the center of the tumor. A small magnitude of rotation error should not significantly compromise the accuracy of the dose distribution. However, when multiple lesions are treated with a single plan, aligning one lesion would result in misalignment of another lesion due to residual rotational errors. Figure 6 illustrates additional margin needed when tumor target is away from the center of image fusion. When two lesions are separated by 10 cm and the center of the image registration is in between the two lesions, 1 mm and 2 mm additional margin is indicated for TALON immobilization and Aktina device, respectively.

| DISCUSSION AND CONCLUSIONS
Over the years, many invasive and non-invasive immobilization devices were created to minimize uncertainty in treatment delivery. 10 Some examples of immobilization devices used over the years include rigid invasive SRS frame, TALON, surface tracking, plastic masks, and Aktina. To maximize patient comfort and compliance to treatment, a non-invasive option is obviously preferred. In the recent years, even though frameless real-time surface imaging-guided radiosurgery facilitated by VisionRT™ (Vision RT Inc. Columbia, MD) or C-Rad™ (C-RAD Group, Uppsala, Sweden) has been reported to be feasible, 11 it is our opinion that immobilization device are still necessary to assist in patient localization. In order to ensure the utmost precision in the delivery of radiation the patient must be accurately posi-  which is considered to be sub-millimeter. Therefore, with a 6 DoF robotic couch, a single plan can be potentially used to treat multiple lesions that are far apart. This would significantly improve the treatment efficiency and reduce the time patient spends on the treatment couch. One thing to note is that not all LINAC manufactures currently support 6 DoF robotic couches. Optical surface image guidance has recently been adopted in intracranial SRS/SRT. The technology provides sub millimeter monitoring of the patient surface and can be useful to monitor intra-fraction motion but has limited potential for reducing inter-fraction setup error beyond the current immobilization devices.