Evaluation of intrafractional head motion for intracranial stereotactic radiosurgery with a thermoplastic frameless mask and ceiling-floor-mounted image guidance device

Intrafraction motion in intra-cranial multi-target stereotactic radiosurgery plans: A multi-institutional investigation on robustness

PURPOSE

Even with modern immobilisation devices, some amount of intrafraction patient motion is likely to occur during stereotactic radiosurgery (SRS) delivery. The aim of this work was to investigate how robustness of plans to intrafraction motion is affected by plan geometry and complexity.

METHODS

In 2018, the Trans-Tasman Radiation Oncology Group conducted a multiple-target SRS international planning challenge, the data from which was utilised in this study. Patient geometry included five intracranial targets with a prescription of 20 Gy. A previously validated in-house algorithm was used to simulate realistic intrafraction patient motion for these plans. Three scenario types were simulated: translational intrafraction motion; rotational motion; and simultaneous rotational and translational motion. Dosimetric impact was assessed using: dose covering 98 % of planning target volume, dose covering 99 % of gross tumour volume (GTV D99%), volume of normal brain receiving 12 Gy and maximum dose covering 0.03 cc brainstem.

RESULTS

GTV D99% was reduced by up to 70 %, with the strongest correlations between planning factors and robustness to intrafraction motion found for plan complexity. Despite only moderate correlation strength at r = 0.4, lower complexity plans had, on average, 5 % - 9 % less intrafraction motion scenarios with failing targets compared to the highest complexity plans.

CONCLUSIONS

SRS plans with lower complexity, in particular larger mean multi-leaf collimator (MLC) gap and MLC aperture irregularity, were shown to improve plan robustness to intrafraction patient motion.

Margin derivation from intrafraction patient motion of multi-target, single isocentre, brain stereotactic radiosurgery treatments

BACKGROUND

Brain metastases are the most common intracranial malignancy and remain a substantial source of morbidity and mortality in cancer patients. Linear accelerator based stereotactic radiosurgery (SRS) is widely used and is frequently delivered by hypo-fractionnated volumetric modulated arc therapy using non-coplanar beams, where geometric accuracy and planning margins are a major concern.

PURPOSE

To give a practical analysis of intrafraction patient motion for multi-target, single isocentre, brain SRS treatments and to derive adapted GTV-to-PTV margins.

METHODS

Data of 154 lesions, spread over 85 fractions from 56 patients treated in our institution with the Varian HyperArc  SRS solution was processed. Intrafraction patient motion were recorded using an Optical Surface Monitoring System during irradiation. The present study focuses on small tumor volumes, roughly equal or inferior to 1.5 cm 3 ${\rm cm}^3$ , and frameless mask-based immobilization. For each treatment session, a tumor displacement vector matrix was calculated from the patient drifts as a function of time. Data were combined together into a representative treatment scenario and the dosimetric impact of GTV displacement was calculated.

RESULTS

Recommended margins due to patient motion range between 0.3 and 1 mm, depending on the distance tumor-isocentre, and the desired GTV edge dose coverage. Those values should be added quadratically with other sources of uncertainty, such as mechanical isocentre and kV-MV misalignment.

CONCLUSION

Thorough analysis of intrafraction patient motion was performed, the dosimetric impact was calculated for different scenarios, and adequate GTV-to-PTV margins were derived. These values vary according to the distance isocentre-to-GTV, as well as the desired dose coverage, and should be chosen adequately.

Insights into the dosimetric and geometric characteristics of stereotactic radiosurgery for multiple brain metastases: A systematic review

BACKGROUND

GammaKnife (GK) and CyberKnife (CK) have been the mainstay stereotactic radiosurgery (SRS) solution for multiple brain metastases (MBM) for several years. Recent technological advancement has seen an increase in single-isocentre C-arm linac-based SRS. This systematic review focuses on dosimetric and geometric insights into contemporary MBM SRS and thereby establish if linac-based SRS has matured to match the mainstay SRS delivery systems.

METHODS

The PubMed, Web of Science and Scopus databases were interrogated which yielded 891 relevant articles that narrowed to 20 articles after removing duplicates and applying the inclusion and exclusion criteria. Primary studies which reported the use of SRS for treatment of MBM SRS and reported the technical aspects including dosimetry were included. The review was limited to English language publications from January 2015 to August 2023. Only full-length papers were included in the final analysis. Opinion papers, commentary pieces, letters to the editor, abstracts, conference proceedings and editorials were excluded. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were followed. The reporting of conformity indices (CI) and gradient indices, V12Gy, monitor units and the impact of translational and rotational shifts were extracted and analysed.

RESULTS

The single-isocentre technique for MBM dominated recent SRS studies and the most studied delivery platforms were Varian. The C-arm linac-based SRS plan quality and normal brain tissue sparing was comparable to GK and CK and in some cases better. The most used nominal beam energy was 6FFF, and optimised couch and collimator angles could reduce mean normal brain dose by 11.3%. Reduction in volume of the healthy brain receiving a certain dose was dependent on the number and size of the metastases and the relative geometric location. GK and CK required 4.5-8.4 times treatment time compared with linac-based SRS. Rotational shifts caused larger changes in CI in C-arm linac-based single-isocentre SRS.

CONCLUSION

C-arm linac-based SRS produced comparable MBM plan quality and the delivery is notably shorter compared to GK and CK SRS.

Quantification of six-degree-of-freedom motion during beam delivery in spine stereotactic body radiotherapy using intra-irradiation cone-beam computed tomography imaging technique

PURPOSE

Quantifying intra-fractional six-degree-of-freedom (6DoF) residual errors or motion from approved patient setups is necessary for accurate beam delivery in spine stereotactic body radiotherapy. However, previously reported errors were not acquired during beam delivery. Therefore, we aimed to quantify the 6DoF residual errors and motions during arc beam delivery using a concurrent cone-beam computed tomography (CBCT) imaging technique, intra-irradiation CBCT.

METHODS

Consecutive 15 patients, 19 plans for various treatment sites, and 199 CBCT images were analyzed. Pre-irradiation CBCT was performed to verify shifts from the initial patient setup using the ExacTrac system. During beam delivery by two or three co-planar full-arc rotations, CBCT imaging was performed concurrently. Subsequently, an intra-irradiation CBCT image was reconstructed. Pre- and intra-irradiation CBCT images were rigidly registered to a planning CT image based on the bone to quantify 6DoF residual errors.

RESULTS

6DoF residual errors quantified using pre- and intra-irradiation CBCTs were within 2.0 mm/2.0°, except for one measurement. The mean elapsed time (mean ± standard deviation [min:sec]) after pre-irradiation CBCT to the end of the last arc beam delivery was 6:08 ± 1:25 and 7:54 ± 2:14 for the 2- and 3-arc plans, respectively. Root mean squares of residual errors for several directions showed significant differences; however, they were within 1.0 mm/1.0°. Time-dependent analysis revealed that the residual errors tended to increase with elapsed time.

CONCLUSION

The errors represent the optimal intra-fractional error compared with those acquired using the pre-, inter-beam, and post-6DoF image guidance and can be acquired within a standard treatment timeslot.

Automatic measurement of beam-positioning accuracy at off-isocenter positions

PURPOSE

This study performed an automatic measurement of the off-axis beam-positioning accuracy at a single isocenter via the TrueBeam Developer mode and evaluated the beam-positioning accuracy considering the effect of couch rotational errors.

METHODS

TrueBeam STx and the Winston-Lutz test-dedicated phantom, with a 3 mm diameter steel ball, were used in this study. The phantom was placed on the treatment couch, and the Winston-Lutz test was performed at the isocenter for four gantry angles (0°, 90°, 180°, and 270°) using an electronic portal imaging device. The phantom offset positions were at distances of 0, 25, 50, 75, and 100 mm from the isocenter along the superior-inferior, anterior-posterior, and left-right directions. Seventeen patterns of multileaf collimator-shaped square fields of 10 × 10 mm² were created at the isocenter and off-axis positions for each gantry angle. The beam-positioning accuracy was evaluated with couch rotation along the yaw-axis (0°, ± 0.5°, and ± 1.0°).

RESULTS

The mean beam-positioning errors at the isocenter and off-isocenter distances (from the isocenter to ±100 mm) were 0.46-0.60, 0.44-0.91, and 0.42-1.11 mm for the couch angles of 0°, ±0.5°, and ±1°, respectively. The beam-positioning errors increased as the distance from the isocenter and couch rotation increased.

CONCLUSION

These findings suggest that the beam-positioning accuracy at the isocenter and off-isocenter positions can be evaluated quickly and automatically using the TrueBeam Developer mode. The proposed procedure is expected to contribute to an efficient evaluation of the beam-positioning accuracy at off-isocenter positions.

Comparison of intrafractional motion with two frameless immobilization systems in surface-guided intracranial stereotactic radiosurgery

Purpose/objectives

The aim of this study is to compare intrafractional motion using two commercial non‐invasive immobilization systems for linac‐based intracranial stereotactic radiosurgery (SRS) under guidance with a surface‐guided radiotherapy (SGRT) system.

Materials/methods

Twenty‐one patients who received intracranial SRS were retrospectively selected. Ten patients were immobilized with a vacuum fixation biteplate system, while 11 patients were immobilized with an open‐face mask system. A setup margin of 1 mm was used in treatment planning. Real‐time surface motion data in 37 treatment fractions using the vacuum fixation system and 44 fractions using the open‐face mask were recorded by an SGRT system. Variances of intrafractional motion along three translational directions and three rotational directions were compared between the two immobilization techniques with Levene's tests. Intrafractional motion variation over time during treatments was also evaluated.

Results

Using the vacuum fixation system, the average and standard deviations of the shifts were 0.01 ± 0.18 mm, ‐0.06 ± 0.30 mm, and  0.02 ± 0.26 mm in the anterior–posterior (AP), superior–inferior (SI), and left–right (LR) directions, and ‐0.02 ± 0.19°, ‐0.01 ± 0.13°, and 0.01 ± 0.13° for rotations in yaw, roll, and pitch, respectively; using the open‐face mask system, the average and standard deviations of the shifts were ‐0.06 ± 0.20 mm, ‐0.02 ± 0.35 mm, and 0.01 ± 0.40 mm in the AP, SI, and LR directions, and were 0.05 ± 0.23°, 0.02 ± 0.21°, and 0.00 ± 0.16° for rotations in yaw, roll, and pitch, respectively. There was a significant increase in intrafractional motion variance over time during treatments.

Conclusion

Patients with the vacuum fixation system had significantly smaller intrafractional motion variation compared to those with the open‐face mask system. Using intrafractional motion techniques such as surface imaging system is recommended to minimize dose deviation due to intrafractional motion. The increase in intrafractional motion over time indicates clinical benefits with shorter treatment time.