Quantifying the Sensitivity of Target Dose on Intrafraction Displacement in Intracranial Stereotactic Radiosurgery.

PURPOSE: Mask-immobilized stereotactic radiosurgery (SRS) using a gating window is an emerging technology. However, the amount of intracranial tumor motion that can be tolerated during treatment while satisfying clinical dosimetric goals is unknown. The purpose of this study was to quantify the sensitivity of target dose to tumor motion. METHODS AND MATERIALS: In clinical SRS plans, where a nose marker was tracked as surrogate for target motion, translational and rotational target movements were simulated using nose-marker displacements of ±0.5 mm, ±1.0 mm, or ±1.5 mm. The effect on minimum dose to 99% of the target (D99) and percent target coverage by prescription dose was quantified using mixed-effect modeling with variables: displacement, target volume, and location. RESULTS: The effect on dose metrics is statistically larger for translational displacements compared with rotational displacements, and the effect of pitch rotations is statistically larger compared with yaw rotations. The mixed-effect model for translations showed that displacement and target volume are statistically significant variables, for rotation the variable target distance to rotation axis is additionally significant. For mean target volume (12.6 cc) and translational nose-marker displacements of 0.5 mm, 1.0 mm, and 1.5 mm, D99 decreased by 2.2%, 7.1%, and 13.0%, and coverage by 0.4%, 1.8%, and 4.4%, respectively. For mean target volume, mean distance midpoint-target to pitch axis (7.6cm), and rotational nose-marker displacement of 0.5 mm, 1.0 mm, and 1.5 mm, D99 decreased by 1.0%, 3.6%, and 6.9%, and coverage by 0.2%, 0.8%, and 1.9%, respectively. For rotational yaw axis displacement, mean distance midpoint-target axis (4.2cm), D99 decreased by 0.3%, 1.2%, and 2.5%, and coverage by 0.1%, 0.2%, and 0.5%, respectively. CONCLUSIONS: Simulated target displacements showed that sensitivity of tumor dose to motion depends on both target volume and target location. Suggesting that patient- and target-specific thresholds may be implemented for optimizing the balance between dosimetric plan accuracy and treatment prolongation caused by out-of-tolerance motion.

Technical note: Personalized treatment gating thresholds in frameless stereotactic radiosurgery using predictions of dosimetric fidelity and treatment interruption.

PURPOSE: Gamma Knife Icon (GKI) enables a user-defined gating threshold for intrafraction motion during stereotactic radiosurgery (SRS). An optimal threshold would ensure dosimetric fidelity of the planned distribution and minimize treatment time extension by gating. A prediction of motion characteristics for a patient based on a retrospective database of motion traces could be beneficial to evaluating the choice of gating threshold. A short acquisition of motion may help to define a personalized threshold that balances dosimetric accuracy and treatment length. This study aims to evaluate the performance of a prediction of motion and the resultant dosimetric consequences for a range of motion gating thresholds. METHODS: A database of 2552 motion traces (776 patients) was analyzed using previously published methods to characterize patient intrafraction motion on the GKI. For a selection of six fractionated SRS patient cases (two patients with single brain metastasis, four vestibular schwannomas), a 10-min sample of motion was used to classify motion and identify traces in the database with similar metrics. The similar motion traces were used to perform a predictive reconstruction of the selected patient's delivered dose for a range of motion thresholds. The remaining fractions were reconstructed and compared to that predicted. From the six cases, 26 fractions were used to predict the number of interruptions (n = 26), change in target coverage (n = 26), and change in brainstem maximum dose (vestibular cases only, n = 20). The difference between mean predicted and reconstructed values was compared for accuracy. RESULTS: The difference between mean prediction and reconstructed values was 0.32 ± 0.38% in target coverage, 2.36 ± 5.06 interruptions, and 0.15 ± 0.24 Gy for the brainstem maximum dose. Sixty-seven of the 72 predictions (26 coverage, 26 interruptions, and 20 brainstem maximum dose) were within one standard deviation of the predicted mean. CONCLUSIONS: Large databases of motion traces were used to characterize patient performance and predict motion performance. Dosimetric deterioration due to motion and extension of treatment duration can be predicted in some cases using only a short acquisition of motion and the treatment plan. This reconstruction may provide benefit in generating a patient-specific motion threshold.

Real-Time Infrared Motion Tracking Analysis for Patients Treated With Gated Frameless Image Guided Stereotactic Radiosurgery.

PURPOSE: The transition from frame-based brain stereotactic radiosurgery (SRS) to frameless delivery is supported by real-time intrafraction monitoring to ensure accurate delivery. The purpose of this study is to characterize these real-time motion traces in a large cohort of patients treated with frameless gated brain SRS and to develop patient-specific predictions of tolerance violations. METHODS AND MATERIALS: SRS patients treated on the Gamma Knife Icon were immobilized using a device-specific thermoplastic head mask. A motion marker was fixed to the patient's nose, with gating and cone beam computed tomography (CBCT)-based corrections to the treatment at excursions from baseline exceeding 1.5 mm. The traces of 1446 fractions were analyzed according to magnitude (932 unique treatment plans for 462 unique individual patients), directional distribution of displacement, and stability. A neural network model was developed to predict interruptions based on a subset of trace data. RESULTS: The average displacement of the marker in the first fraction of all patients was 0.62 ± 0.25 mm with beam CBCT corrections, which would otherwise be modeled at 0.96 ± 0.96 mm without intrafraction motion correction (P < .0001). Twenty-nine percent of fractions delivered were interrupted, of which the Z-axis (superoinferior) motion was the largest contributor to excursion. Baseline corrections significantly compensated for the magnitude of motion in all 3 dimensions (P < .01). The motion relative to the first acquired CBCT was on average seen to consistently increase with treatment time, with the minimum P value occurring at 61.3 minutes. The neural network prediction model was able to predict treatment interruptions with 84% sensitivity on the first 5-minute sample of the trace. CONCLUSIONS: Corrections to marker position significantly decreased marker excursions in all 3 axes compared with a single CBCT alignment. Patient-specific modeling may aid in the optimization of cases selected for frameless radiosurgery to increase the accuracy of planned delivery.

Intra-fraction motion gating during frameless Gamma Knife® Icon™ therapy: The relationship between cone beam CT assessed intracranial anatomy displacement and infrared-tracked nose marker displacement.

METHODS: Gamma Knife Icon™'s high-definition motion management (HDMM) system gates treatment delivery should intra-fraction displacement of a nose marker exceed some user-defined threshold. A method, previously-validated with a phantom, is used to relate intra-fractional displacements of the nose marker to displacements of patient targets. Additionally, novel analysis is performed to ascertain the relationship between nose marker displacement and displacement of a 3D grid of coordinates throughout stereotactic space. This spatial information is used to retrospectively review HDMM threshold levels based upon real target locations. RESULTS: For 41 targets from 22 patients, the mean(standard deviation) and maximum target-to-nose displacement ratio was 0.54(0.32) and 1.65, respectively. On average, displacements typically exceed those of the nose only for coordinates at the most extreme peripheral corner of the investigated 3D grid of points. Allowing target displacement of up to a maximum of 0.8mm, retrospective review indicated that at the locations of the 41 targets a median(range) HDMM threshold of 1.4(1.0-1.9) mm could have been adopted, compared to our standard threshold of 1.0mm. CONCLUSIONS: Intracranial targets typically displace by a magnitude around half that of the nose. Novel analysis to determine the spatial variation of target-to-nose displacement ratio suggests, for our 41 targets, HDMM threshold could have been increased from our standard. Cases for which HDMM threshold could be safely increased would minimise treatment gating events and expedite treatment delivery to offer patient comfort benefits.