Simulation of intrafraction motion and overall geometrical accuracy of a frameless intracranial radiosurgery process

Stereotactic and fractionated stereotactic radiosurgery for single and multiple brain metastases: Results of multicenter planning studies

PURPOSE

Stereotactic and fractionated stereotactic radiosurgery (SRS/fSRS) utilization is growing in India, although planning studies are scarce. This study assessed clinical practices for SRS/fSRS treatment planning for brain metastases (BM) using ICRU-91 and explored the impact of planning tools.

METHODS & MATERIALS

Participants from 23 centers received two anonymized CT datasets with predrawn structures for single met (SM) and four BMs (MM) cases via email. Centers used local protocol to create plans. The plans were evaluated for target coverage, normal brain doses, and ICRU-91 dosimetric indices.

RESULTS

Monaco TPS overestimated mean GTV (PTV) by 3.7 (4.2)% and 2.1 (2.0)% for SM and MM respectively. Some institutions had good conformity and target coverage, whereas others had high OAR doses despite inadequate PTV dose coverage. Conformity index (CI) ranged from 1.07 to 1.45 (SM) and 1.06 to 1.25 (MM), and homogeneity index (HI) ranged from 0.07 to 0.28 (SM) and 0.13 to 0.32 (MM). Significant variation in GI and dose prescription isodose line selection was observed among centers.

CONCLUSIONS

There was a significant heterogeneity in the planning parameters noted among different centers. The study emphasized the importance of established planning protocols and comprehensive training for staff involved in SRS/fSRS. Notably, plans with finer MLC width outperformed, yet wider MLC plans achieved ICRU-91 indices comparable to published literature. The importance of our study is underscored by the absence of a national framework for SRS planning in India.

Innovative margin design and optimized isocenter to minimize the normal tissue in target volumes for single-isocenter multi-target stereotactic radiosurgery

Objective.Treating multiple brain metastases in a single plan is a popular radiosurgery technique. However, targets positioned off-isocenter are subject to rotational uncertainties. This work introduces two new planning target volumes (PTVs) that address this increased uncertainty. The volume of normal tissue included in these PTVs when paired with optimized isocenters are evaluated and compared with conventional methods.Approach.Sets of 1000 random multi-target radiosurgery patients were simulated, each patient with a random number of spherical targets (2-10). Each target had a random volume (0.1-15 cc) and was randomly positioned between 5 and 50 mm or 100 mm from isocenter. Two new PTVs ('LensPTV' and 'SwipePTV') and conventional isotropic PTVs were created using isocenters derived from the center-of-centroids, the center-of-mass, or optimized per PTV type. The total volume of normal tissue in the PTVs for each patient was calculated and compared using 1 mm translations and 0.5°, 1.0°, and 2.0° rotations.Main results.Using the new PTVs and/or using optimized isocenters decreased the total volume of normal tissue in the PTVs per patient. The SwipePTV, in particular, provided the greatest decrease. Compared to the SwipePTV, the LensPTV and the conventional isotropic PTV included an extra 0.68 and 0.73 cc of normal tissue per patient (median), respectively, when using 50 mm max distance to isocenter and 1° max rotation angle. Under these conditions, 25% of patients had extra volume of normal tissue ≥ 0.96 and 1.04 cc. When using 100 mm max distance to isocenter and 2° max rotation angle, 25% of patients had extra volume of normal tissue ≥ 4.35 and 5.75 cc.Significance.PTVs like those presented here, especially when paired with optimized isocenters, can decrease the total volume of included normal tissue and reduce the risk of toxicity without compromising target coverage.

Single-Isocenter Linac-Based Radiosurgery for Brain Metastases with Coplanar Arcs: A Dosimetric and Clinical Analysis

The efficacy of linac-based SRS/fSRS treatments using the single-isocenter coplanar FFF-VMAT technique for both single and multiple BM was investigated. Seventy patients (129 BM) treated with 15-21 Gy in 1 (n = 59) or 27 Gy in 3 (n = 11) fractions were analyzed. For each fraction, plans involving the intra-fractional errors measured by post-treatment CBCT were recalculated. The relationships of BM size, distance-to-isocenter, and barycenter shift with the difference in target coverage were evaluated. Clinical outcomes were assessed using logistic regression and Kaplan-Meier analysis. The median delivery time was 3.78 min (range, 1.83-9.25). The median post-treatment 3D error was 0.5 mm (range, 0.1-2.7) and the maximum rotational error was 0.3° (range, 0.0-1.3). In single BM patients, the GTV D95% was never reduced by >5%, whereas PTV D95% reductions >1% occurred in only 11 cases (29%). In multiple BM patients, dose deficits >5% and >1% occurred in 2 GTV (2%) and 34 PTV (37%), respectively. The differences in target coverage showed a moderate-to-strong correlation only with barycenter shift. Local failure of at least one treated BM occurred in 13 (21%) patients and the 1-year and 2-year local control rates for all lesions were 94% and 90%, respectively. The implemented workflow ensured that the degradation of target and brain dose metrics in delivered treatments was negligible. Along with encouraging clinical outcomes, these findings warrant a reduction in the PTV margins at our institution.

Six degrees of freedom intrafraction cranial motion detection using a novel capacitive monitoring technique: evaluation with human subjects

The purpose of this work is to introduce and evaluate a capacitive monitoring array capable of continuous 6DOF cranial motion detection during high precision radiotherapy. The ring-shaped capacitive array consists of four equally sized conductive sensors positioned at the cranial vertex. The system is modular, non-contact, and provides continuous motion information through the thermoplastic immobilization mask without relying on skin monitoring or use of ionizing radiation. The array performance was evaluated through a volunteer study with a cohort of twenty-five individuals. The study was conducted in a linac suite and the volunteers were fitted with an S-frame thermoplastic mask. Each volunteer took part in one data acquisition session per day for three consecutive days. During the data acquisition, the conductive array was translated and rotated relative to their immobilized cranium in 1-millimetre and 1-degree steps to simulate cranial motion. Capacitive signals were collected at each position at a frequency of 20 Hz. The data from the first acquisition session was then used to train a classifier model and establish calibration equations. The classifier and calibration equations were then applied to data from the subsequent acquisition sessions to evaluate the system performance. The trained classifiers had an average success rate of 92.6% over the volunteer cohort. The average error associated with calibration had a mean value below 0.1 mm or 0.1 deg for all six motions. The capacitive array system provides a novel method to detect translational and rotational cranial motion through a thermoplastic mask.

Reducing ExacTrac intrafraction imaging uncertainty for prostate stereotactic body radiotherapy using a pre-treatment CBCT

This study evaluated the intrafractional auto-matching uncertainties of prostate-implanted fiducial markers when using the ExacTrac v6.5 (Brainlab, Feldkirchen, Germany) X-ray stereoscopic system. A customised phantom with 3 implanted gold seeds was initially positioned at the isocentre using a cone beam CT (CBCT) prior to intrafractional imaging. Progressive offsets were applied to the phantom in all six directions (3 translational, 3 rotational) of 0 mm, 1 mm, 2 mm, 0°, 1° and 2°. Subsequently, the ability of the ExacTrac image-matching functions to detect and correct these offsets was tested. For comparison, this procedure was repeated, but without a CBCT for pre-treatment positioning. The auto-matching uncertainties when a CBCT was introduced into the workflow were significantly reduced, and overall, the auto-matching statistics using the implanted marker (seeds) matching function was found to be more precise than the bony anatomy function in-phantom. The total standard deviations for the translational shifts using the implanted marker and bony anatomy functions respectively were 0.1 mm and 0.3 mm vertically, 0.1 mm and 0.3 mm longitudinally, and 0.1 mm and 0.4 mm laterally. The standard deviations for the rotational shifts using the implanted marker and bony anatomy matching functions respectively were 0.2° and 1.2° for the yaw (angle vert), 0.3° and 1.1° for the pitch (angle long), and 0.2° and 1.2° for the roll (angle lat) directions. The reduced uncertainties from introducing a CBCT for initial localisation resulted in decreased probability of inhibits due to false positives during treatment.

Performance assessment of two motion management systems for frameless stereotactic radiosurgery

BACKGROUND/PURPOSE

Frameless stereotactic radiosurgery (SRS) requires dedicated systems to monitor patient motion in order to avoid inaccurate radiation delivery due to involuntary shifts. The purpose of this study is to assess the accuracy and sensitivity of two distinct motion monitoring systems used for frameless SRS.

METHODS

A surface image-guided system known as optical surface monitoring system (OSMS), and a fiducial marker-based system known as high definition motion management (HDMM) as part of the latest Gamma Knife Icon® were compared. A 3D printer-based cranial motion phantom was developed to evaluate the accuracy and sensitivity of these two systems in terms of: (1) the capability to recognize predefined shifts up to 3 cm, and (2) the capability to recognize predefined speeds up to 3 cm/s. The performance of OSMS, in terms of different reference surfaces, was also evaluated.

RESULTS

Translational motion could be accurately detected by both systems, with an accuracy of 0.3 mm for displacement up to 1 cm, and 0.5 mm for larger displacements. The reference surface selection had an impact on OSMS performance, with flat surface resulting in less accuracy. HDMM was in general more sensitive when compared with OSMS in capturing the motion, due to its faster frame rate, but a delay in response was observed with faster speeds. Both systems were less sensitive in detection of superior-inferior motion when compared to lateral or vertical displacement directions.

CONCLUSION

Translational motion can be accurately and sensitively detected by OSMS and HDMM real-time monitoring systems. However, performance variations were observed along different motion directions, as well as amongst the selection of reference images. Caution is needed when using real-time monitoring systems for frameless SRS treatment.

Technical Note: Rotational positional error corrected intrafraction set-up margins in stereotactic radiotherapy: A spatial assessment for coplanar and noncoplanar geometry

PURPOSE

The aim of this study is to calculate setup margin based on six-dimensional (6D) corrected residual positional errors from kV cone beam computed tomography (CBCT) and from intrafraction projection kV imaging in coplanar and in noncoplanar couch positions in stereotactic radiotherapy.

METHODS

Six dimensional positional corrections were carried out before patient treatments, using a robotic couch and CBCT matching. A CBCT and stereoscopic ExacTrac image were acquired post-table position correction. Further, a series of intrafraction ExacTrac images were obtained for the variable couch position. Translational and rotational errors were identified as lateral (X), longitudinal (Y), vertical (Z); roll (Ɵ°), pitch (Φ°) and yaw (Ψ°). A total of 699 intrafraction image sets (361 coplanar and 338 noncoplanar) for 51 SRS/SRT patients were analysed. Rotational errors were corrected in terms of translational coordinates. Residual set-up margins were calculated from CBCT shifts. ExacTrac shifts give residual + intrafraction setup margins as a function of coplanar and noncoplanar couch positions.

RESULTS

The average residual positional error obtained from CBCT in X, Y, Z, Ɵ, Φ, Ψ were 0.1 ± 0.4 mm, 0.0 ± 0.6 mm, 0.0 ± 0.5 mm, 0.2 ± 0.8°, 0.1 ± 0.6° and -0.1 ± 0.7° respectively. For ExacTrac, the shits were -0.5 ± 0.9 mm, -0.0 ± 1mm, -0.6 ± 1.0mm, 0.4 ± 0.9°, -0.2 ± 0.6°, and -0.0 ± 0.8°. CBCT calculated linear setup margins in X, Y, Z direction were 0.5, 1.2, and 1 mm respectively. ExacTrac yielded coplanar and noncoplanar linear setup margins were 1.2, 1.3, 1.5, 1.4, 1.5, and 2.1 mm respectively.

CONCLUSION

CBCT-based gross residual set-up margin is equal to 1 mm. ExacTrac calculated residual plus intrafraction setup margin falls within a 2 mm range; attributed to intrafraction patient movement, table position inaccuracies, and poor image fusion in noncoplanar geometry. There could be variations in the required additional margin between centers and between machines, which require further studies.

Direct comparison between surface imaging and orthogonal radiographic imaging for SRS localization in phantom

Purpose

Surface imaging (SI) offers a nonionizing, near real time alternative to radiographic imaging for intrafraction radiosurgery localization. In this work, we systematically compared a commercial SI system vs a commercial room mounted x‐ray localization system in phantom.

Methods

An anthropomorphic head phantom with fiducial markers was imaged with linear accelerator on‐board x‐ray imaging, SI, and room mounted x‐ray imaging (RM) at ±45° and ±90° couch angles for three different head tilts and six different isocenters (72 total positions). The shifts generated by the three systems were compared as functions of couch angle, head tilt, and isocenter position with the on‐board imaging shifts used as ground truth. Two sample Kolmogorov–Smirnov tests were used to evaluate equivalence of the groups.

Results

The magnitude of the displacement vectors for RM minus on‐board imaging and SI minus on‐board imaging over all 72 phantom positions were 0.7 ± 0.3 mm for both cases. The RM and SI showed no significant difference based on couch angle or isocenter position. Both systems showed decreasing accuracy with increasing couch angle, but both systems agreed with ground truth to <=1.1 mm at all couch angles. The exaggerated chin‐up head orientation showed significantly different shifts for SI and RM based on increased variance in the SI measurements, although both had submillimeter accuracy on average. The standard deviation of the real time SI displacement vector was <0.06 mm over all measurements, during which the on‐board imaging panels partially blocked the lateral camera pods for half the time.

Conclusions

RM and SI showed similar accuracy over measurements at 72 different phantom positions. SI showed minimal performance loss with camera pods blocked. SI is a feasible option for intra‐fraction radiosurgery localization based on these phantom measurements.

Relationship between pressure levels on the occipital region and intrafraction motion using an individualized head support for intracranial treatment

The purpose of this study was to evaluate the relationship between pressure on the occipital region and intrafraction motion using an individualized vacuum pillow and a thermoplastic mask for intracranial treatment. We calculated head displacement during treatment from 8811 image verifications in 59 patients and divided them into two groups according to the magnitude of the mean and standard deviation (SD) of the displacement in the 59 patients. Pressure was compared between the small (n = 29) and large (n = 30) displacement groups using Welch's t-test for the mean and SD of displacement. The mean head displacement in the small and large groups was (0.3, 0.3, 0.4) and (0.5, 0.6, 0.7) (unit: mm) for the vector length and 10 mm and 30 mm radius targets, respectively. The mean SD of head displacement in the small and large groups was (0.2, 0.2, 0.2) and (0.3, 0.3, 0.4) (unit: mm) for the vector length and 10 mm and 30 mm radius targets, respectively. Significant differences were observed in the SD of the displacement in the vector length and 10 mm radius target between the two groups. The SD of the displacement under a pressure of 15 kPa was smaller than that under a pressure of 11 kPa. The intrafraction motion under a high-pressure level on the occipital region was less than that under a low-pressure level. Management of pressure on the occipital region may result in less intrafraction motion in clinical practice.

To frame or not to frame? Cone-beam CT-based analysis of head immobilization devices specific to linac-based stereotactic radiosurgery and radiotherapy

Purpose

Noninvasive frameless systems are increasingly being utilized for head immobilization in stereotactic radiosurgery (SRS). Knowing the head positioning reproducibility of frameless systems and their respective ability to limit intrafractional head motion is important in order to safely perform SRS. The purpose of this study was to evaluate and compare the intrafractional head motion of an invasive frame and a series of frameless systems for single fraction SRS and fractionated/hypofractionated stereotactic radiotherapy (FSRT/HF‐SRT).

Methods

The noninvasive PinPoint system was used on 15 HF‐SRT and 21 SRS patients. Intrafractional motion for these patients was compared to 15 SRS patients immobilized with Cosman‐Roberts‐Wells (CRW) frame, and a FSRT population that respectively included 23, 32, and 15 patients immobilized using Gill‐Thomas‐Cosman (GTC) frame, Uniframe, and Orfit. All HF‐SRT and FSRT patients were treated using intensity‐modulated radiation therapy on a linear accelerator equipped with cone‐beam CT (CBCT) and a robotic couch. SRS patients were treated using gantry‐mounted stereotactic cones. The CBCT image‐guidance protocol included initial setup, pretreatment and post‐treatment verification images. The residual error determined from the post‐treatment CBCT was used as a surrogate for intrafractional head motion during treatment.

Results

The mean intrafractional motion over all fractions with PinPoint was 0.62 ± 0.33 mm and 0.45 ± 0.33 mm, respectively, for the HF‐SRT and SRS cohort of patients (P‐value = 0.266). For CRW, GTC, Orfit, and Uniframe, the mean intrafractional motions were 0.30 ± 0.21 mm, 0.54 ± 0.76 mm, 0.73 ± 0.49 mm, and 0.76 ± 0.51 mm, respectively. For CRW, PinPoint, GTC, Orfit, and Uniframe, intrafractional motion exceeded 1.5 mm in 0%, 0%, 5%, 6%, and 8% of all fractions treated, respectively.

Conclusions

The noninvasive PinPoint system and the invasive CRW frame stringently limit cranial intrafractional motion, while the latter provides superior immobilization. Based on the results of this study, our clinical practice for malignant tumors has evolved to apply an invasive CRW frame only for metastases in eloquent locations to minimize normal tissue exposure.

Technical Note: Using k-means clustering to determine the number and position of isocenters in MLC-based multiple target intracranial radiosurgery

Purpose

To present the k‐means clustering algorithm as a tool to address treatment planning considerations characteristic of stereotactic radiosurgery using a single isocenter for multiple targets.

Methods

For 30 patients treated with stereotactic radiosurgery for multiple brain metastases, the geometric centroids and radii of each met were determined from the treatment planning system. In‐house software used this as well as weighted and unweighted versions of the k‐means clustering algorithm to group the targets to be treated with a single isocenter, and to position each isocenter. The algorithm results were evaluated using within‐cluster sum of squares as well as a minimum target coverage metric that considered the effect of target size. Both versions of the algorithm were applied to an example patient to demonstrate the prospective determination of the appropriate number and location of isocenters.

Results

Both weighted and unweighted versions of the k‐means algorithm were applied successfully to determine the number and position of isocenters. Comparing the two, both the within‐cluster sum of squares metric and the minimum target coverage metric resulting from the unweighted version were less than those from the weighted version. The average magnitudes of the differences were small (−0.2 cm² and 0.1% for the within cluster sum of squares and minimum target coverage, respectively) but statistically significant (Wilcoxon signed‐rank test, P < 0.01).

Conclusions

The differences between the versions of the k‐means clustering algorithm represented an advantage of the unweighted version for the within‐cluster sum of squares metric, and an advantage of the weighted version for the minimum target coverage metric. While additional treatment planning considerations have a large influence on the final treatment plan quality, both versions of the k‐means algorithm provide automatic, consistent, quantitative, and objective solutions to the tasks associated with SRS treatment planning using a single isocenter for multiple targets.

Comparison of volumetric-modulated arc therapy and dynamic conformal arc treatment planning for cranial stereotactic radiosurgery

The aim was to analyze arc therapy techniques according to the number and position of the brain lesions reported by comparing dynamic noncoplanar conformal arcs (DCA), two coplanar full arcs (RAC) with volumetric-modulated arc therapy (VMAT), multiple noncoplanar partial arcs with VMAT (RANC), and two full arcs with VMAT and 10° table rotation (RAT). Patients with a single lesion (n= 10), multiple lesions (n = 10) or a single lesion close to organs at risk (n = 5) and previously treated with DCA were selected. For each patient, the DCA treatment was replanned with all VMAT techniques. All DCA plans were compared with VMAT plans and evaluated in regard to the different quality indices and dosimetric parameters. For single lesion, homogeneity index (HI) better results were found for the RANC technique (0.17 ± 0.05) compared with DCA procedure (0.27± 0.05). Concerning conformity index (CI), the RAT technique gave higher and better values (0.85 ± 0.04) compared with those obtained with the DCA technique (0.77 ± 0.05). DCA improved healthy brain protection (8.35 ± 5.61 cc vs. 10.52 ± 6.40 cc for RANC) and reduced monitor unit numbers (3046 ± 374 MU vs. 4651 ± 736 for RANC), even if global room occupation was higher. For multiple lesions, VMAT techniques provided better HI (0.16) than DCA (0.24 ± 0.07). The CI was improved with RAT (0.8 ± 0.08 for RAT vs. 0.71 ± 0.08 for DCA). The V10Gy healthy brain was better protected with DCA (9.27 ± 4.57 cc). Regarding the MU numbers: RANC < RAT< RAC < DCA. For a single lesion close to OAR, RAT achieved high degrees of homogeneity (0.27 ± 0.03 vs. 0.53 ± 0.2 for DCA) and conformity (0.72± 0.06vs. 0.56 ± 0.13 for DCA) while sparing organs at risk (Dmax = 12.36 ± 1.05Gyvs. 14.12 ± 0.59 Gy for DCA, and Dmean = 3.96 ± 3.57Gyvs. 4.72 ± 3.28Gy for DCA). On the other hand, MU numbers were lower with DCA (2254 ± 190 MUvs. 3438 ± 457 MU for RANC) even if overall time was inferior with RAC. For a single lesion, DCA provide better plan considering low doses to healthy brain even if quality indexes are better for the others techniques. For multiple lesions, RANC seems to be the best compromise, due to the ability to deliver a good conformity and homogeneity plan while sparing healthy brain tissue. For a single lesion close to organs at risk, RAT is the most appropriate technique.

Development of a real-time monitoring system for intra-fractional motion in intracranial treatment using pressure sensors

This study developed a dedicated real-time monitoring system to detect intra-fractional head motion in intracranial radiotherapy using pressure sensors. The dedicated real-time monitoring system consists of pressure sensors with a thickness of 0.6 mm and a radius of 9.1 mm, a thermoplastic mask, a vacuum pillow, and a baseplate. The four sensors were positioned at superior-inferior and right-left sides under the occipital area. The sampling rate of pressure sensors was set to 5 Hz. First, we confirmed that the relationship between the force and the displacement of the vacuum pillow follows Hook's law. Next, the spring constant for the vacuum pillow was determined from the relationship between the force given to the vacuum pillow and the displacement of the head, detected by Cyberknife target locating system (TLS) acquisitions in clinical application. Finally, the accuracy of our system was evaluated by using the 2  ×  2 confusion matrix. The regression lines between the force, y, and the displacement, x, of the vacuum pillow were given by y = 3.8x, y = 4.4x, and y = 5.0x when the degree of inner pressure was  -12 kPa,-20 kPa, and  -27 kPa, respectively. The spring constant of the vacuum pillow was 1.6 N mm(-1) from the 6D positioning data of a total of 2999 TLS acquisitions in 19 patients. Head motions of 1 mm, 1.5 mm, and 2 mm were detected in real-time with the accuracies of 67%, 84%, and 89%, respectively. Our system can detect displacement of the head continuously during every interval of TLS with a resolution of 1-2 mm without any radiation exposure.

Prolonged treatment time deteriorates positioning accuracy for stereotactic radiosurgery

INTRODUCTION

The accuracy of radiation delivery is increasingly important as radiotherapy technology continues to develop. The goal of this study was to evaluate intrafractional motion during intracranial radiosurgery and the relationship between motion change and treatment time.

METHODS AND MATERIALS

A total of 50 treatment records with 5988 images, all acquired during treatments with the CyberKnife Radiosurgery System, were retrospectively analyzed in this study. We measured translation and rotation motion including superior-inferior (SI), right-left (RL), anterior-posterior (AP), roll, tilt and yaw. All of the data was obtained during the first 45 minutes of treatment. The records were divided into 3 groups based on 15-min time intervals following the beginning of treatment: group A (0-15 min), group B (16-30 min) and group C (31-45 min). The mean deviations, systematic errors, random errors and margin for planning target volume (PTV) were calculated for each group.

RESULTS

The mean deviations were less than 0.1 mm in all three translation directions in the first 15 minutes. Greater motion occurred with longer treatment times, especially in the SI direction. For the 3D vector, a time-dependent change was observed, from 0.34 mm to 0.77 mm (p=0.01). There was no significant correlation between the treatment time and deviations in the AP, LR and rotation axes. Longer treatment times were associated with increases in systematic error, but not in random error. The estimated PTV margin for groups A, B and C were 0.86 / 1.14 / 1.31 mm, 0.75 / 1.12 / 1.20 mm, and 0.43 / 0.54 / 0.81 mm in the SI, RL, and AP directions, respectively.

CONCLUSIONS

During intracranial radiosurgery, a consistent increase in the positioning deviation over time was observed, especially in the SI direction. If treatment time is greater than 15 minutes, we recommend increasing the PTV margins to ensure treatment precision.

Quantifying isocenter measurements to establish clinically meaningful thresholds

A dataset range of isocenter congruency verification tests have been examined from a statistical perspective for the purpose of establishing tolerance levels that are meaningful, based on the fundamental limitation of linear accelerator isocentricity and the demands of a high-precision stereotactic radiosurgery program. Using a laser-defined isocenter, a total of 149 individual isocenter congruency tests were examined with recorded values for ideal spatial corrections to the isocenter test tool. These spatial corrections were determined from radiation exposures recorded on an electronic portal imaging device (EPID) at various gantry, collimator, and treatment couch combinations. The limitations of establishing an ideal isocenter were quantified from each variable which contributed to uncertainty in isocenter definition. Individual contributors to uncertainty, specifically, daily positioning setup errors, gantry sag, multileaf collimator (MLC) offset, and couch walkout, were isolated from isocenter congruency measurements to determine a clinically meaningful isocenter measurement. Variations in positioning of the test tool constituted, on average, 0.38 mm magnitude of correction. Gantry sag and MLC offset contributed 0.4 and 0.16 mm, respectively. Couch walkout had an average degrading effect to isocenter of 0.72 mm. Considering the magnitude of uncertainty contributed by each uncertainty variable and the nature of their combination, an appropriate schedule action and immediate action level were determined for use in analyzing daily isocenter congruency test results in a stereotactic radiosurgery (SRS) program. The recommendations of this study for this linear accelerator include a schedule action level of 1.25 mm and an immediate action level of 1.50mm, requiring prompt correction response from clinical medical physicists before SRS or stereotactic body radiosurgery (SBRT) is administered. These absolute values were derived from considering relative data from a specific linear accelerator and, therefore, represent a means by which a numerical quantity can be used as a test threshold with relative specificity to a particular linear accelerator.

Spine Stereotactic Body Radiation Therapy Residual Setup Errors and Intra-Fraction Motion Using the Stereotactic X-Ray Image Guidance Verification System

Purpose

To determine the precision of our institution’s current immobilization devices for spine SBRT, ultimately leading to recommendations for appropriate planning margins.

Methods

We identified 12 patients (25 treatments) with spinal metastasis treated with spine Stereotactic Body Radiation Therapy (SBRT). The Body-FIX system was used as immobilization device for thoracic (T) and lumbar (L) spine lesions. The head and shoulder mask system was used as immobilization device for cervical (C) spine lesions. Initial patient setup used the infrared positioning system with body markers. Stereotactic X-ray imaging was then performed and correction was made if the initial setup error exceeded predetermined institutional tolerances, 1.5 mm for translation and 2° for rotation. Three additional sets of verification X-rays were obtained pre-, mid-, and post-treatment for all treatments.

Results

Intrafraction motion regardless of immobilization technique was found to be 1.28 ± 0.57 mm. The mean and standard deviation of the variances along each direction were as follows: Superior-inferior, 0.56 ± 0.39 mm and 0.77 ± 0.52 mm, (p = 0.25); Anterior-posterior, 0.57 ± 0.43 mm and 1.14 ± 0.61 mm, (p = 0.01); Left-right, 0.48 ± 0.34 mm and 0.74 ± 0.40 mm, (p = 0.09) respectively. There was a significantly greater difference in the average 3D variance of the BodyFIX as compared to the head and shoulder mask immobilization system, 1.04 ± 0.46 mm and 1.71 ± 0.52 mm; (p = 0.003) respectively.

Conclusions

Overall, our institution’s image guidance system using stereotactic X-ray imaging verification provides acceptable localization accuracy as previously defined in the literature. We observed a greater intrafraction motion for the head and shoulder mask as compared with the BodyFIX immobilization system, which may be a result of greater C-spine mobility and/or the suboptimal mask immobilization. Thus, better immobilization techniques for C-spine SBRT are needed to reduce setup error and intrafraction motion. We are currently exploring alternative C-spine immobilization techniques to improve set up accuracy and decrease intrafraction motion during treatment.

Development of a phantom to evaluate the positioning accuracy of patient immobilization systems using thermoplastic mask and polyurethane cradle

PURPOSE

The purpose of this study was to develop a new phantom to evaluate the positioning accuracy of patient immobilization systems.

METHODS

The phantom was made of papers formed into a human shape, paper clay, and filling rigid polyester. Acrylonitrile butadiene styrene (ABS) pipes were inserted at anterior-posterior (A-P) and right-left (R-L) directions in the phantom to give static load by pulling ropes through the pipes. First, the positioning precision of the phantom utilizing a target locating system (TLS) was evaluated by moving the phantom on a couch along inferior-superior (I-S), A-P, and R-L directions in a range from -5 mm to +5 mm. The phantom's positions detected with the TLS were compared with values measured by a vernier caliper. Second, the phantom movements in a tensile test were chosen from patient movements determined from 15 patients treated for intracranial lesions and immobilized with a thermoplastic mask and polyurethane cradle. The phantom movement was given by minimum or maximum values of patient movements in each direction. Finally, the relationship between phantom movements and the static load in the tensile test was characterized from measurements using the new phantom and the TLS.

RESULTS

The differences in all positions between the vernier caliper measurement and the TLS detected values were within 0.2 mm with frequencies of 100%, 95%, and 90% in I-S, A-P, and R-L directions, respectively. The phantom movements according to patient movements in clinical application in I-S, A-P, and R-L directions were within 0.58 mm, 0.94 mm, and 0.93 mm from the mean value plus standard deviation, respectively. The regression lines between the phantom movements and static load were given by y = 0.359x, y = 0.241x, and y = 0.451x in I-S, A-P, and R-L directions, respectively, where x is the phantom movement (mm) and y is the static load (kgf). The relationship between the phantom movements and static load may represent the performance of inhibiting patient movements, so the accuracy of the immobilization system in the intracranial lesion will be estimated in advance by basic tensile test on the new phantom.

CONCLUSIONS

The newly developed phantom was useful to evaluate the accuracy of immobilization systems for a Cyberknife system for intracranial lesions.

Study of ExacTrac X-ray 6D IGRT setup uncertainty for marker-based prostate IMRT treatment

Novalis Tx ExacTrac X-ray system has the 6D adjustment ability for patient setup. Limited studies exist about the setup uncertainty with ExacTrac X-ray system for IMRT prostate treatment with fiducial markers implanted. The purpose of this study is to investigate the marker-based prostate IMRT treatment setup uncertainty using ExacTrac 6D IGRT ability for patient setup. Forty-three patients with prostate cancers and markers implanted have been treated on the Novalis Tx machine. The ExacTrac X-ray system has been used for the patient pretreatment setup and intratreatment verification. In total, the shifts data for 1261 fractions and 3504 correction times (the numbers of X-ray images were taken from tube 1 and tube 2) have been analyzed. The setup uncertainty has been separated into uncertainties in 6D. Marker matching uncertainty was also analyzed. Correction frequency probability density function was plotted, and the radiation dose for imaging was calculated. The minimum, average, and maximum translation shifts were: -5.12 ± 3.89 mm, 0.20 ± 2.21 mm, and 6.07 ± 4.44 mm, respectively, in the lateral direction; -6.80 ± 3.21 mm, -1.09 ± 2.21 mm, and 3.12 ± 2.62 mm, respectively, in the longitudinal direction; and -7.33 ± 3.46 mm, -0.93 ± 2.70 mm, and 5.93 ± 4.85mm, respectively, in the vertical direction. The minimum, average, and maximum rotation shifts were: -1.23° ± 1.95°, 0.25° ± 1.30°, and 2.38° ± 2.91°, respectively, along lateral direction; -0.67° ± 0.91°, 0.10° ± 0.61°, and 1.51° ± 2.04°, respectively, along longitudinal direction; and -0.75° ± 1.01°, 0.02° ± 0.50°, and 0.82° ± 1.13°, respectively, along vertical direction. On average, each patient had three correction times during one fraction treatment. The radiation dose is about 3 mSv per fraction. With the ExacTrac 6D X-ray system, the prostate IMRT treatment with marker implanted can achieve less than 2 mm setup uncertainty in translations, and less than 0.25° in rotations as overall interfraction mean error. The imaging dose is less than kV (CBCT) for setup verification.

Clinical comparison of positional accuracy and stability between dedicated versus conventional masks for immobilization in cranial stereotactic radiotherapy using 6-degree-of-freedom image guidance system-integrated platform

PURPOSE

To compare the positioning accuracy and stability of two distinct noninvasive immobilization devices, a dedicated (D-) and conventional (C-) mask, and to evaluate the applicability of a 6-degrees-of-freedom (6D) correction, especially to the C-mask, based on our initial experience with cranial stereotactic radiotherapy (SRT) using ExacTrac (ET)/Robotics integrated into the Novalis Tx platform.

MATERIALS AND METHODS

The D- and C-masks were the BrainLAB frameless mask system and a general thermoplastic mask used for conventional radiotherapy such as whole brain irradiation, respectively. A total of 148 fractions in 71 patients and 125 fractions in 20 patients were analyzed for the D- and C-masks, respectively. For the C-mask, 3D correction was applied to the initial 10 patients, and thereafter, 6D correction was adopted. The 6D residual errors (REs) in the initial setup, after correction (pre-treatment), and during post-treatment were measured and compared.

RESULTS

The D-mask provided no significant benefit for initial setup. The post-treatment median 3D vector displacements (interquatile range) were 0.38 mm (0.22, 0.60) and 0.74 mm (0.49, 1.04) for the D- and C-masks, respectively (p<0.001). The post-treatment maximal translational REs were within 1 mm and 2 mm for the D- and C-masks, respectively, and notably within 1.5 mm for the C-mask with 6D correction. The pre-treatment 3D vector displacements were significantly correlated with those for post-treatment in both masks.

CONCLUSIONS

The D-mask confers positional stability acceptable for SRT. For the C-mask, 6D correction is also recommended, and an additional setup margin of 0.5 mm to that for the D-mask would be sufficient. The tolerance levels for the pre-treatment REs should similarly be set as small as possible for both systems.

Dependence of intrafraction motion on fraction duration for pediatric patients with brain tumors

The purpose of this study was to quantify the intrafraction motion of pediatric patients with brain tumors during radiation therapy and investigate any correlation between motion, use of general anesthesia, and daily treatment duration. 100 pediatric patients with a mean age of 8.5 years (range: 1.0 to 17.8) were included in this prospective study. Forty-one patients required general anesthesia during treatment, mean age 4.8 years; 59 patients did not, mean age 11.2 years. Each patient had an intracranial tumor and was treated in the supine position with a thermoplastic facemask and headrest for immobilization. A pretreatment localization CBCT was acquired for each treatment fraction and a post-treatment CBCT was acquired every other fraction. If the magnitude of the patient's position pre-CBCT offset was ≥ 2 mm, the position was corrected. The difference between the patient's position based on the post-CBCT and the assumed position at the start of treatment (either the pre-CBCT offset if the magnitude was < 2 mm, or 0 offset due to correction) was determined and labeled intrafraction motion. Correlations between daily treatment duration and intrafraction motion were examined. There was an average of 14.2 post-CBCTs acquired per patient. The magnitude of the mean intrafraction motion was 1.2 ± 0.8 mm for patients requiring general anesthesia, and 1.5 ± 1.2 mm for those without (p < 0.001). The mean offset in each direction was less than 0.5 mm for both cohorts. There was no correlation between daily treatment duration and the magnitude of intrafraction motion. The intrafraction motion of pediatric patients undergoing external beam therapy for intracranial tumors is small, < 2 mm, and is independent of the daily treatment duration.

The accuracy of frameless stereotactic intracranial radiosurgery

PURPOSE

To determine the accuracy of frameless stereotactic radiosurgery using the BrainLAB ExacTrac system and robotic couch by measuring the individual contributions such as the accuracy of the imaging and couch correction system, the linkage between this system and the linac isocenter and the possible intrafraction motion of the patient in the frameless mask.

MATERIALS AND METHODS

An Alderson head phantom with hidden marker was randomly positioned 31 times. Automated 6D couch shifts were performed according to ExacTrac and the deviation with respect to the linac isocenter was measured using the hidden marker. ExacTrac-based set-up was performed for 46 patients undergoing hypofractionated stereotactic radiotherapy for 135 fractions, followed by verification X-rays. Forty-three of these patients received post-treatment X-ray verification for 79 fractions to determine the intrafraction motion.

RESULTS

The hidden target test revealed a systematic error of 1.5 mm in one direction, which was corrected after replacement of the system calibration phantom. The accuracy of the ExacTrac positioning is approximately 0.3 mm in each direction, 1 standard deviation. The intrafraction motion was 0.35±0.21 mm, maximum 1.15 mm.

CONCLUSION

Intrafraction motion in the BrainLAB frameless mask is very small. Users are strongly advised to perform an independent verification of the ExacTrac isocenter in order to avoid systematic deviations.

Validation of Pinnacle treatment planning system for use with Novalis delivery unit

For an institution that already owns the licenses, it is economically advantageous and technically feasible to use Pinnacle TPS (Philips Radiation Oncology Systems, Fitchburg, WI) with the BrainLab Novalis delivery system (BrainLAB A.G., Heimstetten, Germany). This takes advantage of the improved accuracy of the convolution algorithm in the presence of heterogeneities compared with the pencil beam calculation, which is particularly significant for lung SBRT treatments. The reference patient positioning DRRs still have to be generated by the BrainLab software from the CT images and isocenter coordinates transferred from Pinnacle. We validated this process with the end-to-end hidden target test, which showed an isocenter positioning error within one standard deviation from the previously established mean value. The Novalis treatment table attenuation is substantial (up to 6.2% for a beam directed straight up and up to 8.4% for oblique incidence) and has to be accounted for in calculations. A simple single-contour treatment table model was developed, resulting in mean differences between the measured and calculated attenuation factors of 0.0%-0.2%, depending on the field size. The maximum difference for a single incidence angle is 1.1%. The BrainLab micro-MLC (mMLC) leaf tip, although not geometrically round, can be represented in Pinnacle by an arch with satisfactory dosimetric accuracy. Subsequently, step-and-shoot (direct machine parameter optimization) IMRT dosimetric agreement is excellent. VMAT (called "SmartArc" in Pinnacle) treatments with constant gantry speed and dose rate are feasible without any modifications to the accelerator. Due to the 3 mm-wide mMLC leaves, the use of a 2 mm calculation grid is recommended. When dual arcs are used for the more complex cases, the overall dosimetric agreement for the SmartArc plans compares favorably with the previously reported results for other implementations of VMAT: gamma(3%,3mm) for absolute dose obtained with the biplanar diode array passing rates above 97% with the mean of 98.6%. However, a larger than expected dose error with the single-arc plans, confined predominantly to the isocenter region, requires further investigation.