Clinical evaluation of a robotic 6-degree of freedom treatment couch for frameless radiosurgery

Comparison of imaging modalities for the accurate delineation of arteriovenous malformations (AVM) and evaluation of setup accuracy with reference to non-invasive LINAC-based stereotactic radiosurgery (SRS)

AIMS

To compare the accuracy of nidus delineation using magnetic resonance angiography (MRA) to digital subtraction angiography (DSA) and to evaluate setup accuracy of non-invasive frame SRS treatments.

SETTINGS AND DESIGN

A prospective observational study of 16 patients who underwent non-invasive frame LINAC-based SRS for brain AVMs.

MATERIALS AND METHODS

The nidus was separately delineated using DSA and MRA after co-registration onto CT simulation images and compared with respect to their volume and maximum diameters. During treatment, the setup errors observed in x-, y-, and z-directions were recorded.

STATISTICAL ANALYSIS

Paired t-test (to compare volume and maximum diameter). Wilcoxon signed-rank test (for setup accuracy).

RESULTS

The mean volume of nidus contoured in MRA was 4.16 cc compared to 3.11 cc in DSA (P 0.297). The mean maximum diameters using MRA and DSA, respectively, in antro-posterior, cranio- caudal, and transverse diameters were 21.97 cc vs. 19.46 cc (P 0.2380), 6.59 cc vs. 9.63 cc (P 0.161), and 18.87 cc vs. 16.81 cc (P 0.178). But these modalities can potentially misinterpret the nidus volume, warranting caution for use of either modality alone. The mean translational shift observed in the x-, y-, and z-directions were 0.06 mm, 0.13 mm, and 0.13 mm, respectively, when couch was brought to neutral position after clockwise couch rotation and 0.07, 0, and 0, respectively, after counterclockwise couch rotation.

CONCLUSION

This study could not demonstrate any statistically significant differences in nidus delineation between MRA and DSA. Setup accuracy achieved with non-invasive thermoplastic mask-based immobilization is within acceptable limits for SRS.

Optimization of isocenter position for multiple brain metastases single-isocenter stereotactic radiosurgery to minimize dosimetric variations due to rotational uncertainty

PURPOSE

This paper studied a novel calculation framework that can determine the optimal value isocenter position of single isocenter SRS treatment plan for multiple brain metastases, in order to minimize the dosimetric variations caused by rotational uncertainty.

MATERIALS AND METHODS

21 patients with 2-4 GTVswho received SRS treatment for multiple brain metastases in our institution were selected for the retrospective study. The PTVwas obtained by expanding GTV 1 mm isotropic margin. We studied a stochastic optimization framework, which determined the optimal value isocenter location by maximizing the average target dose coverageC_target,meanwith a rotation error of no more than 1°. We evaluated the performance of the optimal isocenter by comparing theC_target,meanand average dice similarity coefficient (DSC)with the optimal value and the center of mass (CM) respectively as the treatment isocenter. The extra PTV margin to achieve 100% target dose coverage was calculated by our framework.

RESULTS

Compared to the CM method, the optimal value isocenter method increased the averageC_target,meanof all targets from 97.0% to 97.7%and the average DSC from 0.794to 0.799. Throughout all the cases, the average extra PTV margin to obtain full target dose coverage was 0.7 mmwhen using the optimal value isocenter as the treatment isocenter.

CONCLUSION

We studied a novel computational framework using stochastic optimization to determine the optimal isocenter position of SRS treatment plan for multiple brain metastases. At the same time, our framework gave the extra PTV margin to obtain full target dose coverage.

Optimization of isocenter position for multiple targets with nonuniform-margin expansion

PURPOSE

The single isocenter for multiple-target (SIMT) technique has become a popular treatment technique for multiple brain metastases. We have implemented a method to obtain a nonuniform margin for SIMT technique. In this study, we further propose a method to determine the isocenter position so that the total expanded margin volume is minimal.

MATERIALS AND METHOD

Based on a statistical model, the relationship between nonuniform margin and the distance d (from isocenter to target point), setup uncertainties, and significance level was established. Due to the existence of rotational error, there is a nonlinear relationship between the margin volume and the isocenter position. Using numerical simulation, we study the relationship between optimal isocenter position and translational error, rotational error, and target size. In order to find the optimal isocenter position quickly, adaptive simulated annealing (ASA) algorithm was used. This method was implemented in the Pinnacle3 treatment planning system and compared with isocenter at center-of-geometric (COG), center-of-volume (COV), and center-of-surface (COS). Ten patients with multiple brain metastasis targets treated with the SIMT technique was selected for evaluation.

RESULTS

When the size of tumors is equal, the optimal isocenter obtained by ASA and numerical simulation coincides with COG, COV, and COS. When the size of tumors is different, the optimal isocenter is close to the large tumor. The position of COS point is closer to the optimal point than the COV point for nearly all cases. Moreover, in some cases the COS point can be approximately selected as the optimal point. The ASA algorithm can reduce the calculating time from several hours to tens of seconds for three or more tumors. Using multiple brain metastases targets, a series of volume difference and calculating time were obtained for various tumor number, tumor size, and separation distances. Compared with the margin volume with isocenter at COG, the margin volume for optimal point can be reduced by up to 27.7%.

CONCLUSION

Optimal treatment isocenter selection of multiple targets with large differences could reduce the total margin volume. ASA algorithm can significantly improve the speed of finding the optimal isocenter. This method can be used for clinical isocenter selection and is useful for the protection of normal tissue nearby.

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.

Evaluation of plan quality and treatment efficiency in cranial stereotactic radiosurgery treatment plans with a variable source-to-axis distance

INTRODUCTION

Radiotherapy deliveries with dynamic couch motions that shorten the source-to-axis distance (SAD) on a C-arm linac have the potential to increase treatment efficiency through the increase of the effective dose rate. In this investigation, we convert clinically deliverable volumetric modulated arc therapy (VMAT) and dynamic conformal arc (DCA) plans for cranial radiosurgery into virtual isocenter plans through implementation of couch trajectories that maintain the target at a shortened but variable SAD throughout treatment.

MATERIALS AND METHODS

A randomly sampled population of patients treated with cranial radiosurgery from within the last three years were separated into groups with one, two, and three lesions. All plans had a single isocenter (regardless of number of targets), and a single prescription dose. Patient treatment plans were converted from their original delivery at a standard isocenter to a dynamic virtual isocenter in MATLAB. The virtual isocenter plan featured a variable isocenter position based upon the closest achievable source-to-target distance (referred to herein as a virtual source-to-axis distance [vSAD]) which avoided collision zones on a TrueBeam STx platform. Apertures were magnified according to the vSAD and monitor units at a given control point were scaled based on the inverse square law. Doses were calculated for the plans with a virtual isocenter in the Eclipse (v13.6.23) treatment planning system (TPS) and were compared with the clinical plans. Plan metrics (MU, Paddick conformity index, gradient index, and the volume receiving 12 Gy or more), normal brain dose-volume differences, as well as maximum doses received by organs at risk (OARs) were assessed. The values were compared between standard and virtual isocenter plans with Wilcoxon Sign Ranked tests to determine significance. A subset of the plans were mapped to the MAX-HD anthropomorphic phantom which contained an insert housing EBT3 GafChromic film and a PTW 31010 microion chamber for dose verification on a linac.

RESULTS

Delivering plans at a virtual isocenter resulted in an average reduction of 20.9% (p = 3×10^-6 ) and 20.6% (p = 3.0×10^-6 ) of MUs across all VMAT and all DCA plans, respectively. There was no significant change in OAR max doses received by plans delivered at a virtual isocenter. The low dose wash (1.0-2.0 Gy or 5-11% of the prescription dose) was increased (by approximately 20 cc) for plans with three lesions. This was equivalent to a 2.7%-3.8% volumetric increase in normal tissue receiving the respective dose level when comparing the plan with a virtual isocenter to a plan with a standard isocenter. Gamma pass rates with a 5%/1mm analysis criteria were 96.40% ± 2.90% and 95.07% ± 3.10% for deliveries at standard and virtual isocenter, respectively. Absolute point dose agreements were within -0.36% ± 3.45% and -0.55% ± 3.39% for deliveries at a standard and virtual isocenter, respectively. Potential time savings per arc were found to have linear relationship with the monitor units delivered per arc (savings of 0.009 s/MU with an r² = 0.866 when fit to plans with a single lesion).

CONCLUSIONS

Converting clinical plans at standard isocenter to a virtual isocenter design did not show any losses to plan quality while simultaneously improving treatment efficiency through MU reductions.

Robustness of Breast Margins with Volumetric Modulated Arc Therapy without a Six-Degrees-of-Freedom Couch: A Dosimetric Evaluation

In our hospital, a TrueBeam linear accelerator and the PerfectPitch 6-degrees-of-freedom (6-DOF) couch (Varian), with 7 mm margins, are used for volumetric modulated arc therapy (VMAT) of breast cancer (BC). This study tested whether a 3-degrees-of-freedom (3-DOF) couch, i.e., without rotation compensation (such as the Halcyon couch), affected dose metrics. A total of 446 daily extended cone beam computed tomography (CBCT) data of 20 patients who received VMAT for BC were used to recalculate the treatment plans with the session registration (6-DOF) and a simulated matching with 3-DOF. The initial plan provided significantly better coverage for internal mammary chain and clavicular lymph node clinical target volumes (CTVs) than the 6-DOF and 3-DOF CBCT plans. The volumes receiving 110% of the prescribed dose (V110%) were increased for all CTVs with the 6-DOF and 3-DOF CBCT plans, but the difference was significant only for the breast/chest wall CTV (p < 0.05; paired samples t-test). Protection of the heart and lungs was comparable among plans. The dose volume histograms based on the 6-DOF and 3-DOF data were similar for CTVs and organs at risk. Therefore, with a 7 mm margin, VMAT and a 3-DOF couch can be used for BC treatment without any compromise in delivery accuracy.

Evaluation of the accuracy of a six-degree-of-freedom robotic couch using optical surface and cone beam CT images of an SRS QA phantom

Purpose:

To assess the accuracy of the Varian PerfectPitch six-degree-of-freedom (6DOF) robotic couch by using a Varian SRS QA phantom.

Methods:

The stereotactic radiosurgery (SRS) phantom has five tungsten carbide BBs each with 7·5 mm in diameter arranged with the known geometry. Optical surface images and cone beam CT (CBCT) images of the phantom were taken at different pitch, roll and rotation angles. The pitch, roll, and rotation angles were varied from −3 to 3 degrees by inputs from the linac console. A total of 39 Vision RT images with different rotation angle combinations were collected, and the Vision RT software was used to determine the rotation angles and translational shifts from those images. Eight CBCT images at most allowed rotational angles were analysed by in-house software. The software took the coordinates of the voxel of the maximum CT number inside a 7·5-mm sphere surrounding one BB to be the measured position of this BB. Expected BB positions at different rotation angles were determined by multiplying measured BB positions at zero pitch and roll values by a rotation matrix. Applying the rotation matrix to 5 BB positions yielded 15 equations. A linear least square method was used for regression analysis to approximate the solutions of those equations.

Results:

Of the eight calculations from CBCT images, the maximum rotation angle differences (degree) were 0·10 for pitch, 0·15 for roll and 0·09 for yaw. The maximum translation differences were 0·3 mm in the left–right direction, 0·5 mm in the anterior–posterior direction and 0·4 mm in the superior–inferior direction.

Conclusions:

The uncertainties of the 6-DOF couch were examined with the methods of optical surface imaging and CBCT imaging of the SRS QA phantom. The rotational errors were less than 0·2 degree, and the isocentre shifts were less than 0·8 mm.

ISRS Technical Guidelines for Stereotactic Radiosurgery: Treatment of Small Brain Metastases (≤1 cm in Diameter)

PURPOSE

The objective of this literature review was to develop International Stereotactic Radiosurgery Society (ISRS) consensus technical guidelines for the treatment of small, ≤1 cm in maximal diameter, intracranial metastases with stereotactic radiosurgery. Although different stereotactic radiosurgery technologies are available, most of them have similar treatment workflows and common technical challenges that are described.

METHODS AND MATERIALS

A systematic review of the literature published between 2009 and 2020 was performed in Pubmed using the Preferred Reporting Items for Systematic Review and Meta-analyses (PRISMA) methodology. The search terms were limited to those related to radiosurgery of brain metastases and to publications in the English language.

RESULTS

From 484 collected abstract 37 articles were included into the detailed review and bibliographic analysis. An additional 44 papers were identified as relevant from a search of the references. The 81 papers, including additional 7 international guidelines, were deemed relevant to at least one of five areas that were considered paramount for this report. These areas of technical focus have been employed to structure these guidelines: imaging specifications, target volume delineation and localization practices, use of margins, treatment planning techniques, and patient positioning.

CONCLUSION

This systematic review has demonstrated that Stereotactic Radiosurgery (SRS) for small (1 cm) brain metastases can be safely performed on both Gamma Knife (GK) and CyberKnife (CK) as well as on modern LINACs, specifically tailored for radiosurgical procedures, However, considerable expertise and resources are required for a program based on the latest evidence for best practice.

Comparison of dosimetric impact of intra-fractional setup discrepancy between multiple- and single-isocenter approaches in linac-based stereotactic radiotherapy of multiple brain metastases

Introduction

Treatment of multiple brain metastases by linac‐based stereotactic radiotherapy (SRT) can employ either a multiple‐isocenter (MI) or single‐isocenter (SI) approach. The purposes of this study were to evaluate the dosimetric results of MI and SI approaches and compare the impacts of intra‐fractional setup discrepancies on the robustness of respective approaches using isocenter shifts, whether the same magnitude of translational and rotational effects could lead to a significant difference between the two approaches.

Methods

Twenty‐two patients with multiple brain metastases treated by linac‐based SRT were recruited. Treatment plans were computed with both the MI and SI approaches. For the MI approach, the isocenter was located at the geometric center of each planning target volumes (PTVs), whereas the isocenter of the SI approach was located midway between the PTV centroids. To simulate the intra‐fractional errors, isocenter displacements including translational and rotational shifts were hypothetically applied. Apart from the dosimetric outcomes of the two approaches, the impact of the isocenter shifts on PTVs and organs at risk (OARs) were recorded in terms of the differences (δ) in dose parameters relative to the reference plan and was then compared between the MI and SI approaches.

Results

Both MI and SI plans met the plan acceptance criteria. The mean Paddick conformity index (Paddick CI) and D_max of most OARs between MI and SI plans did not show a significant difference, except that higher doses to the left optic nerve and optic chiasm were found in SI plans (p = 0.03). After the application of the isocenter shifts, δCI increased with an increase in the magnitude of the isocenter shift. When comparing between MI and SI plans, the δCIs were similar (p > 0.05) for all extents of translational shifts, but δCIs were significantly higher in SI plans after application of all rotations particularly ±1.5° and ±2.0° shifts. Despite the result that the majority of δD_Max of OARs were higher in the SI plans, only the differences in the left optic nerve and chiasm showed generally consistent significance after both translational ≥±1 mm and rotational shifts of ≥±1∘.

Conclusion

Both MI and SI approaches could produce clinically acceptable plans. However, isocenter shifts brought dosimetric impacts to both MI and SI approaches and the effects increased with the increase of the shift magnitude. Although similar impacts were shown in plans of both approaches after translational isocenter shift, SI plans were relatively more vulnerable than MI plans to rotational shifts.

Optimization of treatment isocenter location in single-isocenter LINAC-based stereotactic radiosurgery for management of multiple brain metastases

PURPOSE

Single-isocenter linear accelerator (LINAC)-based stereotactic radiosurgery (SRS) has become a promising treatment technique for the management of multiple brain metastases. Because of the high prescription dose and steep dose gradient, SRS plans are sensitive to geometric errors, resulting in loss of target coverage and suboptimal local tumor control. Current planning techniques rely on adding a uniform and isotropic setup margin to all gross tumor volumes (GTVs) to account for rotational uncertainties. However, this setup margin may be insufficient, since the magnitude of rotational uncertainties varies and is dependent upon the distance between a GTV and the isocenter. In this study, we designed a framework to determine the optimal isocenter of a single-isocenter SRS plan for multiple brain metastases using stochastic optimization to mitigate potential errors resulting from rotational uncertainties.

METHODS

Planning target volumes (PTVs), defined as GTVs plus a 1-mm margin following common SRS planning convention, were assumed to be originally treated with a prescription dose and therefore covered by the prescription isodose cloud. The dose distribution, including the prescription isodose, was considered invariant assuming small rotations throughout the study. A stochastic optimization scheme was developed to determine the location of the optimal isocenter, so that the prescription dose coverage of rotated GTVs, equivalent to the intersecting volumes between the rotated GTVs and original PTVs, was maximized for any random small rotations about the isocenter. To evaluate the coverage of GTVs, the expected V 100 % undergoing random rotations was approximated as the sample average V 100 % undergoing a predetermined number of rotations. The expected V 100 % of each individual GTV and total GTVs was then compared between the plans using the optimal isocenter and the center-of-mass (CoM), respectively.

RESULTS

Twenty-two patients previously treated for multiple brain metastases in a single institute were included in this retrospective study. Each patient was initially treated for more than three brain metastases (mean: 7.6; range: 3-15) with the average GTV volume of 0.89 cc (range: 0.03-11.78 cc). The optimal isocenter found for each patient was significantly different from the CoM, with the average Euclidean distance between the optimal isocenter and the CoM being 4.36 ± 2.59 cm. The dose coverage to GTVs was also significantly improved (paired t-test; p < 0.001) when the optimal isocenter was used, with the average V 100 % of total GTVs increasing from 87.1% (standard deviation as std: 11.7%; range: 39.9-98.2%) to 94.2% (std: 5.4%; range: 77.7-99.4%). The volume of a GTV was positively correlated with the expected V 100 % regardless of the isocenter used (Spearman coefficient: ρ = 0.66 ; p < 0.001). The distance between a GTV and the isocenter was negatively correlated with the expected V 100 % when the CoM was used ( ρ = - 0.21 ; p = 0.004), however no significant correlation was found when the optimal isocenter was used ( ρ = - 0.11 ; p = 0.137).

CONCLUSION

The proposed framework provides an effective approach to determine the optimal isocenter of single-isocenter LINAC-based SRS plans for multiple brain metastases. The implementation of the optimal isocenter results in SRS plans with consistently higher target coverage despite potential rotational uncertainties, and therefore significantly improves SRS plan robustness against random rotational uncertainties.

Combined effect of dose gradient and rotational error on prescribed dose coverage for single isocenter multiple brain metastases in frameless stereotactic radiotherapy

BACKGROUND

To evaluate the combined effect of rotational error and dose gradient on target dose coverage in frameless stereotactic radiotherapy.

METHODS

Three spherical targets of different diameters (1, 1.5, and 2 cm) were drawn and placed equidistantly on the same axial brain computed tomography (CT) images. To test the different isocenter-target distances, 2.5- and 5-cm configurations were prepared. Volumetric modulated arc therapy plans were created for different dose gradients from the target, in which the dose gradients were modified using the maximum dose inside the target. To simulate the rotational error, CT images and targets were rotated in two ways by 0.5°, 1°, and 2°, in which one rotation was in the axial plane and the other was in three dimensions. The initial optimized plan parameters were copied to the rotated CT sets, and the doses were recalculated. The coverage degradation after rotation was analyzed according to the target dislocation and 12-Gy volume.

RESULTS

A shallower dose gradient reduced the loss of target coverage under target dislocation, and the effect was clearer for small targets. For example, the coverage of the 1-cm target under 1-mm dislocation increased from 93 to 95% by increasing the Paddick gradient index from 5.0 to 7.9. At the same time, the widely accepted necrosis indicator, the 12-Gy volume, increased from 1.2 to 3.5 cm³, which remained in the tolerable range. From the differential dose volume histogram (DVH) analysis, the shallower dose gradient ensured that the dose-deficient under-covered target volume received a higher dose similar to that in the prescription.

CONCLUSIONS

For frameless stereotactic brain radiotherapy, the gradient, alongside the margin addition, can be adjusted as an ancillary parameter for small targets to increase target coverage or at least limit coverage reduction in conditions with probable positioning error.

Finite element analysis of a capacitive array for 6D intrafraction motion detection during stereotactic radiosurgery

This work presents a non-contact, non-ionizing solution for the continuous detection and characterization of intrafraction cranial motion with six-degrees of freedom (DoF). This capacitive monitoring system is a modular tool capable of detecting the cranial position through a thermoplastic mask without the use of skin as a surrogate. The purpose of this investigation is to develop an array of capacitive monitoring sensor plates capable of detecting translational and rotational cranial motion during radiotherapy. This study compares the performance of different capacitive monitoring array designs for their potential to detect intrafraction cranial translations and rotations. To this end, a finite element analysis (FEA) model of the human cranium was used to calculate the system capacitance while simulating translational (superior-inferior, lateral, anterior-posterior) and rotational (roll, pitch, yaw) cranial motion. The model was validated by comparing simulation results against experimental results acquired with the help of human volunteers. The verified FEA model was then used to compare multiple potential array designs. The arrays' sensitivities to translational and rotational motion and uniqueness of response were compared to determine the most promising design for six-DoF motion detection. The most promising array design was chosen for a clinical volunteer study.

Frameless immobilization system with roll correction for stereotactic radiosurgery of intracranial brain metastases

The objectives of this study were to develop a frameless immobilization system that allows roll rotation corrections and to investigate the performance of this system for stereotactic radiosurgery (SRS) treatment. We designed the support frame of a frameless immobilization system based on the commercial Brainlab immobilization system. The support frame consisted of a fixed component and a rotating component. With rack and pinion gears and guide holes installed in the system, the rotating component was configured to be rotated along the longitudinal axis of the patient with respect to the fixed component. To evaluate the performance of the system, the six degree-of-freedom (6D) positioning corrections (translational and rotational corrections) were assessed by image verification between planning computed tomography (CT) and cone-beam computed tomography (CBCT) images. The commercial immobilization system was evaluated in the same manner for comparison. The mean translational shifts for the commercial system were 0.68 ± 0.19 mm, 0.73 ± 0.24 mm and 0.78 ± 0.19 mm, while those for the developed system were 0.44 ± 0.31 mm, 0.43 ± 0.25 mm and 0.60 ± 0.14 mm in the lateral, longitudinal and vertical directions, respectively. The mean rotational shifts for the commercial system were 0.37° ± 0.12°, 0.32° ± 0.16° and 0.38° ± 0.14°, while those for the developed system were 0.04° ± 0.04°, 0.11° ± 0.06° and 0.15° ± 0.12° along the lateral, vertical and longitudinal axes of the patient, respectively. For institutions that do not have 6D robotic couches installed, the use of the developed immobilization system can provide 6D corrections, resulting in shorter treatment times and higher patient positioning accuracy.

Investigating the impacts of intrafraction motion on dosimetric outcomes when treating small targets with virtual cones

Purpose

Intrafraction patient motion is a well‐documented phenomenon in radiation therapy. In stereotactic radiosurgery applications in which target sizes can be very small and dose gradients very steep, patient motion can significantly impact the magnitude and positional accuracy of the delivered dose. This work investigates the impact of intrafraction motion on dose metrics for small targets when treated with a virtual cone.

Materials and Methods

Monte Carlo simulations were performed to calculate dose kernels for treatment apertures ranging from 1 × 2.5 mm² to 10 × 10 mm². The phantom was an 8.2‐cm diameter sphere and isotropic voxels had lengths of 0.25 mm. Simulated treatments consisted of 3 arcs: 1 axial arc (360° gantry rotation, couch angle 0°) and 2 oblique arcs (180° gantry rotation, couch angle ±45°). Dose distributions were calculated via superposition of the rotated kernels. Two different collimator orientations were considered to create a virtual cone: (a) each treatment arc was delivered twice, once each with a static collimator angle of ±45°, and (b) each treatment arc was delivered once, with dynamic collimator rotation throughout the arc. Two different intrafraction motion patterns were considered: (a) constant linear motion and (b) sudden, persistent motion. The impact of motion on dose distributions for target sizes ranging from 1 to 10 mm diameter spheres was quantified as a function of the aperture size used to treat the lesions.

Results

The impact of motion on both the target and the surrounding tissue was a function of both aperture shape and target size. When a 0.5‐mm linear drift along each dimension occurred during treatment, targets ≥5 mm saw less than a 10% decrease in coverage by the prescription dose. Smaller apertures accrued larger penalties with respect to dosimetric hotspots seen in the tissues surrounding the target volume during intrafraction motion. For example, treating a 4‐mm‐sized target that undergoes 2.60 mm (3D vector) of continuous linear motion, the D₅ in the concentric shells that extend 1, 2, and 3 mm from the surface of the target was 39%, 24%, and 14% smaller, respectively when comparing the delivery of a larger aperture (6 × 10 mm²) to a smaller aperture (2 × 5 mm²). Using a static collimator for shaping a virtual cone during treatment minimized the dosimetric impact of motion in the majority of cases. For example, the volume that is covered by 70% or more of the prescription dose is smaller in 60.4% of cases when using the static collimator. The volume covered by 50, and 30% or more of the prescription dose is also smaller when treating with a static collimator, but the clinical significance of this finding is unknown.

Conclusions

In this work, the dosimetric trade‐offs between aperture size and target size when irradiating with virtual cones has been demonstrated. These findings provide information about the tradeoffs between target coverage and normal tissue sparing that may help inform clinical decision making when treating smaller targets with virtual cones.

Time motion study to evaluate the impact of flattening filter free beam on overall treatment time for frameless intracranial radiosurgery using Varian TrueBeam® linear accelerator

Background

The aim was to study the impact of the flattening filter free (FFF) beam on overall treatment time for frameless intracranial radiosurgery using TrueBeam^® LINAC.The development of frameless stereotactic radiosurgery (SRS) is possible due to the incorporation of image guidance in the delivery of treatment. It is important to analyze the cost and benefits of FFF beams for treating SRS by understanding the impact of FFF beams in reducing the treatment time.

Materials and methods

Dynamic conformal arc (DCA ) and volumetric arc therapy (VMAT) plans were generated using 6 MV with a flattening filter (FF) and FFF beams. Overall treatment time was divided into beam on time (BOT) and beam off time (BFT). Percentage beam on time reduction (PBOTR) and Percentage total time reduction (PTTR) factors were defined for the comparison.

Results

BOT reduction was observed to be significant for higher dose per fraction but subjected to the treatment technique and modulation differences. PBOTR values are much higher than PTTR values. The 39.9% of PBOTR resulted in only 8% PTTR for DCA and 65.3% resulted in 15.9% PTTR for VMAT.

Conclusion

Major BFT was utilized for imaging and verification. FFF beam significantly reduced the beam on time and was found to be most effective if the fractional dose was as high as that for SRS. Newly defined PBOTR and PTTR factors are very useful indicators to evaluate the efficacy of FFF beams in terms of time reduction.

Distance to isocenter is not associated with an increased risk for local failure in LINAC-based single-isocenter SRS or SRT for multiple brain metastases

PURPOSE

To evaluate the impact of the distance between treatment isocenter and brain metastases on local failure in patients treated with a frameless linear-accelerator-based single-isocenter volumetric modulated arc (VMAT) SRS/SRT for multiple brain metastases.

METHODS AND MATERIALS

Patients treated with SRT for brain metastases (BM) between April 2014 and May 2019 were included in this retrospective study. BM treated with a single-isocenter multiple-target (SIMT) SRT were evaluated for local recurrence-free intervals in dependency to their distance to the treatment isocenter. A Cox-regression model was used to investigate different predictor variables for local failure. Results were compared to patients treated with a single-isocenter-single-target (SIST) approach.

RESULTS

In total 315 patients with a cumulative number of 1087 BM were analyzed in this study of which 140 patients and 708 BM were treated with SIMT SRS/SRT. Median follow-up after treatment was 13.9 months for SIMT approach and 11.9 months for SIST approach. One-year freedom from local recurrence was 87% and 94% in the SIST and SIMT group, respectively. Median distance to isocenter (DTI) was 4.7 cm (range 0.2-10.5) in the SIMT group. Local recurrence-free interval was not associated with the distance to the isocenter in univariable or multivariable Cox-regression analysis. Multivariable analysis revealed only volume as an independent significant predictor for local failure (p-value <0.05).

CONCLUSION

SRS/SRT using single-isocenter VMAT for multiple targets achieved high local metastases control rates irrespective of distance to the isocenter, supporting efficacy of single-isocenter stereotactic radiation therapy for multiple brain metastases.

Commissioning cranial single-isocenter multi-target radiosurgery for the Versa HD

PURPOSE

Brainlab's Elements Multiple Brain Mets SRS (MBMS) is a dedicated treatment planning system for single-isocenter multi-target (SIMT) cranial stereotactic radiosurgery (SRS) treatments. The purpose of this study is to present the commissioning experience of MBMS on an Elekta Versa HD.

METHODS

MBMS was commissioned for 6 X, 6 FFF, and 10 FFF. Beam data collected included: output factors, percent depth doses (PDDs), diagonal profiles, collimator transmission, and penumbra. Beam data were processed by Brainlab and resulting parameters were entered into the planning system to generate the beam model. Beam model accuracy was verified for simple fields. MBMS plans were created on previously treated cranial SRS patient data sets. Plans were evaluated using Paddick inverse conformity (ICI), gradient indices (GI), and cumulative volume of brain receiving 12 Gy. Dosimetric accuracy of the MBMS plans was verified using microDiamond, Gafchromic film, and SRS Mapcheck measurements of absolute dose and dose profiles for individual targets. Finally, an end-to-end (E2E) test was performed with a MR-CT compatible phantom to validate the accuracy of the simulation-to-delivery process.

RESULTS

For square fields, calculated scatter factors were within 1.0% of measured, PDDs were within 0.5% past dmax, and diagonal profiles were within 0.5% for clinically relevant off-axis distances (<10 cm). MBMS produced plans with ICIs < 1.5 and GIs < 5.0 for targets > 10 mm. Average point doses of the MBMS plans, measured by microDiamond, were within 0.31% of calculated (max 2.84%). Average per-field planar pass rates were 98.0% (95.5% minimum) using a 2%/1 mm/10% threshold relative gamma analysis. E2E point dose measurements were within 1.5% of calculated and Gafchromic film pass rates were 99.6% using a 5%/1 mm/10% threshold gamma analysis.

CONCLUSION

The experience presented can be used to aid the commissioning of the Versa HD in the Brainlab MBMS treatment planning system, to produce safe and accurate SIMT cranial SRS treatments.

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

PURPOSE

To evaluate intrafractional head motion (IFM) in patients who underwent intracranial stereotactic radiosurgery with the ExacTrac X-ray system (ETX) and a frameless mask.

METHODS

A total of 143 patients who completed a pre-treatment examination for IFM were eligible for this study. The frameless mask type B R408 (Klarity Medical & Equipment Co., Ltd., Guangzhou, China), which covers the back of the head, and the entire face, was used for patient immobilization. After the initial 6D correction and first X-ray verification (IFM₁), X-ray verification was performed every 3 min for a duration of 15 min. The IFM_p (2 ≤ p ≤ 6) was calculated as the positional difference from IFM₁. In addition, the inter-phase IFM (IP-IFM) and IFM_m were calculated. The IP-IFM was defined as |IFM_p - IFM_p-1|, and IFM_m as the difference between the values after all patients were asked to move their heads intentionally with the frameless mask on.

RESULTS

Both translational IFM_p and IP-IFM exceeded 1 mm for a single patient, whereas, for all patients, the translational IFM_m values were kept to within 1 mm in all directions. The proportions of the rotational IFM_p, IP-IFM, and IFM_m values within 0.5° were greater than 94.4%, 98.6%, and 90.2% for all of the rotational axes, respectively.

CONCLUSIONS

A frameless mask achieved highly accurate patient positioning in combination with ETX and a 6°-of-freedom robotic couch; however, a deviation over 1 mm and 0.5° was observed with low frequency. Therefore, X-ray verification and correction are required during treatment.

ESTRO ACROP guideline for target volume delineation of skull base tumors

BACKGROUND AND PURPOSE

For skull base tumors, target definition is the key to safe high-dose treatments because surrounding normal tissues are very sensitive to radiation. In the present work we established a joint ESTRO ACROP guideline for the target volume definition of skull base tumors.

MATERIAL AND METHODS

A comprehensive literature search was conducted in PubMed using various combinations of the following medical subjects headings (MeSH) and free-text words: "radiation therapy" or "stereotactic radiosurgery" or "proton therapy" or "particle beam therapy" and "skull base neoplasms" "pituitary neoplasms", "meningioma", "craniopharyngioma", "chordoma", "chondrosarcoma", "acoustic neuroma/vestibular schwannoma", "organs at risk", "gross tumor volume", "clinical tumor volume", "planning tumor volume", "target volume", "target delineation", "dose constraints". The ACROP committee identified sixteen European experts in close interaction with the ESTRO clinical committee who analyzed and discussed the body of evidence concerning target delineation.

RESULTS

All experts agree that magnetic resonance (MR) images with high three-dimensional spatial accuracy and tissue-contrast definition, both T2-weighted and volumetric T1-weighted sequences, are required to improve target delineation. In detail, several key issues were identified and discussed: i) radiation techniques and immobilization, ii) imaging techniques and target delineation, and iii) technical aspects of radiation treatments including planning techniques and dose-fractionation schedules. Specific target delineation issues with regard to different skull base tumors, including pituitary adenomas, meningiomas, craniopharyngiomas, acoustic neuromas, chordomas and chondrosarcomas are presented.

CONCLUSIONS

This ESTRO ACROP guideline achieved detailed recommendations on target volume definition for skull base tumors, as well as comprehensive advice about imaging modalities and radiation techniques.

Frameless radiosurgical third ventriculostomy: Technical report

Background:

We describe the technical report and results of the first image-guided, linear accelerator, frameless radiosurgical third ventriculostomy.

Methods:

We report a 20 years old man, with diplopia, balance disturbances, and limitation for gaze supraversion. Magnetic resonance imaging resonance imaging of the brain and cranial computed tomography showed showed a left thalamic-midbrain lesion that caused partial compression of the Silvio aqueduct and mild ventricular dilatation. The biopsy revealed the diagnosis of pleomorphic xanthoastrocytoma. Before radical treatment of the tumor with fractionated stereotactic radiotherapy, the patient underwent to frameless radiosurgical third ventriculostomy, on the TrueBeam STX® platform with the ExacTrac localization system. The target used was the one defined on the floor of the third ventricle, at the midpoint between the mammillary bodies and the infundibular recess. The prescription dose was 120 Gy, given using a monoisocentric technique of multiple noncoplanar circular arches. The geometric arrangement of the plan consisted of 15 arches, with a 4 mm cone, distributed over a 110° table.

Results:

There was symptomatic and image improvement two days after radiosurgery. On CT, a reduction in ventricular dilation was observed with a reduction in the Evans index from 0.39 (initial CT) to 0.29 (CT at 15 days). In 3.0T magnetic resonance image at 3 months, we showed the third ventriculostomy. There have been no treatment failures or complications.

Conclusion:

It is possible to effectively perform the frameless radiosurgical third ventriculostomy without associated morbidity in the short term.

A retrospective analysis of setup and intrafraction positional variation in stereotactic radiotherapy treatments

PURPOSE

The aim of this study was to provide a comprehensive assessment of patient intrafraction motion in linac-based frameless stereotactic radiosurgery (SRS) and radiotherapy (SRT).

METHODS

A retrospective review was performed on 101 intracranial SRS/SRT patients immobilized with the Klarity stereotactic thermoplastic mask (compatible with the Brainlab frameless stereotactic system) and aligned on a 6 Degree of Freedom (DoF) couch with the Brainlab ExacTrac image guidance system. Both pretreatment and intrafraction correction data are provided as observed by the ExacTrac system. The effects of couch angle and treatment duration on positioning outcomes are also explored.

RESULTS

Initial setup data for patients is shown to vary by up to ±4.18 mm, ±2.97°, but when corrected with a single x-ray image set with ExacTrac, patient positions are corrected to within ±2.11 mm, ±2.27°. Intrafraction patient motion is shown to be uniformly random and independent of both time and couch angle. Patient motion was also limited to within approximately 3 mm, 3° by the thermoplastic mask.

CONCLUSIONS

Our results indicate that since patient intrafraction motion is unrelated to couch rotation and treatment duration, intrafraction patient monitoring in 6 DoF is required to minimize intracranial SRS/SRT margins.

Selection of single-isocenter for multiple-target stereotactic brain radiosurgery to minimize total margin volume

Treating multiple brain metastases with a single isocenter improves efficiency but requires margins to account for rotation induced shifts that increase with target-to-isocenter distance. A method to select the single isocenter position that minimizes the total volume of normal tissue treated during multi-target stereotactic radiosurgery (SRS) is presented. A statistical framework was developed to quantify the impact of uncertainties on planning target volumes (PTV). Translational and rotational shifts were modeled with independent, zero mean, Gaussian distributions in three dimensions added in quadrature. The standard deviations of errors were varied from 0.5-2.0 mm and 0.5°-2.0°. The volume of normal tissue treated due to margin expansions required to maintain a 95% probability of target coverage was computed. Tumors were modeled as 4-40 mm diameter spheres. Target separation distance was varied from 40-100 mm for two- and three-lesion scenarios. The percent increase in PTV was determined relative to an isocenter at the geometric centroid of the targets for the optimal isocenter that minimized the total normal tissue treated, and isocenters at the center-of-mass (COM) and center-of-surface-area (CSA). For two targets, isocenter placement at the optimal location, COM, and CSA, reduced the total margin versus an isocenter at midline up to 17.8%, 17.7%, and 17.8%, respectively, for 0.5 mm and 0.5° errors. For three targets, optimal isocenter placement reduced the margin volume up to 21%, 19%, and 14%, for uncertainties of (0.5 mm, 0.5°), (1.0 mm, 1.0°), and (2.0 mm, 2.0°), respectively. COM and CSA provide useful approximations to select the optimal isocenter for multi-target single-isocenter SRS for two or three targets with maximum dimensions ⩽ 40 mm and separation distances ⩽ 100 mm when uncertainties are ⩽ 1.0 mm and ⩽ 1.0°. CSA provides a more accurate approximation than COM. Optimal treatment isocenter selection for multiple targets of large size differences can significantly reduce total margin volume.

Intra-fraction and Inter-fraction analysis of a dedicated immobilization device for intracranial radiation treatment

Background

Immobilization devices are crucial to minimize patient positioning uncertainties in radiotherapy (RT) treatments. Accurate inter and intra-fraction motions is particularly important for intracranial and stereotactic radiation treatment which require high precision in dose delivery. Recently, a new immobilization device has been developed specifically for the radiation treatment of intracranial malignancies. To date, no data are available on the use of this device in daily clinical practice. The aim of this study is to investigate the intra and inter-fraction variations, patient comfort and radiographer confidence of the immobilization system from two distinct institutions: HagaZiekenhuis, Den Haag, Netherlands and IRCCS Ospedale Sacro Cuore Don Calabria, Negrar, Italy.

Material and method

Sixteen patients (10 diagnosed with brain metastases and 6 with primary central nervous systemic tumor) from IRCCS Ospedale Sacro Cuore Don Calabria and 17 patients (all diagnosed with brain metastases tumor) from HagaZiekenhuis were included in this study. The median target volume was 436 cc (range 3.2–1628 cc) and 4.58 cc (range 0.4–27.19 cc) for IRCCS and Haga, respectively. For patients treated in IRCCS Sacro Cuore Don Calabria, the median dose prescription was 30 Gy (range 27–60 Gy) and median number of fractions 10 (range 3–30). In Haga the median dose prescription was 21 Gy (range 8–21 Gy) and the median number of fraction was 1 (range 1–3). The immobilization device was assembled during CT simulation. A short interview to the patient regarding the device’s comfort level was conducted at the end of the simulation procedure. Additionally, simulation setup time and radiographer (RTT) procedures (i.e. mask preparation) were evaluated. Prior to radiation treatment delivery, an automatic rigid match on the cranial bones between cone beam computed tomography (CBCT) and planning-CT was performed. A couch shift was performed subsequently. An extra post-treatment CBCT was acquire after the treatment delivery. This post-treatment CBCT was matched with pre-treatment CBCT to identify any possible intra-fraction motion. All online matches were validated by experienced radiation oncologist or RTT. A total of 126 CBCT’s were analyzed offline by radiation oncologist/medical physicist. The data of the pre-treatment CBCT match was used to quantify inter-fraction motion. The post-treatment CBCT was matched with pre-treatment CBCT to identify any possible intra-fraction motion.

Results

During the molding of the mask, all patients responded positive to the comfort. Median time required by the RTTs to assemble the immobilization system was 9 min (range 6–12 min). In terms of comfort, all patients reported a good-to high level of satisfaction. The RTTs also respond positively towards the use of the locking mechanism and clips. Results of positioning uncertainties were comparable between the two institutes. The mean inter-fraction motion for all translational and rotational directions were < 2 mm (SD < 4 mm) and < 0.5°(SD < 1.5°), respectively, while the mean intra-fraction motions were < 0.4 mm (SD < 0.6 mm) and 0.3° (SD < 0.5°).

Conclusions

This study demonstrates the efficacy and feasibility of the immobilization device in the intracranial radiation treatment. Both patient comfort and preparation time by RTTs are considered adequate. In combination with online daily imaging procedure, this device can achieve submillimeter accuracy required for intracranial and stereotactic treatments.

A review of Image Guided Radiation Therapy in head and neck cancer from 2009-201 - Best Practice Recommendations for RTTs in the Clinic

Radiation therapy (RT) is beneficial in Head and Neck Cancer (HNC) in both the definitive and adjuvant setting. Highly complex and conformal planning techniques are becoming standard practice in delivering increased doses in HNC. A sharp falloff in dose outside the high dose area is characteristic of highly complex techniques and geometric uncertainties must be minimised to prevent under dosage of the target volume and possible over dosage of surrounding critical structures. CTV-PTV margins are employed to account for geometric uncertainties such as set up errors and both interfraction and intrafraction motion. Robust immobilisation and Image Guided Radiation Therapy (IGRT) is also essential in this group of patients to minimise discrepancies in patient position during the treatment course. IGRT has evolved with increased 2-Dimensional (2D) and 3-Dimensional (3D) IGRT modalities available for geometric verification. 2D and 3D IGRT modalities are both beneficial in geometric verification while 3D imaging is a valuable tool in assessing volumetric changes that may have dosimetric consequences for this group of patients. IGRT if executed effectively and efficiently provides clinicians with confidence to reduce CTV-PTV margins thus limiting treatment related toxicities in patients. Accumulated exposure dose from IGRT vary considerably and may be incorporated into the treatment plan to avoid excess dose. However, there are considerable variations in the application of IGRT in RT practice. This paper aims to summarise the advances in IGRT in HNC treatment and provide clinics with recommendations for an IGRT strategy for HNC in the clinic.

Evaluating the positional uncertainty of intrafraction, adjacent fields, and daily setup with the BrainLAB ExacTrac system in patients who are receiving craniospinal irradiation

Purpose

To investigate the daily setup, interfraction motion, variability in the junction areas, and dosimetric effect in craniospinal irradiation (CSI) patients.

Methods

Fifteen CSI patients who had undergone split‐field IMRT were followed in the study. Previous, middle, and posttreatment, each target volume position was evaluated using the ExacTrac system. Interfraction and intrafraction motions, the margin of the junction in adjacent targets volumes, and the dosimetric effect of the longitudinal residual error were analyzed.

Results

The lowest attainment rate within the tolerance of the initial setup error was 66.79% in six directions. The values of the initial error were within 15 mm (SD 4.5 mm) in the translation direction and 5° (SD 1.3°) in the rotation direction after the transposition of the treatment isocenter. With the guidance of the ExacTrac system, the interfraction and intrafraction residual errors were almost within the tolerance after correction, the margin of CTV‐to‐PCTV was in the range of target expansion criteria. The residual longitudinal errors resulted in only slight changes in the mean doses of PGTV and PCTV, while the maximum dose of the spinal cord increased by 16.1%. The patients did not exhibit any side‐effects by the overall treatment during the follow‐up period.

Conclusions

Position correction is necessary after setup and the transposition of the treatment isocenter. Intra‐fraction motion in the lateral direction should be monitored throughout treatment. The position errors in junction areas are almost within the tolerance after correction. The patients did not exhibit any side‐effects by the overall treatment.

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

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.

Dosimetric impact of rotational errors on the quality of VMAT-SRS for multiple brain metastases: Comparison between single- and two-isocenter treatment planning techniques

Purpose

In the absence of a 6D couch and/or assuming considerable intrafractional patient motion, rotational errors could affect target coverage and OAR‐sparing especially in multiple metastases VMAT‐SRS cranial cases, which often involve the concurrent irradiation of off‐axis targets. This work aims to study the dosimetric impact of rotational errors in such applications, under a comparative perspective between the single‐ and two‐isocenter treatment techniques.

Methods

Ten patients (36 metastases) were included in this study. Challenging cases were only considered, with several targets lying in close proximity to OARs. Two multiarc VMAT plans per patient were prepared, involving one and two isocenters, serving as the reference plans. Different degrees of angular offsets at various orientations were introduced, simulating rotational errors. Resulting dose distributions were evaluated and compared using commonly employed dose‐volume and plan quality indices.

Results

For single‐isocenter plans and 1^⁰ rotations, plan quality indices, such as coverage, conformity index and D_95%, deteriorated significantly (>5%) for distant targets from the isocenter (at> 4–6 cm). Contrarily, for two‐isocenter plans, target distances to nearest isocenter were always shorter (≤4 cm), and, consequently, 1^⁰ errors were well‐tolerated. In the most extreme case considered (2^⁰ around all axes) conformity index deteriorated by on‐average 7.2%/cm of distance to isocenter, if one isocenter is used, and 2.6%/cm, for plans involving two isocenters. The effect is, however, strongly associated with target volume. Regarding OARs, for single‐isocenter plans, significant increase (up to 63%) in D_max and D_0.02cc values was observed for any angle of rotation. Plans that could be considered clinically unacceptable were obtained even for the smallest angle considered, although rarer for the two‐isocenter planning approach.

Conclusion

Limiting the lesion‐to‐isocenter distance to ≤4 cm by introducing additional isocenter(s) appears to partly mitigate severe target underdosage, especially for smaller target sizes. If OAR‐sparing is also a concern, more stringent rotational error tolerances apply.

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.

Dosimetric effect of rotational setup errors in stereotactic radiosurgery with HyperArc for single and multiple brain metastases

Purpose

In stereotactic radiosurgery (SRS) with single‐isocentric treatments for brain metastases, rotational setup errors may cause considerable dosimetric effects. We assessed the dosimetric effects on HyperArc plans for single and multiple metastases.

Methods

For 29 patients (1–8 brain metastases), HyperArc plans with a prescription dose of 20–24 Gy for a dose that covers 95% (D_95%) of the planning target volume (PTV) were retrospectively generated (Ref‐plan). Subsequently, the computed tomography (CT) used for the Ref‐plan and cone‐beam CT acquired during treatments (Rot‐CT) were registered. The HyperArc plans involving rotational setup errors (Rot‐plan) were generated by re‐calculating doses based on the Rot‐CT. The dosimetric parameters between the two plans were compared.

Results

The dosimetric parameters [D_99%, D_95%, D_1%, homogeneity index, and conformity index (CI)] for the single‐metastasis cases were comparable (P > 0.05), whereas the D_95% for each PTV of the Rot‐plan decreased 10.8% on average, and the CI of the Rot‐plan was also significantly lower than that of the Ref‐plan (Ref‐plan vs Rot‐plan, 0.93 ± 0.02 vs 0.75 ± 0.14, P < 0.01) for the multiple‐metastases cases. In addition, for the multiple‐metastases cases, the Rot‐plan resulted in significantly higher V_10Gy (P = 0.01), V_12Gy (P = 0.02), V_14Gy (P = 0.02), and V_16Gy (P < 0.01) than those in the Ref‐plan.

Conclusion

The rotational setup errors for multiple brain metastases cases caused non‐negligible underdosage for PTV and significant increases of V_10Gy to V_16Gy in SRS with HyperArc.

Real-time intrafraction motion monitoring in external beam radiotherapy

Radiotherapy (RT) aims to deliver a spatially conformal dose of radiation to tumours while maximizing the dose sparing to healthy tissues. However, the internal patient anatomy is constantly moving due to respiratory, cardiac, gastrointestinal and urinary activity. The long term goal of the RT community to 'see what we treat, as we treat' and to act on this information instantaneously has resulted in rapid technological innovation. Specialized treatment machines, such as robotic or gimbal-steered linear accelerators (linac) with in-room imaging suites, have been developed specifically for real-time treatment adaptation. Additional equipment, such as stereoscopic kilovoltage (kV) imaging, ultrasound transducers and electromagnetic transponders, has been developed for intrafraction motion monitoring on conventional linacs. Magnetic resonance imaging (MRI) has been integrated with cobalt treatment units and more recently with linacs. In addition to hardware innovation, software development has played a substantial role in the development of motion monitoring methods based on respiratory motion surrogates and planar kV or Megavoltage (MV) imaging that is available on standard equipped linacs. In this paper, we review and compare the different intrafraction motion monitoring methods proposed in the literature and demonstrated in real-time on clinical data as well as their possible future developments. We then discuss general considerations on validation and quality assurance for clinical implementation. Besides photon RT, particle therapy is increasingly used to treat moving targets. However, transferring motion monitoring technologies from linacs to particle beam lines presents substantial challenges. Lessons learned from the implementation of real-time intrafraction monitoring for photon RT will be used as a basis to discuss the implementation of these methods for particle RT.

Frameless Stereotactic Radiosurgery on the Gamma Knife Icon: Early Experience From 100 Patients

BACKGROUND

The Gamma Knife (GK) Icon (Elekta AB) uses a cone-beam computed tomography (CBCT) scanner and an infrared camera system to support the delivery of frameless stereotactic radiosurgery (SRS). There are limited data on patients treated with frameless GK radiosurgery (GKRS).

OBJECTIVE

To describe the early experience, process, technical details, and short-term outcomes with frameless GKRS at our institution.

METHODS

We reviewed our patient selection and described the workflow in detail, including image acquisition, treatment planning, mask-based immobilization, stereotactic CBCT localization, registration, treatment, and intrafraction monitoring. Because of the short interval of follow-up, we provide crude rates of local control.

RESULTS

Data from 100 patients are reported. Median age is 67 yr old. 56 patients were treated definitively, 21 postoperatively, and 23 had salvage GKRS for recurrence after surgery. Forty-two patients had brain metastases, 26 meningiomas, 16 vestibular schwannomas, 9 high-grade gliomas, and 7 other histologies. Median doses to metastases were 20 Gy in 1 fraction (range: 14-21), 24 Gy in 3 fractions (range: 19.5-27), and 25 Gy in 5 fractions (range: 25-30 Gy). Thirteen patients underwent repeat SRS to the same area. Median treatment time was 17.7 min (range: 5.8-61.7). We found an improvement in our workflow and a greater number of patients eligible for GKRS because of the ability to fractionate treatments.

CONCLUSION

We report a large cohort of consecutive patients treated with frameless GKRS. We look forward to studies with longer follow-up to provide valuable data on clinical outcomes and to further our understanding of the radiobiology of hypofractionation in the brain.

Development of a rotational set-up correction device for stereotactic head radiation therapy: A performance evaluation

We developed a new head supporting device to provide accurate correction of rotational setup during image-guided radiation therapy (IGRT), evaluating its correction performance and the efficacy of dose distribution in stereotactic radiotherapy (SRT) using a helical tomotherapy (HT) system. The accuracy of rotational motion was measured using an electronic inclinometer; we compared device angles and measurement values from 0.0° to 3.0°. The correction accuracy was investigated based on the distance between rotational centers in the device and on megavoltage computed tomography (MVCT); the correction values were compared using distances in the range of 0.0-9.0 cm using a head phantom with a rotational error of 1.5°. For an SRT with a simultaneous integrated boost plan and a rotational error of 3.0° in yaw angle using a head phantom, and for a single-isocenter SRT for multiple brain metastases in the data of three patients, dosimetric efficacy of the HT unit was evaluated for calculated dose distributions with MVCT after rotational correction. This device can correct pitch and yaw angles within 0.3° and can be corrected to within 0.5° for each rotational angle according to the result of MVCT correction regardless of the rotational center position. In the head phantom study, the device had a beneficial impact on rotational correction; D99% for the target improved by approximately 10% with rotational correction. Using patient data with the device, the mean difference based on the treatment planning data was 0.3% for D99% and -0.1% for coverage index to the target. Our rotational setup correction device has high efficacy, and can be used for IGRT.

The impact of the three degrees-of-freedom fiducial marker-based setup compared to soft tissue-based setup in hypofractionated intensity-modulated radiotherapy for prostate cancer

Purpose

We evaluated the setup accuracy of a three‐degree‐of‐freedom fiducial marker (3DOF‐FM)‐based setup compared to a soft tissue (ST)‐based setup in hypofractionated intensity‐modulated radiotherapy (IMRT) for prostate cancer.

Materials and Methods

We analyzed the setup accuracy for 17 consecutive prostate cancer patients with three implanted FMs who underwent hypofractionated IMRT. The 3DOF‐ST‐based setup using cone‐beam computed tomography (CT) was performed after a six DOF‐bony structure (BS)‐based setup using an ExacTrac x‐ray system. The 3DOF‐FM‐based matching using the ExacTrac x‐ray system was done during the BS‐ and ST‐based setups. We determined the mean absolute differences and the correlation between the FM‐ and ST‐based translational shifts relative to the BS‐based setup position. The rotational mean shifts detected by the ExacTrac x‐ray system were also evaluated.

Results

The mean differences in the anterior‐posterior (AP), superior‐inferior (SI), and left‐right (LR) dimensions were 0.69, 0.0, and 0.30 mm, respectively. The Pearson correlation coefficients for both shifts were 0.92 for AP, 0.91 for SI, and 0.68 for LR. The percentages of shift agreements within 2 mm were 85% for AP, 93% for SI, and 99% for LR. The absolute values of rotational shifts were 0.1° for AP, 0.3°, and 1.2° for LR.

Conclusions

The setup accuracy of the 3DOF‐FM‐based setup has the potential to be interchangeable with a ST‐based setup. Our data are likely to be useful in clinical practice along with the popularization of the hypofractionated IMRT in prostate cancer.

TomoTherapy treatments of multiple brain lesions: an in-phantom accuracy evaluation

Aim of the present study was to evaluate the accuracy which can be obtained with helical TomoTherapy^® (HT, Accuray) systems in the case of multiple intracranial targets treatments. Set-up accuracy was measured, for different registration options and MegaVoltage CT (MVCT) slice thickness, by applying known misalignments to an ad-hoc developed phantom. End-to-end (E2E) tests were performed to assess the delivery accuracy in phantoms containing multiple targets by using radiochromic films: measured dose distribution centroids were compared with physical and calculated target positions on axial and coronal planes. A Gamma index analysis was carried out on planned and measured planar dose maps. The bone and tissue algorithm with the fine MVCT reconstruction grid gave the best results among the automatic options. The most accurate registration modality resulted to be the manual one with a sub-voxel accuracy shifts and a capability in the detection of rotations within 0.3°. For the E2E along the coronal plane (six targets), a mean deviation between measured dose distribution centroids and physical barycenters of 0.6 mm (range 0.1 mm-1.3 mm) was observed. Along the axial plane (five targets), a mean deviation of 1.2 mm (range 0.7 mm-2.1 mm) was found for the centroids shifts. Gamma index (5%, 1 mm, local) passing rates higher than 87.5% between planned and delivered dose distributions were measured. These results demonstrate that multiple brain lesion HT treatments are feasible with an accuracy at least comparable to frameless linac-based delivery, when a set-up capable to assure angular corrections and a reliable patient immobilization is employed.

Commissioning and Evaluation of a Third-Party 6 Degrees-of-Freedom Couch Used in Radiotherapy

Purposes:

The newly released Protura 6 degrees-of-freedom couch (CIVCO) has limited quality assurance protocols and pertinent publications. Herein, we report our experiences of the Protura system acceptance, commissioning, and quality assurance.

Methods:

The Protura system integration was tested with peripheral equipment on the following items: couch movement range limit, 6 degrees-of-freedom movement accuracy, weight test and couch sagging, system connection with Linac, isocentricity of couch and rotation alignment, kV and cone-beam computed tomography imaging of HexaCHECK with MIMI phantom (Standard Imaging), and an in-house custom 6 degrees-of-freedom quality assurance phantom. A couch transmission measurement was also performed.

Results:

The vertical, longitudinal, and lateral ranges of the 6 degrees-of-freedom couch pedestal are 43.9 to 0.0 cm, 24.6 to 149.5 cm, −20.6 to 20.7 cm, respectively. The couch movement accuracy was within 1 mm in all directions. The couch sagging with a 200 lbs (∼91 kg) evenly distributed object is 1.0 cm and 0.4° pitch in the distal end of the couch. The isocentricity of the couch was about 0.5 mm in diameter of all crosshair projections on the couch isocenter level, and the largest couch rotation alignment observed was (0.3°) at the couch angle of 90°. The deviation from the reference position (zero position) of the HexaCHECK phantom, measured by matching the cone-beam computed tomography with the reference planning computed tomography, was found to be below 0.2 mm in the anterior–posterior and right–left dimensions, 0.4 mm in superior–inferior dimension, and 0.1° in roll, pitch, and yaw directions.

Conclusions:

A 6 degrees-of-freedom quality assurance phantom is helpful for the commissioning and routine quality assurance tests. Due to the third-party integration with Linac, the system is prone to “double-correction” errors. A rigorous quality assurance program is the key to a successful clinical implementation of the Protura system.

High-precision quality assurance of robotic couches with six degrees of freedom

A robotic couch capable of six degrees of freedom (6-DoF) of motion was introduced for state-of-the-art radiation therapy. Patient treatment requires precise quality assurance (QA) of 6-DoF. Unfortunately, conventional methods do not provide the requisite accuracy and precision. Therefore, we developed a high-precision automated QA system using a visual tracking system (VTS). The VTS comprises four motion-sensing cameras, a cube with infrared reflective markers. To acquire data in treatment room coordinates, a transformation matrix from VTS coordinates to treatment room coordinates was determined. The mean error and standard deviation of linear and rotational motions, as well as couch sagging were analyzed from continuously acquired images in the moving couch. The accuracy of VTS was 0.024 mm deviation for the sinusoidal motion, and the accuracy of the transformation matrix was 0.02 mm. In a cross-comparison, the difference between Laser Tracker (FARO) measurements was 0.14 ± 0.12 mm for translation and 0.032 ± 0.026° on average for yaw rotation. The new system provides QA of yaw, pitch and roll motion as well as sagging of the couch and sub-millimeter/degree accuracy together with precision.

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.

Evaluation of the Efficacy of Rotational Corrections for Standard-Fractionation Head and Neck Image-Guided Radiotherapy

PURPOSE

Modern linear accelerators are equipped with cone beam computed tomography and robotic couches that can correct for errors in the translational (X, Y, Z) and rotational (α, β, γ) axes prior to treatment delivery. Here, we compared the positional accuracy of 2 cone beam registration approaches: (1) employing translational shifts only in 3 degrees of freedom (X, Y, Z), versus; (2) using translational-rotational shifts in 6 degrees of freedom (X, Y, Z, α, β, γ).

METHODS

This retrospective study examined 140 interfraction cone beam images from 20 patients with head and neck cancer treated with standard intensity-modulated radiation therapy. The cone beam images were matched to planning simulation scans in 3, then in 6 degrees of freedom, using the mandible, clivus, and C2 and C7 vertebrae as surrogate volumes. Statistical analyses included a generalized mixed model and was used to assess whether there were significant differences in acceptable registrations between the 2 correction methods.

RESULTS

The rates of improvement with corrections in 6 degrees of freedom for the mandible with a 5-mm expansion margin were 54.55% ( P = .793), for the clivus 85.71% ( P = .222), and for C7 87.50% ( P = .015). There was a 100% increase in acceptability for the C2 vertebra within the 5-mm margin ( P < .001). For the 3-mm expansion margin, the rates of improvement for the mandible, clivus, C2, and C7 were 63.16% ( P = .070), 91.30% ( P = .011), 84.21% ( P = .027), and 76.92% ( P < .001), respectively.

CONCLUSIONS

Significant registration improvements with the use of rotational corrections with a 5-mm expansion margin are only seen in the C7 vertebra. At the 3-mm margin, significant improvements are found for the C2, C7, and clivus registrations, suggesting that intensity-modulated radiotherapy treatments for head and neck cancers with 3-mm planning target volume margins may benefit from corrections in 6 degrees of freedom.

Quality assurance for a six degrees-of-freedom table using a 3D printed phantom

Purpose

To establish a streamlined end‐to‐end test of a 6 degrees‐of‐freedom (6DoF) robotic table using a 3D printed phantom for periodic quality assurance.

Methods

A 3D printed phantom was fabricated with translational and rotational offsets and an imbedded central ball‐bearing (BB). The phantom underwent each step of the radiation therapy process: CT simulation in a straight orientation, plan generation using the treatment planning software, setup to offset marks at the linac, registration and corrected 6DoF table adjustments via hidden target test, delivery of a Winston‐Lutz test to the BB, and verification of table positioning via field and laser lights. The registration values, maximum total displacement of the combined Winston‐Lutz fields, and a pass or fail criterion of the laser and field lights were recorded. The quality assurance process for each of the three linacs were performed for the first 30 days.

Results

Within a 95% confidence interval, the overall uncertainty values for both translation and rotation were below 1.0 mm and 0.5° for each linac respectively. When combining the registration values and other uncertainties for all three linacs, the average deviations were within 2.0 mm and 1.0° of the designed translation and rotation offsets of the 3D print respectively. For all three linacs, the maximum total deviation for the Winston‐Lutz test did not exceed 1.0 mm. Laser and light field verification was within tolerance every day for all three linacs given the latest guidance documentation for table repositioning.

Conclusion

The 3D printer is capable of accurately fabricating a quality assurance phantom for 6DoF positioning verification. The end‐to‐end workflow allows for a more efficient test of the 6DoF mechanics while including other important tests needed for routine quality assurance.

Towards frameless maskless SRS through real-time 6DoF robotic motion compensation

Stereotactic radiosurgery (SRS) uses precise dose placement to treat conditions of the CNS. Frame-based SRS uses a metal head ring fixed to the patient's skull to provide high treatment accuracy, but patient comfort and clinical workflow may suffer. Frameless SRS, while potentially more convenient, may increase uncertainty of treatment accuracy and be physiologically confining to some patients. By incorporating highly precise robotics and advanced software algorithms into frameless treatments, we present a novel frameless and maskless SRS system where a robot provides real-time 6DoF head motion stabilization allowing positional accuracies to match or exceed those of traditional frame-based SRS. A 6DoF parallel kinematics robot was developed and integrated with a real-time infrared camera in a closed loop configuration. A novel compensation algorithm was developed based on an iterative closest-path correction approach. The robotic SRS system was tested on six volunteers, whose motion was monitored and compensated for in real-time over 15 min simulated treatments. The system's effectiveness in maintaining the target's 6DoF position within preset thresholds was determined by comparing volunteer head motion with and without compensation. Comparing corrected and uncorrected motion, the 6DoF robotic system showed an overall improvement factor of 21 in terms of maintaining target position within 0.5 mm and 0.5 degree thresholds. Although the system's effectiveness varied among the volunteers examined, for all volunteers tested the target position remained within the preset tolerances 99.0% of the time when robotic stabilization was used, compared to 4.7% without robotic stabilization. The pre-clinical robotic SRS compensation system was found to be effective at responding to sub-millimeter and sub-degree cranial motions for all volunteers examined. The system's success with volunteers has demonstrated its capability for implementation with frameless and maskless SRS treatments, potentially able to achieve the same or better treatment accuracies compared to traditional frame-based approaches.

[Description of the GORTEC 2017-03 study: Postoperative stereotactic radiotherapy for early stage oropharyngeal and oral cavity cancer with high risk margin (PHRC-K-16-164)]

The GORTEC 2017-03-Stereo-postop study is a phase 2, multicentric, nationwide study, funded by the hospital clinical research program (PHRC). The sponsor is Centre Jean-Perrin in Clermont-Ferrand, in partnership with the GORTEC. The principal investigators are Dr J Biau and Dr M Lapeyre. The main objective is to study severe late toxicity of postoperative stereotactic radiotherapy (6×6Gy) for early stage oropharyngeal and oral cavity cancer with high risk margins. The secondary objectives include acute toxicity, efficacy, nutritional impact and quality of life. The population is adult patients, with pT1 or pT2 squamous cell carcinoma of the oropharynx or oral cavity (except lips), without indication of neck irradiation or concomitant chemotherapy, with at risk margin (R1, less than 5mm or uncertain). Ninety patients will be included over a 2-year period; this was calculated to limit the rate of 2-year severe toxicity at 5 to 15%, with a 2-year local control of at least 80 to 90%. If this study is considered as positive, stereotactic radiotherapy (6×6Gy) could become the third therapeutic option, with brachytherapy and normofractionated intensity-modulated radiotherapy (IMRT), for postoperative irradiation of oropharyngeal and oral cavity cancer with high risk margins.

An evaluation of the BrainLAB 6D ExacTrac/Novalis Tx System for image-guided intracranial radiotherapy

Purpose

Stereotactic-fractionated radiotherapy and radiosurgery (RS) for benign and malignant intracranial lesions relies on a very high degree of accuracy in dose alignment due to the ablative dose delivered, and therefore requires a high-precision image guidance modality. The aim of this review is to investigate the localisation and verification accuracy performance of ExacTrac (ET) and Novalis Tx System.

Materials and methods

A systematic review of the database Science Direct was carried out using search terms ‘stereotactic radiotherapy (SRT)’ and ‘ET’. All articles before 2000 were excluded. Only articles that involved intracranial lesions, with the exception of one article, were included in the final review.

Results

Results from gold standard Hidden Target Tests and patient data show that patient position can be reproduced within 1·0 mm with the use of ET imaging. In addition, the 6 degrees of freedom algorithm function of ET allows for better translational accuracy as well optimal positioning when rotations are corrected for. Studies showed excellent correlation (p<0·01) between bony ET images and cone beam computed tomography (CBCT) soft tissue registration, evidencing the safe reliance of bony anatomy for image guidance via ET. Shifts were found to be comparable between CBCT and ET.

Conclusion

There is the need for regular calibration to prevent systematic errors and potential geographic miss. However, due to ET’s additional benefits, including reduced concomitant dose and faster imaging time, ET is the superior image guidance modality for RS/SRT in the treatment of intracranial lesions.

Cone beam CT-based set-up strategies with and without rotational correction for stereotactic body radiation therapy in the liver

BACKGROUND

Accurate patient positioning is crucial in stereotactic body radiation therapy (SBRT) due to a high dose regimen. Cone-beam computed tomography (CBCT) is often used for patient positioning based on radio-opaque markers. We compared six CBCT-based set-up strategies with or without rotational correction.

MATERIAL AND METHODS

Twenty-nine patients with three implanted markers received 3-6 fraction liver SBRT. The markers were delineated on the mid-ventilation phase of a 4D-planning-CT. One pretreatment CBCT was acquired per fraction. Set-up strategy 1 used only translational correction based on manual marker match between the CBCT and planning CT. Set-up strategy 2 used automatic 6 degrees-of-freedom registration of the vertebrae closest to the target. The 3D marker trajectories were also extracted from the projections and the mean position of each marker was calculated and used for set-up strategies 3-6. Translational correction only was used for strategy 3. Translational and rotational corrections were used for strategies 4-6 with the rotation being either vertebrae based (strategy 4), or marker based and constrained to ±3° (strategy 5) or unconstrained (strategy 6). The resulting set-up error was calculated as the 3D root-mean-square set-up error of the three markers. The set-up error of the spinal cord was calculated for all strategies.

RESULTS

The bony anatomy set-up (2) had the largest set-up error (5.8 mm). The marker-based set-up with unconstrained rotations (6) had the smallest set-up error (0.8 mm) but the largest spinal cord set-up error (12.1 mm). The marker-based set-up with translational correction only (3) or with bony anatomy rotational correction (4) had equivalent set-up error (1.3 mm) but rotational correction reduced the spinal cord set-up error from 4.1 mm to 3.5 mm.

CONCLUSIONS

Marker-based set-up was substantially better than bony-anatomy set-up. Rotational correction may improve the set-up, but further investigations are required to determine the optimal correction strategy.