Degree Of Freedom Couch Research Articles

Evaluation of corrective effect of 6 degree of freedom couch on setup errors in intensity modulated radiotherapy for postoperative rectal cancer patients.

To explore the corrective effect of 6 degree of freedom couch on rotation errors in intensity modulated radiotherapy (IMRT) for postoperative rectal cancer patients, further to probe into the clinical application value of 6 degree of freedom couch in radiotherapy. From January 1, 2020 to December 1, 2020, 30 patients with rectal cancer receiving postoperative intensity modulated radiotherapy in The First Hospital of Hebei Medical University were included in this retrospective study. The setup error values in all direction of patients before and after 6 degree of freedom correction were collected during each radiotherapy session. In this study, a total of 382 data before and after the correction of 6 degree of freedom couch were collected. It was found that the setup errors in the Y direction gradually increased, was maximal in the third week, and then became smaller, and the setup errors in the other directions increased with the extension of radiotherapy time and reached the maximum at the 5th week. In the translation direction, the setup errors value in Z direction occurred more frequently than that in X and Y directions between the range of 0.21-0.80cm. In the rotation direction, the setup errors value in rotation X direction occurred more frequently than that in rotation Y and Z directions between the range of 0.21°-2.99°. In addition, after the correction of the 6 degree of freedom couch in real time, the setup errors in patients were significantly reduced in all directions (P < 0.05). In summary, it was recommended to clinically use 6 degree of freedom couch combined with IMRT for real-time correction of placement errors in patients with rectal cancer undergoing radiotherapy. At the same time, it was necessary to observe the tumor size and body weight changes of patients on the 5th week. If necessary, radiotherapy positioning and planning should be performed in time.

Comparison of Nodal CTV Coverage with Alignment to Bladder-Rectal Interface vs. Pelvic Bone in Post-Prostatectomy Radiotherapy

<h3>Purpose/Objective(s)</h3> In patients receiving post-prostatectomy radiation therapy (RT), daily image guidance is used to align to the bladder-rectal interface (BRI) which varies depending on bladder and rectal filling. Alignment to pelvic bone is often used for nodal irradiation targets. The impact of alignment prioritized to the BRI on coverage of concurrent nodal targets is unknown. We hypothesized that the nodal clinical target volume (CTVn) would remain within nodal planning target volume (PTV) expansions with alignment to BRI versus bone. <h3>Materials/Methods</h3> Cone-beam CTs (CBCT) from patients receiving prostate bed RT using a 6 degrees of freedom couch were analyzed using transformation matrices to calculate differences in offsets. The CBCTs with translational offsets of ≥5mm were further evaluated for impact on nodal coverage. The CTVn was contoured according to consensus guidelines and two PTVs generated, using 5 and 7mm symmetric expansions (PTV5=5mm and PTV7=7mm) on the treatment planning CT. Each CBCT of interest was extended superiorly to evaluate the full nodal volumes. Two separate registrations of each CBCT to the treatment planning CT were performed, one with alignment prioritized to the BRI and the other to pelvic bone. The CTVn and PTVs were copied to each CBCT registration (BRI and bone) for analysis (e.g., PTVn_BRI is where the planned nodal dose is delivered when the treatment is matched to BRI). The inclusion of the CTVn within each PTV was evaluated. Boolean functions were used to compare the overlap and intersections of the nodal volumes between alignment to BRI versus bone. <h3>Results</h3> A set of 139 CBCTs from 14 patients were initially evaluated, of which 13 CBCTs had maximal translational offsets of ≥5mm. The mean CTVn was 324.3cc, PTV5 was 655.2cc, and PTV7 was 801.4cc. When alignment to the BRI was prioritized, the CTVn was completely included within the PTV7 in 9 of 13 CBCTs. At least 95% of the CTVn was within the PTV5 or PTV7 in all but one CBCT in which 92% of the CTVn was within the PTV5. The volume of the CTVn outside the PTV5 ranged from 2.7-28.4cc (mean 2.6%, range 0.6-8%) and outside the PTV7 from 0-15.4cc (mean 0.7%, range 0-4.3%). The most common location for incomplete inclusion of the CTVn within the PTV was in the most caudal and posterior nodal regions (between the prostate bed and the presacral space), primarily occurring in the event of pitch related mismatch between BRI and bone alignment. When the intersection between PTV_(bone) and PTV_(BRI) was evaluated, the overlap ranged between 80-98% and did not differ between comparisons for 5 and 7mm expansions. <h3>Conclusion</h3> These data from CBCTs with translational offsets of ≥5mm suggest that even with larger disagreement between alignment to BRI versus bone, the vast majority of the CTVn remains encompassed within the PTV, particularly when a 7mm expansion is used. The predominant site of incomplete inclusion raises concern about use of smaller posterior nodal PTV margins. These results are informative for concurrent prostate bed and nodal irradiation.

The development and testing of a novel spherical radiotherapy phantom system for the commissioning and patient-specific quality assurance of mono-isocentric multiple mets SRS plans.

To develop a single radiotherapy phantom system capable of performing both patient-specific quality assurance (QA) measurements and commissioning measurements for mono-isocentric LinAc-based stereotactic radiosurgery ("mLSRS") treatment plans. Design A 20-cm diameter spherical phantom was designed which contained within it a film cartridge. The surface of the sphere was machined to display a set of angular markings at both the equator and the meridian representing a spherical coordinate system. A stand was designed which allows for free rotation about any vector passing through the center of the sphere. A program was created using Python 3 to: (a) Compute the measurement setup necessary to intersect exactly one film plane with three user specified dicom points contained within the QA plan; (b) Extract the intersection dose plane from the three dimensional DICOM dose file and; (c) Generate a synthetic computed tomography (CT) in the exact measurement geometry which is subsequently used for phantom positioning during the QA measurement. To assess the functionality of the phantom system dynamic conformal arc mLSRS plans that were generated by a clinically commissioned multiple metastasis treatment planning system (BrainLab Elements version 2.0) using patient-specific data. A total of seven patient plans were created that contained a total of 31 targets {<Volume>=(0.382±0.534)cc: Range [0.051, 2.05]cc, <Off-Axis Distance>=(30±16)mm: Range[0, 55]mm} 27 of which were directly measured with film and analyzed. Each planned isocenter was mapped to the phantom's center and the dose was recomputed. From the phantom dose distribution dicom points of interest were selected in sets of three and input into the provided software. The software computed the plane that intersects with the entered three points and instructed the user on the setup geometry to place the film in the intersecting plane. The software then generated a synthetic CT scan with embedded fiducial markers rotated into the setup orientation. This CT was then used as the setup reference image in ExacTrac image guidance system (tolerance 0.7mm & 0.5deg). All plans were delivered on a Varian Truebeam linear accelerator with HDMLC, Exactrac and a 6 degree of freedom couch. After delivery of each test plan a 10×10 reference field was delivered to a known dose approximately equal to the maximum dose contained within the plan for film calibration. The test film was scanned simultaneously with the 10×10 reference film and a film that received zero dose using an Epson 10000XL flatbed scanner after waiting 24hours. The test film was scaled according to the reference film and analyzed via the gamma analysis (3%, 1mm, 10%) implemented in Ashland Film QA Pro software. The spherical phantom system was able to perform validation measurements on a variety of patient-specific plan geometries. The average gamma pass-rate γ(3%, 1mm,10%) for all measurements was 96.7% (σ=3.6%). A novel spherical radiotherapy phantom system has been designed and tested on clinically relevant test plans.

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

Purpose and ObjectivesThis 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 MethodsTwenty 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.ResultsFor 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.ConclusionsWith 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.

Residual Setup Errors in Cranial Stereotactic Radiosurgery without 6DoF Robotic Couch

This IRB-approved study was to evaluate the residual interfractional setup errors and intrafractional motion of patients treated with cranial stereotactic radiosurgery without 6 degree of freedom (DoF) couch. We compared a frameless non-invasive vacuum-suction immobilization to a rigid immobilization. Twenty consecutive patients treated by medical linear accelerator or tomotherapy were selected for data collection. The dose and number of fractions received by each patient ranged from 18Gy in 1 fraction (SRS) to 25Gy in 5 fractions (SRT). 12 patients were immobilized using PinPoint, a frameless suction system (Aktina Medical, New York) and 8 patients were immobilized using the Talon rigid screw system. Customized head cushions were used in all patients. 6 out of 12 Atkina patients received pre and post treatment cone beam CT (CBCT) to evaluate the intrafractional motion of the Aktina system. The intrafractional motion with the Talon rigid screw system was previously reported to be negligible thus not repeated in this study. All patients received pre treatment CBCT or megavoltage CT (MVCT) to assess interfractional 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. Residual rotational and translational errors were analyzed retrospectively using image registration software. For frameless Aktina system, mean and standard deviation of the intrafractional motions were -0.5±0.7mm (lateral), 0.1±0.9mm (vertical), -0.5±0.6mm (longitudinal), -0.04±0.18°(pitch), -0.1±0.23°(yaw), and -0.03±0.17°(roll) indicating negligible intrafractional motion. Interfractional rotation errors were 0.20±0.69° (pitch), 0.34±0.56° (yaw), 0.35±0.82° (roll) for frameless vacuum-suction immobilization versus -0.10±0.25° (pitch), -0.08±0.16° (yaw), -0.20±0.41° (roll) for Talon rigid screw immobilization showing rigid immobilization setup was more reproducible than frameless immobilization. Without rotational correction by 6DoF couch, residual setup error exists and increases from the image registration center. In a 3D vector space, a tumor located 5cm 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. With image-guided radiotherapy, translational setup errors can be corrected by image registration between pre-treatment setup CT and planning CT. However, without 6 DoF couch, rotational errors cannot be eliminated. Our study showed that the frameless Aktina immobilization system had negligible intrafractional motion. The interfractional rotation setup error using Aktina was larger than rigid immobilization with the Talon system. For a single lesion away from the center of image registration or for multiple lesions far apart, additional margin may be needed to account for the un-correctable rotational setup errors without 6 DoF couch.

Intra-Fraction Motion Analysis for Frameless Linac-Based Stereotactic Radiosurgery with Three-Dimension (3D) Optical Surface Imaging for Intra-Fractional Motion Management

There has been a rapid increase in the use of frameless SRS in place of the frame-based SRS for treatment of brain metastases (BM). Compared to the frame-based SRS, intra-fraction motion management is a major challenge for the frameless SRS. With advanced immobilization and patient monitoring technologies, intra-fraction motion can be reduced. The purpose of this study is to analyze intra-fraction motion with LINAC-based SRS using a frameless radiosurgery platform that uses an integrated 3D optical surface monitoring system for intra-fraction motion management. This retrospective study includes 97 LINAC-SRS treatments delivered to 60 patients of BM with an integrated 3D surface imaging software for intra-fraction monitoring. Every treatment was delivered with a 6MV FFF beam at a dose-rate of 1400MU/min. Each treatment had at least 3 non-coplanar arcs with a minimum of 3 couch rotations. The patients were immobilized using an open-faced rigid SRS-thermoplastic mask with an opening encompassing the eyes, nose, and part of their forehead. Each patient was first aligned using a high-quality cone-beam CT (CBCT) with 1mm slice thickness and positioned using a 6 degrees of freedom couch. The patient’s face was monitored during treatment delivery through the surface monitoring system. During delivery, beam holds occurred if patient motion exceeded pre-defined tolerance (1mm translation/1° rotation) for > 5 seconds. An additional CBCT was performed following the delivering of two arcs to assess for any intra-fraction motion. Each CBCT was then registered to the planning CT using the automated intensity-based registration algorithm, followed by confirmation with a manual check. The intra-fraction shifts (RL, AP, and SI) and rotation (yaw, roll, and pitch) data were retrospectively recorded and analyzed. The total magnitude of the patient intra-fraction motion for all ninety-seven SRS treatments as recorded through shifts were 0.4 mm (mean) ± 0.3 mm (stv). Translational shifts were 0.0±0.3 mm in RL, 0.0±0.3 mm in AP, 0.0±0.3 mm in SI, while the rotational variations were 0.0±0.4° in yaw, 0.0±0.2° in roll, and 0.0±0.2° in pitch. In total, 93.8%(n=91/97) delivered treatments had intra-fraction motion that was ≤1mm and ≤1 degree. Minimal intra-fraction motion was observed during frameless-based LINAC-SRS treatments when using an integrated 3D surface image tracking; Only 6% percent of treatments experienced motion that was >1mm or >1 degree. A planning target margin of ≤1 mm may be considered adequate during the planning process to account for intra-fraction motion in the majority of all cases.

The use of six degrees of freedom couch is only clinically beneficial in stereotactic radio surgery.

The use of six degrees of freedom couch is only clinically beneficial in stereotactic radio surgery.

3 Single shot stereotactic body radiotherapy for localized prostate cancer: Case report

Introduction We report on the quality assurance (QA) and treatment management of the first patient included in the ONE SHOT trial. ONE SHOT, single shot stereotactic body radiotherapy (SBRT) for localized prostate cancer, is a multicenter single arm phase I/II trial which will enrolled 45 patients in total. Its main objective is to determine the safety and efficacy of a single SBRT fraction at a dose of 19 Gy in patients with localized prostate cancer. Indeed, recent data showed good tolerance and biochemical control after single interstitial application of 19/20 Gy to the prostate or after SBRT in five fractions, so that ultra-hypofractionated single-dose SBRT employing imaged-guided radiotherapy and real-time tumour tracking with implanted electromagnetic transponders (Calypso® with DynamicEdge™ gating) may be feasible with a safe toxicity profile and an optimal long-term tumor control. Methods Three Calypso beacons were implanted in the patient prostate 4 days before the planning CT scan. The patient was imaged with empty rectum and full bladder with 1 mm slices thickness. For urethra contouring and visualization during treatment, a non-radiopaque catheter was inserted during CT simulation and before the treatment delivery. The prescription dose was 19 Gy in a single fraction to the whole prostate gland ±proximal seminal vesicles and 17 Gy to the urethra planning-risk volume (PRV). The plan consisted into 2 full VMAT arcs using 10MV FFF beam from the TrueBeam linac with 6 degrees of freedom couch. It was optimized on Eclipse TPS (Varian) with Rapidplan™, a knowledge-based planning software, and the dose was calculated with AAA v13026 algorithm. QA controls included portal dosimetry (PD) (Varian, PDIP v13026), absolute dose measurements with Diamond detector (PTW) inserted in Lucy 3D QA phantom (Standard Imaging) and Gafchromic film measurements (EBT-XD films). Results The treatment plan of the first ONE SHOT patient satisfied the dose-volume constraints for all OARs and the coverage requirement for the PTV and PRV. The total number of monitor units for the two arcs was 7125. The results of the PD QA were within the tolerance, with Gamma index 1%/1 mm = 98.7%. For absolute dose measurement, the difference between expected and measured dose was 1.7% inside PRV and 3% in the PTV region. Film gamma index with 4%/1 mm criteria was 97.4%. Patient positioning before treatment required two cone-beam CT (CBCT) before the first arc, because Calypso detected a 2 mm shift in the longitudinal direction after the couch adjustment made after the 1st CBCT. The first arc was treated without interruption and a third CBCT was acquired between the arcs. However, no shift was observed and the second arc was treated without patient realignment. Conclusions QA results were all within the tolerance for SBRT treatment and the first ONE SHOT patient was treated successfully as planned.

Impact of rectal distension on prostate CBCT-based positioning assessed with 6 degrees-of-freedom couch

IntroductionProstate requires a daily correction of its 3-dimensional position in relation with rectal distension. In this study, we sought to determine whether rectal distension with respect to the rectal behavior might have an impact on prostate translations and/or rotations during prostate image guided radiation therapy using a 6 degrees-of-freedom (DOF) couch. Methods and materialsWe reviewed the data from 39 patients with localized prostate cancer treated with protracted external radiation therapy using a 6 DOF couch. Before each fraction, a kilovoltage cone beam computed tomography (kV-CBCT) scan was performed. The automatic fusion algorithm was set to fuse on soft tissue and allowed correction for translations in 3 dimensions and rotations in the longitudinal axis (“roll”) and lateral axis (“pitch”). After contouring the rectum on each kV-CBCT, we determined the cross-sectional area (CSA) and relative CSA (CSArel) by dividing with the CSA of planning CT. The standard deviation of CSArel per patient was used to classify the patients in 2 groups: patients with a stable rectum and patients with an unstable rectum. The CSArel was compared between these 2 groups with a linear mixed model with group as fixed effect and patient as random effect. ResultsA total of 616 kV-CBCT were analyzed, and 2 subgroups of patients could be defined a posteriori: 19 patients had a stable rectum, mean CSArel (1.06 ± 0.08); the other 20 patients had an unstable rectum, mean CSArel (1.43 ± 0.08). The average pitch in the group with a stable rectum was 0.36° (±0.21) versus 0.40° (±0.20) (P = .898). The pitch was not correlated with the CSArel (P = -.065, r = 0.119). The average roll in the group with a stable rectum was 0.27° (±0.16) versus 0.05° (±0.16) (P = .137). The roll was not correlated with the CSA (P = .094, r = 0.068). The average CSArel was higher (P = .0013) and more variable (P = .035) in the unstable group. ConclusionRectal distension had no impact on the pitch or on the roll, which suggest that a 6 DOF couch has little interest in daily practice for prostate image guided radiation therapy.

How much margin do we need for pelvic lymph nodes irradiation in the era of IGRT?

Background and purpose: Image guided radiotherapy (IGRT) without 6 degree of freedom couch can only correct the translational setup errors of pelvic radiotherapy. But errors introduced by rotation and deformation of CTV can't be adjusted in most of IGRT systems. This article is to evaluate these errors and to provide recommendations on the margin needed in the era of IGRT.Material and methods: 218 patients who received pelvic radiotherapy in PUMC Hospital from 2012 to 2014 were included. A simulation CT and a CBCT were acquired for every patient. 3D and 6D registrations of CT and CBCT were applied. 9 bony landmarks were marked and distances of each landmark between CT and CBCT were measured in three directions.Results: Without image guidance, movements of landmarks in the directions of LR, AP and SI were 0.4 ± 2.5 mm, 1.3 ± 3.8 mm and 1.5 ± 5.0 mm respectively, with 3D-registration, movements were 0.0 ± 1.5 mm, 0.7± 2.8 mm and 0.6± 3.2 mm, and with 6D-registration, movements were 0.0 ± 0.5 mm, 0.2 ± 1.0 mm and 0.2 ± 1.1 mm in each direction.Conclusions: IGRT could reduce setup errors. IGRT with 6D treatment couches could further reduce setup errors compared to 3D couches. For centers without IGRT, we suggest CTV-PTV margins of 6 mm, 9 mm and 12 mm in LR, AP and SI directions respectively, margins of 3 mm, 6.5 mm and 7 mm for the use of daily IGRT with 3D couch and 2 mm, 3 mm and 3 mm for 6D couch.

A Dosimetric Study of a Novel Rotating Gamma Ray System for Stereotactic Body Radiation Therapy

Stereotactic radiosurgery (SRS) unites with current state of art techniques have provided excellent conformity, precision, and versatility for small tumor lesions. Arguably the best known systems are the Gamma ray systems like GammaKnife and the linac based robotic systems like CyberKnife. However, GammaKnife systems are limited to only treating intracranial targets. In this study, we investigate a novel rotating gamma ray system for SRS of intracranial targets and stereotactic body radiation therapy (SBRT) of extracranial body sites as compared to the CyberKnife machine. The RT Pro planning system (version 1.00.4557, Prowess, Concord, CA) was used to generate treatment plans for a prototype CybeRay system for 12 patients, who were previously treated on the CyberKnife. The new CybeRay system (OUR United RT Group., Xian, China) has a rotation gantry containing 16 Co-60 sources focusing at the isocenter. The prototype implemented in RT Pro included 4 different changeable collimators while an improved version included 7 collimators. The treatment can also swing 35° in the superior direction. CybeRay is equipped with an electronic portal imaging device, a cone-beam CT system, and a 6 degrees of freedom couch. Treatment plans from both treatment systems were compared based on isodose distributions, dose volume histograms, conformity index (CI), and the estimated treatment time. CybeRay and CyberKnife results were compared using Wilcoxon signed-rank tests. The planned target volume (PTV) ranged from 2.7cc to 59.5cc. The estimated treatment delivery time for CybeRay plans ranged from 3.7-60.2 minutes as compared to 13-67minutes with Cyberknife (p=0.05). The mean value of CI was 1.23 for Cyberknife as compared to 1.27 for CybeRay (p=0.2). On the other hand, CybeRay was superior with regards to peripheral doses appearing as excessive isodose lines extending to distal normal tissues. To quantify the difference between the two systems, a ring structure (D5cm) was drawn at 5cm distant to the PTV. The maximum dose received by D5cm averaged over 12 patients was 7Gy for CybeRay and 13.4Gy for CyberKnife (p=0.0024). A newly designed rotating gamma ray machine was evaluated through a retrospective dosimetric study using patients previously treated on CyberKnife system. The new machine exhibited high precision stereotaxy and accurate target localization with promising dosimetric capabilities for stereotactic radiation therapy of extra-cranial lesions.Abstract 3565Vol (cc)Dose Gy/fxtreat timeCID5cmCRCKCRCKCRCKAdrenal2736/343.26421.291.188.718.911.850/360.2531.31.236.224.117.745/356.2521.161.2211.415.1Liver4.460/321.2341.191.194.220.44550/528.1381.091.0912.720.3pancreas7.527/311.6441.211.192.85.549.110/131.6381.381.141.53.110.210/145.5331.581.571.33.6Breast25.114/73.69131.211.211112.259.514/74.94191.211.366.28.8Lung12.648/425501.291.188.615.82.754/317.2671.371.259.213.1Mean29401.271.23713.4 Open table in a new tab

Computed tomography images with different resolutions in Sentinel system: a setup error analysis

Objective To study the impact of setup error caused by computed tomography (CT) images with different resolutions in the Sentinel system on clinical treatment. Methods A phantom was scanned by large-aperture positioning CT with two different resolutions (CT1: 0.5 mm×0.5 mm×1.0 mm, FOV 256 mm, Matrix 512, thickness 1 mm; CT3: 1.0 mm×1.0 mm×3.0 mm, FOV 500 mm, Matrix 512, thickness 3 mm). The CT images were transferred to the planning system. The radiation fields were designed and transferred to MOSAIQ and Sentinel systems. Ten fixed setup errors were applied to a six degree of freedom couch. The Sentinel system was used to position the two groups of CT images and generate the setup errors. The comparison of two datasets was made by paired t-test. Cone-beam CT was used for independent verification. Results The setup errors in x-, y-, and z-directions were significantly smaller on CT1 than on CT3(0.19±0.11 vs. 0.33±0.16 mm, P=0.061; 0.59±0.79 vs. 1.07±1.09 mm, P=0.008; 0.67±0.75 vs. 1.16±1.30 mm, P=0.043). There were no significant differences in rotational errors in x-, y-, or z-directions between the two datasets (P=0.494; P=0.182; P=0.298). Conclusions The Sentinel system has a higher setup accuracy in the 0.5 mm×0.5 mm×1.0 mm mode than in the 1.0 mm×1.0 mm×3.0 mm mode. However, the later mode is still an acceptable choice in clinical treatment. Key words: CT resolution; Setup errors; Sentinel system; Radiotherapy

SU‐F‐J‐37: Commissioning a Six Degree of Freedom Couch

Purpose:The proper operation of a six degree of freedom couch (6DOFC) to position a patient for treatment requires that a number systems on the treatment unit work together correctly. Commissioning a 6DOFC requires not only the commissioning of each separate system but also commissioning the interactions between the systems.Methods:To commission a 6DOFC, we tested the resolvers that readout the linear and angular positions; the orthogonality of the axes of the laser positioning system, the imaging system, the gantry and the couch as well as their relationship to each other. An automated Winston‐Lutz test was used verify the coincidence of the origins of the laser system, imaging system and treatment unit. The IsoCal phantom was used to verify the consistency of axes orientations between systems. The operation of the couch and imaging system was verified under simulated load conditions and an end to end test was performed to verify correct operation of the system.Results:The tests on the couch resolvers, orthogonality and coincidence of the axes of each system were successfully completed and showed excellent agreement. The automated Winston‐Lutz test showed that the maximum displacement of the center of the radiation field with respect to the isocenter was &lt;0.6 mm (radius) at isocenter. The analysis of the IsoCal phantom showed that the maximum variation of the radiation field with respect to known locations in the phantom was ∼0.9 mm for all the BBs in the phantom. Load tests showed maximum errors of 1 mm.Conclusion:We tested all system associated with the 6DOFC and found the uncertainty due to imaging was &lt;.9mm and the uncertainty due to loading ∼1 mm, resulting in a total error of ∼1.4 mm.

EP-1786: Rectal distension impact on prostate CBCT-based positioning assessed with 6 degrees of freedom couch

EP-1786: Rectal distension impact on prostate CBCT-based positioning assessed with 6 degrees of freedom couch

SU‐E‐T‐234: Daily Quality Assurance for a Six Degrees of Freedom Couch Using a Novel Phantom

Purpose:To test the accuracy and reproducibility of both translational and rotational movements for a couch with six degrees of freedom (6DoF) using a novel phantom designMethods:An end‐to‐end test was carried out using two different phantoms. A 6 cm3 cube with a central fiducial BB (WL‐QA Sun Nuclear) and a custom fabricated rectangular prism (31 cm × 8 cm × 8 cm), placed on a baseplate with known angular offsets for pitch, roll and yaw with a central fiducial BB and unique surface structures for registration purposes, were used. The end‐to‐end test included an initial CT simulation for a reference study, setup to an offset mark on each phantom, registration of the reference CT to the acquired cone‐beam CT, and final Winston‐Lutz delivery at four cardinal gantry angles. Results for both translational and rotational movements were recorded and compared for both phantoms.Results:Translational and rotational measurements were performed with a PerfectPitch (Varian) couch for 10 trials for both phantoms. Distinct translational shifts were [−5.372±0.384mm, −10.183±0.137mm, 14.028±0.155mm] for the cube and [7.520±0.159mm, −9.117±0.101mm, 16.273±0.115mm] for the prototype phantom for lateral, longitudinal, and vertical shifts, respectively. Distinct rotational adjustments were [1.121±0.102o, −1.067±0.235o, −2.662±0.380o] for the cube and [2.534±0.059o, 1.994±0.025o, 2.094±0.076o] for the prototype for pitch, roll, and yaw, respectively. Winston‐Lutz test results performed after 6DoF couch correction from each cardinal gantry angle ranged from 0.26–0.72mm for the cube and 0.55–0.86mm for the prototype.Conclusion:The prototype phantom is more precise for both translational and rotational adjustments compared to a commercial phantom. The design of the prototype phantom allows for a more discernible visual confirmation of correct translational and rotational adjustments with the prototype phantom. Winston‐Lutz results are more accurate for the commercial phantom but are still within tolerance for the prototype phantom.

SU‐E‐J‐46: Evaluation of the Accuracy of a Six Degree of Freedom Robotic Couch Using ConeBeam CT Images of the Isocal Phantom

PurposeThe accuracy of Varian PerfectPitch six degree of freedom (DOF) robotic couch was examined using Varian Isocal phantom and cone‐beam CT (CBCT) system.MethodsCBCT images of the Isocal phantom were taken at different pitch and roll angles. The pitch and roll angles were varied from 357 to 3 degrees in one degree increments by input from service console, generating a total of 49 combinations with couch angle (yaw) zero. The center positions of the 16 tungsten carbide BBs contained in the Isocal were determined with in‐house image processing software. Expected BBs positions at different rotation angles were determined mathematically by applying a combined translation/rotation operator to BB positions at zero pitch and roll values. A least square method was used to minimize the difference between the expected BB positions and their measured positions. In this way rotation angles were obtained and compared with input values from the console.ResultsA total of 49 CBCT images with voxel sizes 0.51 mm × 0.51 mm × 1 mm were used in analysis. Among the 49 calculations, the maximum rotation angle differences were 0.1 degree, 0.15 degree, and 0.09 degree, for pitch, roll, and couch rotation, respectively. The mean ± standard‐deviation angle differences were 0.028±0.001 degree, −0.043±0.003 degree, and −0.009±0.001 degree, for pitch, roll, and couch rotation, respectively. The maximum isocenter shifts were 0.3 mm, 0.5 mm, 0.4 mm in x, y, z direction respectively following IEC6127 convention. The mean isocenter shifts were 0.07±0.02 mm, −0.05±0.06 mm, and −0.12±0.02 mm in x, y and z directions.ConclusionThe accuracy of the Varian PerfectPitch six DOF couch was studied with CBCTs of the Isocal phantom. The rotational errors were less than 0.15 degree and isocenter shifts were less than 0.5 mm in any direction. This accuracy is sufficient for stereotactic radiotherapy clinical applications.

SU‐E‐J‐28: Assessment of Rotational Misalignment in Cervical Spine Treatments and Potential for 6DOF Couch Corrections

Purpose:To quantify the magnitude of angular misalignments that may arise during spinal radiosurgery for metastases to cervical vertebrae and assess the ability of six‐degree of freedom (6DOF) alignment systems to apply appropriate corrections.Methods:A retrospective analysis of the distributions of rotational misalignments of cervical vertebrae based on planning CT and CBCT acquired for verification of patient positioning was performed. Fifteen patients being treated for cancers of the head and neck were immobilized using Type‐S head‐only thermoplastic masks mounted to an overlay board. Rigid registration was performed using commercially available software for individual vertebrae of each patient. Distributions of rotations were tabulated across all patients and vertebrae, modeled with a normal distribution, and 95% confidence intervals were estimated. Dosimetric consequences of maximal and 6DOF correctable misalignments are simulated.Results:Across all patients and vertebrae, 2/105 pitch rotations were &gt;3° (1.9%), including 2/15 (13%) patients. Upper and lower 95% confidence intervals for pitch, yaw, and roll misalignments were [−2.3°, 3.3°], [−1.8°, 1.4°], and [−2.6°, 2.2°], respectively. Rotations for each vertebra were also considered. The greatest need for rotation appears to be for pitch correction of vertebra C4, with misalignment of 0.2° ± 1.9° observed, which predicts that a rotation &gt;3° would be necessary 11.5% of the time that this vertebra is of interest.Conclusion:Empirical and model data presented here suggest that approximately 2–4% of vertebrae would require a rotation &gt;3° in the sagittal plane, with some vertebrae more prone to rotational misalignments than others. Uncorrected rotational misalignment has the potential to alter dose distributions with respect to both treatment volumes and organs at risk and rotations &gt;3° may require physical repositioning of the patient. Implementation of six‐degree of freedom couch systems may facilitate clinical flow after rotational misalignment is detected, and allow expeditious, automated setup corrections.Varian and Dartmouth‐Hitchcock Medical Center are parties to a customer acceptance evaluation agreement regarding the Varian 6 degree of freedom couch.

SU‐E‐J‐156: Preclinical Inverstigation of Dynamic Tumor Tracking Using Vero SBRT Linear Accelerator: Motion Phantom Dosimetry Study

Purpose:Following the ‘end‐to‐end testing’ paradigm of Dynamic Target Tracking option in our Image‐Guided dedicated SBRT VeroTM linac, we verify the capability of the system to deliver planned dose to moving targets in the heterogeneous thorax phantom (CIRSTM). The system includes gimbaled C‐band linac head, robotic 6 degree of freedom couch and a tumor tracking method based on predictive modeling of target position using fluoroscopically tracked implanted markers and optically tracked infrared reflecting external markers.Methods:4DCT scan of the motion phantom with the VisicoilTM implanted marker in the close vicinity of the target was acquired, the ‘exhale’=most prevalent phase was used for planning (iPlan by BrainLabTM). Typical 3D conformal SBRT treatment plans aimed to deliver 6‐8Gy/fx to two types of targets: a)solid water‐equivalent target 3cm in diameter; b)single VisicoilTM marker inserted within lung equivalent material. The planning GTV/CTV‐to‐PTV margins were 2mm, the block margins were 3 mm. The dose calculated by MonteCarlo algorithm with 1% variance using option Dose‐to‐water was compared to the ion chamber (CC01 by IBA Dosimetry) measurements in case (a) and GafchromicTM EBT3 film measurements in case (b). During delivery, the target 6 motion patterns available as a standard on CIRSTM motion phantom were investigated: in case (a), the target was moving along the designated sine or cosine4 3D trajectory; in case (b), the inserted marker was moving sinusoidally in 1D.Results:The ion chamber measurements have shown the agreement with the planned dose within 1% under all the studied motion conditions. The film measurements show 98.1% agreement with the planar calculated dose (gamma criteria: 3%/3mm).Conclusion:We successfully verified the capability of the SBRT VeroTM linac to perform real‐time tumor tracking and accurate dose delivery to the target, based on predictive modeling of the correlation between implanted marker motion and external surrogate of breathing motion.

Assessment of patient setup errors in IGRT in combination with a six degrees of freedom couch

PurposeThe range of patient setup errors in six dimensions detected in clinical routine for cranial as well as for extracranial treatments, were analyzed while performing linear accelerator based stereotactic treatments with frameless patient setup systems. Additionally, the need for re-verification of the patient setup for situations where couch rotations are involved was analyzed for patients treated in the cranial region. Methods and MaterialsA total of 2185 initial (i.e. after pre-positioning the patient with the infrared system but before image guidance) patient setup errors (1705 in the cranial and 480 in the extracranial region) obtained by using ExacTrac (BrainLAB AG, Feldkirchen, Germany) were analyzed. Additionally, the patient setup errors as a function of the couch rotation angle were obtained by analyzing 242 setup errors in the cranial region. Before the couch was rotated, the patient setup error was corrected at couch rotation angle 0° with the aid of image guidance and the six degrees of freedom (6DoF) couch. For both situations attainment rates for two different tolerances (tolerance A: ±0.5mm, ±0.5°; tolerance B: ±1.0mm, ±1.0°) were calculated. ResultsThe mean (± one standard deviation) initial patient setup errors for the cranial cases were -0.24±1.21°, -0.23±0.91° and -0.03±1.07° for the pitch, roll and couch rotation axes and 0.10±1.17mm, 0.10±1.62mm and 0.11±1.29mm for the lateral, longitudinal and vertical axes, respectively. Attainment rate (all six axes simultaneously) for tolerance A was 0.6% and 13.1% for tolerance B, respectively. For the extracranial cases the corresponding values were -0.21±0.95°, -0.05±1.08° and -0.14±1.02° for the pitch, roll and couch rotation axes and 0.15±1.77mm, 0.62±1.94mm and -0.40±2.15mm for the lateral, longitudinal and vertical axes. Attainment rate (all six axes simultaneously) for tolerance A was 0.0% and 3.1% for tolerance B, respectively. After initial setup correction and rotation of the couch to treatment position a re-correction has to be performed in 77.4% of all cases to fulfill tolerance A and in 15.6% of all cases to fulfill tolerance B. ConclusionThe analysis of the data shows that all six axes of a 6DoF couch are used extensively for patient setup in clinical routine. In order to fulfill high patient setup accuracies (e.g. for stereotactic treatments), a 6DoF couch is recommended. Moreover, re-verification of the patient setup after rotating the couch is required in clinical routine.