Linac Couch Research Articles

Dosimetric Evaluation of Physical Radiation Delivery Limits of a Weak Magnetic Field Generator on GBM Dose Distributions

<h3>Purpose/Objective(s)</h3> To demonstrate the feasibility of meeting dose prescription criteria when using a weak magnetic field generator to support glioblastoma multiforme (GBM) radiation therapy. <h3>Materials/Methods</h3> Weak magnetic fields extend free radical life, and recent experimental results have shown that adding weak magnetic fields (20G-40G) to radiation therapy delivery enhances free radical production, increases tumor cell death, and slows tumor growth. Based on these results, we have built the first clinical unit, consisting of two 63 cm ID Helmholtz coils, spaced 35 cm apart. The coil system is placed such that the patient and Linac couch are within the coils, and the Linac isocenter is coincident with the center of the coil system. We propose to use GBM for our first clinical trial. The limitation imposed by the coil system is a fixed Linac couch angle, constraining the gantry rotation to slightly more than 180°, from lateral to lateral. The impact on treatment plan quality of the corresponding reduction in available beam angles was unknown, so we conducted a treatment planning study to determine if we could provide clinically equivalent treatment plans with the coil in place. We evaluated the treatment plans for 10 GBM patients that had been treated with arc therapy. The prescription was 60 Gy (2 Gy x 30 fractions) for 6 patients, 55.8 Gy (1.8 Gy x 31 fractions) for 2 patients, 54 Gy (1.8 Gy x 30 fractions) for 1 patient, and 40.05 Gy (2.67 Gy x 15 fractions) for 1 patient. The dose was delivered to 95% of the planning target volume (PTV). The PTV margin was 1.3-2 cm surrounding the gross tumor volume. Critical structures, such as the brainstem, optic nerves, and optic chiasm, were contoured and assigned constraints of 62 Gy, 54 Gy, and 54 Gy, respectively. We repeated the treatment planning session using only coplanar arcs that spanned from 90° to 270° (half-arcs). <h3>Results</h3> All the treatment plans generated with the half-arcs met the clinical criteria. The minimum PTV doses for the half arc and clinical plans were 50.0 ± 6.1 Gy and 51.4 ± 7.1 Gy, and the maximum PTV doses were 61.9 ± 7.1 Gy and 62.7 ± 8.6 Gy, respectively. Table 1 summarizes the results of four reported critical structures. All the critical structures met the criteria, and the half-arc plans were neither better nor worse than the clinical plans. <h3>Conclusion</h3> The physical limits imposed by the magnetic field generator on the Linac beam angles do not degrade treatment planning for GBM radiation therapy. The prescribed dose can be adequately delivered while avoiding critical structures. With this groundwork, we can develop clinical trials to evaluate the impact of utilizing weak magnetic fields in radiation therapy.

Dosimetric effect of modelling non-homogeneous LINAC couch using cone-beam computed tomography on quality assurance (QA) results

Abstract Aim: To evaluate the dosimetric effect of modelling a non-homogeneous couch on patients’ quality assurance (QA) gamma pass rates for intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) techniques. Materials and Methods: A non-homogeneous treatment couch (TxT 550 TTM, CIVCO, USA) was imaged using the LINAC mounted cone-beam computer tomography (CBCT) system. Modelling this couch in different situations, including incomplete (homogeneous model), correct model and not defined situations in the treatment planning system (TPS), was performed based on the geometrical and material densities data extracted from the CBCT images. Calculated gamma pass rates between TPS dose calculations and the measurements in a phantom for different couch models were obtained and compared at two gamma criteria (2%-2 mm and 3%-3 mm). Results: Comparing TPS calculations for the correct modelled couch and the measurements showed high gamma pass rates for both the IMRT and VMAT techniques (96·5 ± 0·9%, 99·2 ± 0·5% for IMRT in 2%-2 mm and 3%-3 mm criteria; 97·5 ± 0·8%, 99·4 ± 0·5% for VMAT). The overall gamma pass rate of the IMRT plan QAs was reduced by about 2% and 3% on average for incomplete and no couch modelling, respectively. These reductions for VMAT techniques were 2·5% and 4·3%, respectively. Conclusions: Non-homogeneous couches have different parts with different attenuations, which can be correctly defined using LINAC CBCT. Modelling of treatment couch has a significant effect on patient QA results for VMAT and IMRT plans, especially in radiation fields/subfield transmitting from the couch. We suggest using LINAC CBCTs as an appropriate device for couch modelling in modulated radiotherapy techniques.

Validation of a commercial deformable image registration for surface-guided radiotherapy using an ad hoc-developed deformable phantom.

The use of optical surface systems (OSSs) for patient setup verification in external radiation therapy is increasing. To manage potential deformations in a patient's anatomy, a novel deformable image registration (DIR) tool has been applied in a commercial OSS. In this study we investigate the accuracy of the DIR as compared to rigid image registration (RR). The positioning accuracy of the DIR and RR implemented in the OSS was investigated using an ad hoc-developed anthropomorphic deformable phantom, named Mary. The phantom consists of 33 slices of expanded polystyrene slabs shaped thus to simulate part of a female body. Anatomical details, simulating the ribs and spinal cord, together with 10 inner targets at different depths are included in thorax and abdominal parts. Mary is capable of realistic body movements and deformations, such as head and arm rotations, body torsion and moderate breast/abdomen swelling. The accuracy of DIR and RR was investigated for four internal targets after deliberately deforming the phantom nine times. Breast and abdomen enlargements and torsions around x, y, and z axes were applied. For reference purposes, rigid displacements (where Mary's anatomy was kept intact) were included. The phantom was positioned on the linac couch under the OSS guidance and for each target and displacement a CBCT was acquired. The accuracy of DIR and RR was assessed evaluating the difference in means of absolute values between CBCT and the OSS registration parameters (lateral, longitudinal, vertical, rot, pitch, and roll), using both a reference surface extracted from CT (CTr) or acquired with the OSS (OSSr). A comparison of the four different combinations, DIR+OSSr, DIR+CTr, RR+OSSr, and RR+CTr, was carried out to evaluate the position accuracy for the various combinations. Finally, the positioning accuracy of the different target positions using only OSSr was investigated for the DIR. A paired sample Wilcoxon signed-rank test (P<0.05) and a two-tailed Mann-Whitney test (P<0.05) were carried out. The DIR in combination with OSSr showed significantly (P<0.05) improved positioning accuracy in the lateral and longitudinal directions and in pitch, compared to RR, when deformations were applied to Mary. The positioning accuracy improved from 1.9±1.5mm, 1.1±0.8mm to 1.1±1.2mm, 0.6±0.5mm in lateral and longitudinal directions, respectively, and from 0.8±0.6° to 0.4±0.4° in pitch, using DIR compared to RR. Both the DIR and RR showed a similar positioning accuracy when rigid displacements of Mary were applied. For DIR, the OSSr generally showed improved calculation accuracy compared to CTr. Independent of the reference image used, the target position influenced the registration accuracy, and hence, one target could not be evaluated using RR due to its inability to calculate the correct position. Improved positioning accuracy was observed for DIR with respect to RR when deformations of Mary's anatomy were applied. For both DIR and RR, improved positioning accuracy was observed using OSSr as compared to CTr. The position of the target inside the phantom influenced the positioning accuracy for DIR.

3D Surface Imaging as a Viable Replacement for Permanent Tattoos for Patients Undergoing Breast EBRT - A Feasibility Study on a Phantom

Cutaneous tattoos have historically been required for the setup and alignment of patients undergoing breast RT. This point-based approach yields low resolution of the underlying surface anatomy and leaves patients with permanent markings that can be psychologically distressing. Moreover, the validity of tattoo-based setup with DRR matching to internal bony landmarks can be challenging when used to align an external mobile target. The issue is particularly pronounced for patients with larger body habitus or pendulous breasts. The emerging 3D surface imaging can theoretically address the technical shortcomings of tattoos and simultaneously provide for an improved patient experience since no permanent markings are left behind. A feasibility study was therefore designed to investigate whether 3D surface imaging can replace permanent tattoos for breast RT. A phantom was immobilized on a Linac couch and five tattoos were made on the right breast per our institutional simulation protocol. These were aligned with in-room lasers. Couch coordinates at this position were recorded as the ground truth for the purpose of comparison. A reference surface image was then captured by a 3D surface imaging system. Five ROIs were designed for image registration and phantom shift calculation. ROI1 = RT breast; ROI2 = RT breast + 1/3 LT breast; ROI3 = RT breast + 1/3 LT breast + RT lateral side; ROI4 = RT breast + 1/3 LT breast + RT lateral side + RT superior side (up to inferior border of supraclavicular fossa) and ROI5 = RT breast + 1/3 LT breast + RT lateral side + RT superior side + RT inferior side. Except for ROI1, the others were asymmetric about the beam central axis. For each ROI, the phantom was repeatedly aligned to its pre-defined treatment position from a lowest random couch position with lasers off. A good alignment was defined as translations≤0.3 mm and rotations≤0.3°, as measured by the 3D surface imaging system. The elapsed time from start to an acceptable alignment and the final couch coordinates were recorded. Three measurements were made for each ROI. The same measurements were also performed for the tattoo cohort with lasers only. The mean setup times for each approach were: 86.3s (tattoos), 102.7s (ROI1), 86.7s (ROI2), 57.3s (ROI3), 55.7s (ROI4), and 48.7s (ROI5). For the 3D surface imaging cohort, the setup time was inversely proportional to the ROI size. Fitting a linear regression model yielded a linear coefficient of R2 = 0.892. The larger the ROI, the more robust and faster the setup became. The more asymmetric the ROI, the more effective and sensitive the ROI was in detecting rotational shifts. Post-setup residual errors as measured by couch coordinates were comparable in magnitude for each. With a properly-designed asymmetric ROI, 3D surface imaging may be able to replace tattoos as a viable and effective alternative for breast RT patient setup. The accuracy of this novel approach can be further enhanced by optimizing the topographical height of the skin surface.

A collision prediction framework for noncoplanar radiotherapy planning and delivery.

PurposeNoncoplanar radiotherapy can provide significant dosimetric benefits. However, clinical implementation of such techniques is not fully realized, partially due to the absence of a collision prediction tool integrated into the clinical workflow. In this work, the feasibility of developing a collision prediction system (CPS) suitable for integration into clinical practice has been investigated.MethodsThe CPS is based on a geometric model of the Linear Accelerator (Linac), and patient morphology acquired at the simulator using a combination of the planning CT scan and 3‐D vision camera (Microsoft, Kinect) data. Physical dimensions of Linac components were taken to construct a geometric model. The Linac components include the treatment couch, gantry, and imaging devices. The treatment couch coordinates were determined based on a correspondence among the CT couch top, Linac couch, and the treatment isocenter location. A collision is predicted based on dot products between vectors denoting points in Linac components and patient morphology. Collision test cases were simulated with the CPS and experimentally verified using ArcCheck and Rando phantoms to simulate a patient.ResultsFor 111 collision test cases, the sensitivity and specificity of the CPS model were calculated to be 0.95 and 1.00, respectively. The CPS predicted collision states that left conservative margins, as designed, relative to actual collision locations. The average difference between the predicted and measured collision states was 2.3 cm for lateral couch movements. The predicted couch rotational position for a collision between the gantry and a patient analog differed from actual values on average by 3.8°. The magnitude of these differences is sufficient to account for interfractional patient positioning variations during treatment.ConclusionThe feasibility of developing a CPS using geometric models and standard vector algebra has been investigated. This study outlines a framework for potential clinical implementation of a CPS for noncoplanar radiotherapy.

Early Experience with Mobile CT Used for External Beam Radiation Therapy Simulation and in-Room Image Guidance

To demonstrate feasibility of using mobile CT for CT-simulation and in-room image guidance for external beam radiation therapy (EBRT). An electron density (ED) phantom was scanned using mobile CT to create a ED-HU table for TPS commissioning. A custom lock-bar was fabricated to allow for attachment of radiation therapy immobilization devices to the mobile CT table. To address larger table sag of the mobile CT compared to that of conventional CT simulators, a bubble level was attached to the inferior end of the table so that an operator could correct the table rotation (pitch and roll) after a patient is loaded. Due to the lack of external moving lasers, a traditional three points-setup was utilized for CT simulation. To test feasibility of in-room image guidance using mobile CT, the mobile CT was detached from its table and moved into a linac treatment room where it was positioned beside the linac couch. With the column rotated 90 degrees, the mobile CT was used to acquire a set of volumetric images (CTmobile). Thereafter, the couch was rotated back to the default position and a reference CBCT image was acquired (CBCTref). The first rigid body image registration was performed to register CTmobile to CBCTref, thus placing the CTmobile into the treatment room coordinates (CTmobile-reg). Benefiting from high soft tissue contrast of CTmobile-reg, the second image registration was performed between the planning CT and CTmobile-reg to compute the couch shift values. In-house sequential registration software was developed to perform three-image registration as described above and to calculate couch shifts. The ED-HU table generated by the mobile CT was very similar to those generated by the conventional CT simulators in our clinic. CT simulations using mobile CT were successfully performed for 83 patients with various treatment sites such as brain (13 patients), head and neck (7), thorax (11), spine (21), abdomen (3) and pelvis (28), where a moving laser system and 4D imaging were not required. A phantom test of in-room image guidance using the mobile CT demonstrated that the proposed three-image registration-based workflow is feasible and results in improved soft tissue contrast when compared to CBCT. Our in-house registration software can calculate the couch shift values accurately (< 1 mm and < 0.3° difference compared to commercial localization software). With minor customization, mobile CT can successfully be used for CT simulation in external beam radiation therapy. In-room image guidance using mobile CT allows for improvement of soft tissue contrast and is feasible if combined with a daily CBCT imaging used for the intermediate registration. In future, we plan to develop an optical tracking system to geometrically correlate the mobile CT with the treatment coordinates which will remove the need of the intermediate CBCT scan.

Patient specific methods for room-mounted x-ray imagers for monoscopic/stereoscopic prostate motion monitoring.

PurposeTo investigate the improvement of combined monoscopic/stereoscopic prostate motion monitoring with room‐mounted dual x‐ray systems by adopting patient specific methods.MethodsThe linac couch was used as a motion stage to simulate 40 highly dynamic real patient prostate trajectories. For each trajectory, 40 s pretreatment and 120 s treatment periods were extracted to represent a typical treatment fraction. Motion was monitored via continuous stereoscopic x‐ray imaging of a single gold fiducial and images were retrospectively divided into periods of stereoscopic and monoscopic imaging to simulate periodic blocking of the room‐mounted system by the gantry during arc‐based therapy. The accuracy of the combined motion monitoring was assessed by comparison with the linac couch log files. To estimate 3‐D marker position during monoscopic imaging, the use of population statistics was compared to both maximum likelihood estimation and stereoscopic localization based estimation of individualized prostate probability density functions (PDFs) from the pretreatment period. The inclusion of intrafraction updating was compared to pretreatment initialization alone.ResultsCombined mono/stereoscopic localization was successfully implemented. During the transitions from stereoscopic to monoscopic imaging, fiducial localization exhibits sharp discontinuities when population PDFs were employed. Patient specific PDFs successfully reduced the localization error when estimated from stereoscopic localizations, whereas maximum likelihood estimation (MLE) was too unstable in the room‐mounted geometry. Intrafraction stereoscopic updating provided further increases in accuracy. Residual error tended to decrease throughout the treatment fraction, as the patient‐specific PDFs became more refined.ConclusionsThis is the first demonstration of toggled monoscopic/stereoscopic localization using room‐mounted dual x‐ray imagers, enabling continuous intrafraction motion monitoring for these systems. We showed that both pretreatment individualization and intrafraction updating should be used to provide the most accurate motion monitoring.

O15. Modelling of the Siemens Linac couch in the Monaco planning system

Introduction Carbon fibre material is used for the Linac couch since it is light, strong and rigid. It is presumed to be radio-transparent for mega-voltage photons. Literature has indicated that the attenuation properties of carbon fibre couches may not be negligible, especially with intensity modulated radiotherapy (IMRT) treatments where more beams intersect the couch. According to Spezi et al. (Radiother Oncol, 89:114-122, 2008), inclusion of the Siemens couch in head and neck IMRT can lead to a target dose reduction of 2% and organ at risk (OAR) dose increase of 5% and might be more pronounced if heavily weighted posterior beams are used. This study investigates the influence of the couch on treatment plans in the Monaco planning system. Materials and Methods The couch was computed tomography (CT) scanned and imported into the Monaco planning system in the treatment couch library. Appropriate densities were assigned to the delineated contours of the couch (carbon fibre and inner foam) and were validated by absolute dosimetric measurements. Treatment plans calculated with and without the couch model were compared. Results Point dose measurements were done in a circular phantom, 10 × 10 cm2 field size and 6 MV beam. Measurements indicated attenuation of the beam of up to 4.03 ± 0.28% due to the influence of the couch. After appropriate densities of the couch were assigned, measured, compared to planned values, where within a range of 0.15 ± 0.11% to 0.97 ± 0.20%, excluding two angles which had partial interception of the central axis with the couch edge. Conclusion This study showed that the presence of the couch does affect dosimetry. Protocols will be adjusted to include the couch in patient treatment planning, if this study’s outcome is comparable to literature.

SU‐F‐J‐36: Comparison of Ball Bearing and Iso‐Cube Phantoms for KV‐MV Iso‐Center Coincidence Check

Purpose:To compare two different quality assurance tools for kV‐MV isocenter coincidence check.Methods:Ball‐Bearing device (BBD) (Elekta) provided along with a CBCT equipped LINAC and isocenter cube phantom (CP) (a commercial product of Modus Medical Devices) are utilized to check the coincidence between the MV and kV beam isocenters. The microstepping meter of the BBD allows precision adjustment to better than 0.01mm in three directions. The BBD is aligned to the MV‐isocenter with the lasers and followed by taking MV‐images of at two collimator angles at each cardinal gantry, then its position adjusted. This process takes in iterative steps until 0.25mm tolerance is achieved. Four planar kV‐images are taken at cardinal angles and deviations of BBD position from kVisocenter are evaluated. CP is positioned on the couch with the lasers at known offset from the isocenter. CBCT is utilized to set the cube to its kV isocenter with the precision of linac's couch (1mm). MV‐images are taken at various gantry, collimator, and couch angles and evaluated with the CP manufacturer provided QA software to determine coincidence of the 6mm steel ball at cube center with MV‐isocenter. Three sets of measurements, one after another, were performed on the same day for each phantom on the same linac.Results:Assuming little variation between kV and MV isocenters in a short time, BBD measurements are more reproducible than CP. With comparable conditions both methods agree within 0.5 mm in each direction, while for BBD average standard deviation was 0.07mm and for iso‐cube 0.3mm.Conclusion:Since BBD is more reliable and its results are more reproducible, it should be used during the monthly QA. Since CP is a more efficient device, it should be used for daily QA. A comparison study between the two devices should be periodically performed.

A.33 - Linac couch top attenuation in rotational radiotherapy treatments: Clinical consequences

A.33 - Linac couch top attenuation in rotational radiotherapy treatments: Clinical consequences

Plan robustness in field junction region from arcs with different patient orientation in total marrow irradiation with VMAT

BackgroundThe cranial-caudal (CC) maximum LINAC couch shift is usually around 130–150 cm; therefore total marrow (and lymph-nodes) irradiation (TMI-TMLI) requires two different plans, one with patient head-first-supine (“body plan” – skull-thighs) and a second feet-first-supine (“legs plan” – feet-femurs head). A challenging planning is required to manage the region in which the radiation come from the two plans (that cannot be automatically optimized together). We studied a robust way to produce a plan sum in this field junction region without creating under/over dosage on PTV and hotspots out. Material and methodsTwenty-one patients candidates to bone marrow transplantation were treated with TMI-TMLI on TrueBeam using RapidArc technique. All body bones were defined as PTV and, for TMLI, lymph-nodes and spleen were included, too. The two plans according to ALARA principle were optimized. In particular, in the overlapping region (PTVJ), two specular sigmoid dosimetric shapes were adopted for obtaining homogeneous integral dose. Furthermore, 144 plans from four patients were calculated to evaluate plan robustness. ResultsIn all patients, 95% of the prescription dose covered >99% of PTVJ. Regarding the robustness study, differences <1% were found for mean doses to PTVJ and surrounding healthy tissue (HT) for 5 mm shifts. Maximum dose increased up to 21.4% for 10 mm shifts in CC. PTVJ V95% decreased of around 9% (range −3.4% to 24.3%) revealing possible target under-dosage. ConclusionsDosimetric field junction from two different plans is possible for TMI-TMLI with optimal target coverage. The correct repositioning between the two plans is fundamental, in particular in CC direction.

SU‐E‐J‐48: Imaging Origin‐Radiation Isocenter Coincidence for Linac‐Based SRS with Novalis Tx

PurposeTo implement and evaluate an image‐based Winston‐Lutz (WL) test to measure the displacement between ExacTrac imaging origin and radiation isocenter on a Novalis Tx system using RIT V6.2 software analysis tools. Displacement between imaging and radiation isocenters was tracked over time. The method was applied for cone‐based and MLC‐based WL tests.MethodsThe Brainlab Winston‐Lutz phantom was aligned to room lasers. The ExacTrac imaging system was then used to detect the Winston‐ Lutz phantom and obtain the displacement between the center of the phantom and the imaging origin. EPID images of the phantom were obtained at various gantry and couch angles and analyzed with RIT calculating the phantom center to radiation isocenter displacement. The RIT and Exactrac displacements were combined to calculate the displacement between imaging origin and radiation isocenter. Results were tracked over time.ResultsMean displacements between ExacTrac origin and radiation isocenter were: VRT: −0.1mm ± 0.3mm, LNG: 0.5mm ± 0.2mm, LAT: 0.2mm ± 0.2mm (vector magnitude of 0.7 ± 0.2mm). Radiation isocenter was characterized by the mean of the standard deviations of the WL phantom displacements: σVRT: 0.2mm, σLNG: 0.4mm, σLAT: 0.6mm. The linac couch base was serviced to reduce couch walkout. This reduced σLAT to 0.2mm. These measurements established a new baseline of radiation isocenter‐imaging origin coincidence.ConclusionThe image‐based WL test has ensured submillimeter localization accuracy using the ExacTrac imaging system. Standard deviations of ExacTrac‐radiation isocenter displacements indicate that average agreement within 0.3mm is possible in each axis. This WL test is a departure from the tradiational WL in that imaging origin/radiation isocenter agreement is the end goal not lasers/radiation isocenter.

SU-F-BRB-16: A Spreadsheet Based Automatic Trajectory GEnerator (SAGE): An Open Source Tool for Automatic Creation of TrueBeam Developer Mode Robotic Trajectories

Purpose: To automate creation and delivery of robotic linac trajectories with TrueBeam Developer Mode, an open source spreadsheet-based trajectory generation tool has been developed, tested and made freely available. The computing power inherent in a spreadsheet environment plus additional functions programmed into the tool insulate users from the underlying schema tedium and allow easy calculation, parameterization, graphical visualization, validation and finally automatic generation of Developer Mode XML scripts which are directly loadable on a TrueBeam linac. Methods: The robotic control system platform that allows total coordination of potentially all linac moving axes with beam (continuous, step-and-shoot, or combination thereof) becomes available in TrueBeam Developer Mode. Many complex trajectories are either geometric or can be described in analytical form, making the computational power, graphing and programmability available in a spreadsheet environment an easy and ideal vehicle for automatic trajectory generation. The spreadsheet environment allows also for parameterization of trajectories thus enabling the creation of entire families of trajectories using only a few variables. Standard spreadsheet functionality has been extended for powerful movie-like dynamic graphic visualization of the gantry, table, MLC, room, lasers, 3D observer placement and beam centerline all as a function of MU or time, for analysis of the motions before requiring actual linac time. Results: We used the tool to generate and deliver extended SAD “virtual isocenter” trajectories of various shapes such as parameterized circles and ellipses. We also demonstrated use of the tool in generating linac couch motions that simulate respiratory motion using analytical parameterized functions. Conclusion: The SAGE tool is a valuable resource to experiment with families of complex geometric trajectories for a TrueBeam Linac. It makes Developer Mode more accessible as a vehicle to quickly translate research ideas into machine readable scripts without programming knowledge. As an open source initiative, it also enables researcher collaboration on future developments. I am a full time employee at Varian Medical Systems, Palo Alto, California

SU-E-T-197: Helical Cranial-Spinal Treatments with a Linear Accelerator

Purpose: Craniospinal irradiation (CSI) of systemic disease requires a high level of beam intensity modulation to reduce dose to bone marrow and other critical structures. Current helical delivery machines can take 30 minutes or more of beam-on time to complete these treatments. This pilot study aims to test the feasibility of performing helical treatments with a conventional linear accelerator using longitudinal couch travel during multiple gantry revolutions. Methods: The VMAT optimization package of the Eclipse 10.0 treatment planning system was used to optimize pseudo-helical CSI plans of 5 clinical patient scans. Each gantry revolution was divided into three 120° arcs with each isocenter shifted longitudinally. Treatments requiring more than the maximum 10 arcs used multiple plans with each plan after the first being optimized including the dose of the others (Figure 1). The beam pitch was varied between 0.2 and 0.9 (couch speed 5- 20cm/revolution and field width of 22cm) and dose-volume histograms of critical organs were compared to tomotherapy plans. Results: Viable pseudo-helical plans were achieved using Eclipse. Decreasing the pitch from 0.9 to 0.2 lowered the maximum lens dose by 40%, the mean bone marrow dose by 2.1% and the maximum esophagus dose by 17.5%. (Figure 2). Linac-basedmore » helical plans showed dose results comparable to tomotherapy delivery for both target coverage and critical organ sparing, with the D50 of bone marrow and esophagus respectively 12% and 31% lower in the helical linear accelerator plan (Figure 3). Total mean beam-on time for the linear accelerator plan was 8.3 minutes, 54% faster than the tomotherapy average for the same plans. Conclusions: This pilot study has demonstrated the feasibility of planning pseudo-helical treatments for CSI targets using a conventional linac and dynamic couch movement, and supports the ongoing development of true helical optimization and delivery.« less

4-Pi Non-Coplanar Converging Beams Yield Superior Therapeutic Ratio Than Conventional Coplanar Beams

The 4-Pi non-coplanar converging beam delivery system tends to reduce OAR doses and improve the PTV dose conformality. Given the collision-free feature of the proposed system, it can be further developed to a hypefractionation treatment modality with fast delivery, precise targeting as well as reduced radiation toxicity. • It has been derived and validated in our previous study that the orientation of a 4-Pi beam can be realized by a combination of LINAC gantry rotation and couch rotation in Pinnacle TPS [1].

Feasibility of an online adaptive replanning method for cranial frameless intensity-modulated radiosurgery

To introduce an approach for online adaptive replanning (i.e., dose-guided radiosurgery) in frameless stereotactic radiosurgery, when a 6-dimensional (6D) robotic couch is not available in the linear accelerator (linac). Cranial radiosurgical treatments are planned in our department using intensity-modulated technique. Patients are immobilized using thermoplastic mask. A cone-beam computed tomography (CBCT) scan is acquired after the initial laser-based patient setup (CBCTsetup). The online adaptive replanning procedure we propose consists of a 6D registration-based mapping of the reference plan onto actual CBCTsetup, followed by a reoptimization of the beam fluences (“6D plan”) to achieve similar dosage as originally was intended, while the patient is lying in the linac couch and the original beam arrangement is kept. The goodness of the online adaptive method proposed was retrospectively analyzed for 16 patients with 35 targets treated with CBCT-based frameless intensity modulated technique. Simulation of reference plan onto actual CBCTsetup, according to the 4 degrees of freedom, supported by linac couch was also generated for each case (4D plan). Target coverage (D99%) and conformity index values of 6D and 4D plans were compared with the corresponding values of the reference plans. Although the 4D-based approach does not always assure the target coverage (D99% between 72% and 103%), the proposed online adaptive method gave a perfect coverage in all cases analyzed as well as a similar conformity index value as was planned. Dose-guided radiosurgery approach is effective to assure the dose coverage and conformity of an intracranial target volume, avoiding resetting the patient inside the mask in a “trial and error” way so as to remove the pitch and roll errors when a robotic table is not available.

WE-G-BRA-06: Calibrating an Ionisation Chamber: Gaining Experience Using a Dosimetry 'flight Simulator'.

Recently there has been great interest in the use of simulation training, with the view to enhance safety within radiotherapy practice. We have developed a Virtual Environment for Radiotherapy Training (VERT) which facilitates this, including the simulation of a number of 'Physics practices'. One such process is the calibration of an ionisation chamber for use in Linac photon beams. The VERT system was used to provide a life sized 3D virtual environment within which we were able to simulate the calibration of a departmental chamber for 6MV and 15 MV beams following the UK 1990 Code of Practice. The characteristics of the beams are fixed parameters in the simulation, whereas default (Absorbed dose to water) correction factors of the chambers are configurable thereby dictating their response in the virtual x-ray beam. When the simulation is started, a random, realistic temperature and pressure is assigned to the bunker. Measurement and chamber positional errors are assigned to the chambers. A virtual water phantom was placed on the Linac couch and irradiated through the side using a 10 × 10 field. With a chamber at the appropriate depths and irradiated iso-centrically, the Quality Indices (QI) of the beams were obtained. The two chambers were 'inter-compared', allowing the departmental chamber calibration factor to be calculated from that of the reference chamber. For the virtual 6/15 MV beams, the QI were found to be 0.668/ 0.761 and the inter-comparison ratios 0.4408/ 0.4402 respectively. The departmental chamber calibration factors were calculated; applying these and appropriate environmental corrections allowed the output of the Linac to be confirmed. We have shown how a virtual training environment can be used to demonstrate practical processes and reinforce learning. The UK CoP was used here, however any relevant protocol could be demonstrated. Two of the authors (Beavis and Ward) are Founders of Vertual Ltd, a spin-out company created to commercialise the research presented in this abstract.

SU-E-J-20: Evaluation of Image Qualities and Registration of Varian KV-CBCT Images Reconstructed from the Reduced Number of Projections.

To quantitatively investigate image qualities of the kilo-voltage cone-beam CT images reconstructed with reduced number of projections on the Varian OBI system. Evaluate the registration accuracy using those CBCT images against the reference CT images. CBCT images were obtained from Varian OBI system using standard dose head, pelvis and pelvis spotlight modes. CBCT reconstructions were performed with full, 1/2, 1/4, 1/6 and 1/8 of the full set of projections. Catphan® 504 phantom was used to evaluate high-contrast spatial resolution, low-contrast visibility and uniformity. Rando phantom was imaged for rigid registration study. Rando was set up on the linac couch deliberately shifted by 1cm in vertical, lateral, and longitudinal (no rotational) directions from the reference position. Automatch followed by manual adjustment was conducted 5 times to obtain the average shifts. The same method of analysis in the Rando study was used for the clinical registration study. One patient was imaged with pelvis mode and two patients were imaged with pelvis spotlight mode. The Catphan study indicates that high-contrast spatial resolution and uniformity are virtually not affected by the lowest projection-number (1/8) reconstruction scheme. However low-contrast visibility degrades when the projection number used for reconstruction is as low as 1/6. Rando study shows that registration accuracy can be achieved with images reconstructed with 1/6 of the full set of projections. Patient study shows similar results exhibited in Rando study. However, noisy images and streak artifacts are more pronounced with fewer projections (approximate 1/6), which decreases viewer's ability to visualize soft tissues in pelvic sites. This study shows that KV-CBCT reconstructed with fewer number (approximately as low as 1/6) of regular projections can be used for registration against the reference CT. Although the results are encouraging, more clinical cases should be evaluated in the future. None.

SU‐HH‐BRB‐09: Real‐Time Soft‐Tissue Imaging Concurrent with External Beam Radiation Therapy Delivery

Purpose: The challenge of real‐time visualization, localization, and tracking of organ motion and deformation concurrently with external beam radiation therapy (EBRT) delivery remains unmet and unattainable by existing image guidance technologies. We propose to address this challenge in the development of a novel, minimally interfering, robotic ultrasound image guidance system that can be integrated with existing medical linear accelerators. Method and Materials: The span of ultrasound transducer motion required for trans‐abdominal prostate imaging was quantified via optical tracking of a probe during free‐hand acquisitions. This information was incorporated into the design of a customized human‐safe robotic manipulator to remotely control the transducer position and pressure while avoiding LINAC gantry collisions. The effect of accelerator‐induced electromagnetic interference on the robot was investigated. In addition, a treatment plan with beam directions restricted to sectors that do not interfere with the ultrasound transducer was evaluated. Results: Prostate image integrity was sensitive to probe pitch, thus the manipulator was designed to actively control abdominal pressure (0–10N) and transducer pitch (−40°−90°). Probe placement in other directions remains adjustable to accommodate anatomy variations. Human‐safety requirements translated into the design via a torque limiting friction clutch, lightweight backdriveable links, and remote center of motion pitch mechanism. Collision avoidance was confirmed by rotating the gantry 360° around the robot mounted on the LINAC couch in a typical treatment setup. The control system of the robot behaved similarly with the LINAC beam off and on, with mean servo cycle intervals of 1000.03 μs and 1000.06μs, respectively. For a representative prostate setup, remotely‐controlled robotic ultrasound image acquisition was successfully demonstrated on a volunteer. Dose volume histogram (DVH) for an IMRT plan with restricted sectors remained virtually identical to a clinically employed DVH. Conclusion: Remotely‐controlled robotic ultrasound imaging is feasible and may offer real‐time intra‐fractional soft‐tissue guidance concurrent with EBRT delivery.

Development of a frameless stereotactic radiosurgery system based on real-time 6D position monitoring and adaptive head motion compensation

Stereotactic radiosurgery delivers radiation with great spatial accuracy. To achieve sub-millimeter accuracy for intracranial SRS, a head ring is rigidly fixated to the skull to create a fixed reference. For some patients, the invasiveness of the ring can be highly uncomfortable and not well tolerated. In addition, placing and removing the ring requires special expertise from a neurosurgeon, and patient setup time for SRS can often be long. To reduce the invasiveness, hardware limitations and setup time, we are developing a system for performing accurate head positioning without the use of a head ring. The proposed method uses real-time 6D optical position feedback for turning on and off the treatment beam (gating) and guiding a motor-controlled 3D head motion compensation stage. The setup consists of a central control computer, an optical patient motion tracking system and a 3D motion compensation stage attached to the front of the LINAC couch. A styrofoam head cast was custom-built for patient support and was mounted on the compensation stage. The motion feedback of the markers was processed by the control computer, and the resulting motion of the target was calculated using a rigid body model. If the target deviated beyond a preset position of 0.2 mm, an automatic position correction was performed with stepper motors to adjust the head position via the couch mount motion platform. In the event the target deviated more than 1 mm, a safety relay switch was activated and the treatment beam was turned off. The feasibility of the concept was tested using five healthy volunteers. Head motion data were acquired with and without the use of motion compensation over treatment times of 15 min. On average, test subjects exceeded the 0.5 mm tolerance 86% of the time and the 1.0 mm tolerance 45% of the time without motion correction. With correction, this percentage was reduced to 5% and 2% for the 0.5 mm and 1.0 mm tolerances, respectively.