Beam Gantry Research Articles

Developments towards an Advanced Ion Therapy Research Center in the US

The purpose of these developments is to establish an Advanced Ion Therapy Research Center in the US. The proposed center will be the first linac-based ion therapy facility in the world, with the advantage of much desired fast energy and ion beam species switching capability. Such a facility would prove a unique platform to stage the development of pre-clinical studies to prepare for FDA clearance for carbon and other ion beam therapies in the US, and pave the way to establishing clinical ion therapy facilities in this country. The proposed center will enable a breadth of research and applications such as cellular radiobiology, comparative studies of different ion beam therapies and the development of real-time imaging for precise and accurate dose delivery. A multi-institutional collaboration is proposing an innovative center for ion therapy research that will combine an advanced compact carbon ion linac, a compact superconducting carbon beam gantry and real-time MRI-guided beam delivery for therapy. Research and Development towards the realization of the proposed center is underway along three major axes. First, the development of high-gradient accelerating structures for the linac, second, the development of compact curved, combined-function bending magnets and beam scanning magnets for the gantry, and third, the development to combine real-time MRI imaging with ion beam delivery systems. A conceptual design for a compact carbon ion therapy linac has been recently developed. The linac is 45 meters long and capable of accelerating ion beams from proton to neon up to an energy of 450 MeV/u. Prototypes of high-gradient accelerating structures required for the linac are being fabricated and tested. A design of a compact superconducting carbon gantry was also developed and a compact carbon beam scanner magnet is being prototyped. A preliminary concept for MRI-guided ion beam delivery was most recently developed. Other imaging options and range verification techniques, including prompt gamma, positron emission tomography, ionoacoustics and ion beam CT are being considered. Progress made in these areas of development will be presented. Linac-based technology for ion therapy allows more flexibility in beam tuning than synchrotron-based technology with much faster energy modulation and ion beam switching. The reported developments will enable this technology and pave the way to establishing the proposed therapy research center as a national and international resource.

Implementation of Machine Learning-Based Treatment Planning Tool for Whole Breast Radiotherapy Using Irregular Surface Compensator Technique

To implement the machine learning (ML)-based treatment planning tool and compare plan quality and planning efficiency with the manual treatment process for whole breast radiotherapy using irregular surface compensator technique. Thirty whole breast or chest wall cases were planned by using the irregular surface compensator technique with fluence maps manually and iteratively edited to achieve uniform dose distribution to the target. Patients were treated with these manual plans after physician’s approval. For each case, an in-house ML-based automated treatment planning tool was implemented in a programming interface and generated the fluence maps with the same beam parameters such as beam energy, gantry angle, collimator angle, and aperture shape as manual plans.An analytical algorithm software with 0.25cm grid size was used in dose calculation for all plans. Breast or chest wall clinical target volume (CTV) coverage based on the percentage CTV volume receiving 95% of the prescribed dose (V95%) and high-dose volume based on V105% were compared to evaluate the plan quality as well as the planning efficiency. Two-tailed Wilcoxon Signed-Rank test was performed to test the null hypothesis that the two planning schemes yield equivalent plan quality. The mean planning time was 110.2 min with standard deviation (SD) of 62.8 min for the manual planning with the range from 25 to 270 min, and 6.4 min with SD of 2.1 min for the ML-based planning with the range from 4 to 12 min (p<0.01). CTV mean V95% was 96.7% (SD: 5.0%) for the manual planning and 96.7% (SD: 4.8%) for the ML-based planning (p=0.89). CTV mean V105% was 21.6% (SD: 29.8%) for the manual planning and 20.4% (SD: 30.5%) for the ML-based planning (p=0.22). Differences in doses to heart and lungs were negligible between the paired plans as the two planning schemes used the same beam parameters. This study successfully implemented the ML-based automated treatment planning tool through scripting software. Abiding to the same plan quality as manual process, the automated tool significantly reduced the planning time as the ML-based tool automate the iterative fluence editing process. This technique demonstrated promising potential to significantly improve efficiency of the treatment planning for whole breast or chest wall radiotherapy.

Study of intensity suppression in a gantry beamline for SC200 proton therapy system

ABSTRACT For proton therapy, a beam intensity of 0.4 nA to 4 nA is typically used with a beam transmission factor of 10. However, because of energy degrading, emittance matching, and energy spread selecting, the ratio of beamline transmission (I Max /I Min) for our SC200 proton therapy facility (200 MeV/500 nA) should be far more than 10. Therefore, a novel intensity suppression scheme, based on the installation of two new collimators placed symmetrically with respect to the gantry coupling point, is proposed and verified. First, the feasibility of delivering a beam with variable size at the gantry isocenter is demonstrated. The beam delivery method uses waist-to-waist imaging optics, a symmetric beam method, and the principle of beam line inversion. The relationship between energy and isocenter beam size (in air) is then verified using an empirical formula and a Monte Carlo method. The intensity suppression and subsequent optimization of the gantry beam optics were accomplished through simulations. The results indicate that increases in the entrance beam radius at a specific collimator results in significant changes in the beam optics and intensity behind the collimator. This passive scheme, which produces intentional losses at the gantry coupling point, is demonstrated to be viable.

Beam focal spot intrafraction motion and gantry angle dependence: A study of Varian linac focal spot alignment

The characteristics of the focal spot of the linear accelerator (linac) play a role in determining the resulting dose distribution within the patient, and hence probability of treatment success. A direct measurement of focal spot position is not recommended by AAPM Task Group 142, but factors influenced by focal spot position, such as beam symmetry and isocentre position, are. Traditional methods of measuring focal spot position are time consuming and can only be performed at gantry 0°. The presented method has been proposed using a phantom of novel design to accurately measure the position of the focal spot relative to the collimator’s axis of rotation (CAX) at any gantry angle, and to measure the intra-fraction movement of the focal spot relative to the mean position during treatment. The method was reproducible to within 0.012 mm/0.029 mm (mean/max) for the three Varian linacs tested. The focal spot position was shown to deviate from the CAX by up to 0.386 mm during gantry rotation. The focal spot position was more unstable at the start of treatment, with the worst performing linac having an initial displacement of up to 0.15 mm from its mean position before stabilizing to within 0.01 mm after 3 s. The method proposed is a beneficial addition to the quality assurance (QA) schedule of any clinic, allowing quick determination of source position and movement at any gantry angle. Measurement of focal spot allows the possibility of fine-tuning the electron beam steering system to improve the standard of the photon beam and of stereotactic treatments.

Impact of ion source stability for a medical accelerator

MedAustron is a synchrotron-based hadron therapy center located in Lower Austria. Accelerated proton beams with energies of 62–252 MeV/u are used to treat patients since 2016. The carbon ion beam is currently under commissioning and will provide treatment in 2019 with energies of 120–400MeV/u. Two of the four irradiation rooms are used for clinical treatment while the preparation of the Gantry beam line is ongoing. Proton beams of up to 800 MeV will be provided for non-clinical research. The Injector features three identical ECRIS from Pantechnik, two of which are used to generate the proton and the carbon beam respectively. The medical environment of the accelerator puts strict requirements on the ion source long-term stability operation. The extracted beam current from the source allow for maximum current fluctuations on the order of ±2.5% on continuous run. In this work we discuss the impact of the ion source performances on the characteristics and stability of the entire accelerator. Further, we discuss the latest progress on carbon commissioning and the future perspectives with particular emphasis on the source requirements.

Quality assurance of non-coplanar, volumetric-modulated arc therapy employing a C-arm linear accelerator, featuring continuous patient couch rotation

PurposeTo perform quality assurance of non-coplanar, volumetric-modulated arc therapy featuring continuous couch rotation (CCR-VMAT) using a C-arm linear accelerator.MethodsWe planned and delivered CCR-VMAT using the TrueBeam Developer Mode. Treatment plans were created for both a C-shaped phantom and five prostate cancer patients using seven CCR trajectories that lacked collisions; we used RayStation software (ver. 4.7) to this end. Subsequently, verification plans were generated. The mean absolute error (MAE) between the center of an MV-imaged steel ball and the radiation field was calculated using the Winston–Lutz test. The MAEs between planned and actual irradiation values were also calculated from trajectory logs. In addition, correlation coefficients (r values) among the MAEs of gantry angle, couch angle, and multi-leaf collimator (MLC) position, and mechanical parameters including gantry speed, couch speed, MLC speed, and beam output, were estimated. The dosimetric accuracies of planned and measured values were also assessed using ArcCHECK.ResultsThe MAEs ±2 standard deviations as revealed by the Winston–Lutz test for all trajectories were 0.3 ± 0.3 mm in two dimensions. The MAEs of the gantry, couch, and MLC positions calculated from all trajectory logs were within 0.04°, 0.08°, and 0.02 mm, respectively. Deviations in the couch angle (r = 0.98, p < 0.05) and MLC position (r = 0.86, p < 0.05) increased significantly with speed. The MAE of the beam output error was less than 0.01 MU. The mean gamma passing rate ± 2 SD (range) of the 3%/3 mm, 3%/1 mm, and 5%/1 mm was 98.1 ± 1.9% (95.7–99.6%), 87.2 ± 2.8% (80.2–96.7%), and 96.3 ± 2.8% (93.9–99.6%), respectively.ConclusionsCCR-VMAT delivered via the TrueBeam Developer Mode was associated with high-level geometric and mechanical accuracy, thus affording to high dosimetric accuracy. The CCR-VMAT performance was stable regardless of the trajectory chosen.

Semi-automated IGRT QA using a cone-shaped scintillator screen detector for proton pencil beam scanning treatments

To promote accurate image-guided radiotherapy (IGRT) for a proton pencil beam scanning (PBS) system, a new quality assurance (QA) procedure employing a cone-shaped scintillator detector has been developed for multiple QA tasks in a semi-automatic manner. The cone-shaped scintillator detector (XRV-124, Logos Systems, CA) is sensitive to both x-ray and proton beams. It records scintillation on the cone surface as a 2D image, from which the geometry of the radiation field that enters and exits the cone can be extracted. Utilizing this feature, QA parameters that are essential to PBS IGRT treatment were measured and analyzed. The first applications provided coincidence checks of laser, imaging and radiation isocenters, and dependencies on gantry angle and beam energies. The analysis of the Winston–Lutz test was made available by combining the centricity measurements of the x-ray beam and the pencil beam. The accuracy of the gantry angle was validated against console readings provided by the digital encoder and an agreement of less than 0.2° was found. The accuracy of the position measurement was assessed with a robotic patient positioning system (PPS) and an agreement of less than 0.5 mm was obtained. The centricity of the two onboard x-ray imaging systems agreed well with that from the routinely used Digital Imaging Positioning System (DIPS), up to a consistent small shift of (−0.5 mm, 0.0 mm, −0.3 mm). The pencil beam spot size, in terms of σ of Gaussian fitting, agreed within 0.2 mm for most energies when compared to the conventional measurements by a 2D ion-chamber array (MatriXX-PT, IBA Dosimetry, Belgium). The cone-shaped scintillator system showed advantages in making multi-purpose measurements with a single setup. The in-house algorithms were successfully implemented to measure and analyze key QA parameters in a semi-automatic manner. This study presents an alternative and more efficient approach for IGRT QA for PBS and potentially for linear accelerators.

An investigation of clinical treatment field delivery verification using cherenkov imaging: IMRT positioning shifts and field matching.

Cherenkov light emission has been shown to correlate with ionizing radiation dose delivery in solid tissue. An important clinical application of Cherenkov light is the real-time verification of radiation treatment delivery invivo. To test the feasibility of treatment field verification, Cherenkov light images were acquired concurrent with radiation beam delivery to standard and anthropomorphic phantoms. Specifically, we tested two clinical treatment scenarios: (a) Observation of field overlaps or gaps in matched 3D fields and (b) Patient positioning shifts during intensity modulated radiation therapy (IMRT) field delivery. Further development of this technique would allow real-time detection of treatment delivery errors on the order of millimeters so that patient safety and treatment quality can be improved. Cherenkov light emission was captured using a PI-MAX4 intensified charge coupled device (ICCD) system (Princeton Instruments). All radiation delivery was performed using a Varian Trilogy linear accelerator (linac) operated at 6MV or 18MV for photon and 6MeV or 16MeV for electron studies. Field matching studies were conducted with photon and electron beams at gantry angles of 0°, 15°, and 45°. For each modality and gantry angle, a total of three data sets were acquired. Overlap and gap distances of 0, 2, 5, and 10mm were tested and delivered to solid phantom material of 30×30×5cm3 . Phantom materials used were white plastic water and brown solid water. Tests were additionally performed on an anthropomorphic phantom with an irregular surface. Positioning shift studies were performed using IMRT fields delivered to a thoracic anthropomorphic phantom. For thoracic phantom measurements, the camera was placed laterally to observe the entire right side of the phantom. Fields were delivered with known translational patient positioning shifts in four directions. Changes in the Cherenkov fluence were evaluated through the generation of difference maps from unshifted Cherenkov images. All images were evaluated using ImageJ, Python, and MATLAB software packages. For matched fields, Cherenkov images were able to quantitate matched field separations with discrepancies between 2 and 4mm, depending on gantry angle and beam energy or modality. For all photon and electron beams delivered at a gantry angle of 0°, image analysis indicated average discrepancies of less than 2mm for all field gaps and overlaps, with 83% of matched fields exhibiting discrepancies less than 1mm. Beams delivered obliquely to the phantom surface exhibited average discrepancies as high as 4mm for electron beams delivered at large oblique angles. Finally, for IMRT field delivery, vertical and lateral patient positioning shifts of 2mm were detected in some cases, indicating the potential detectability threshold of using this technique alone. Our study indicates that Cherenkov imaging can be used to support and bolster current treatment delivery verification techniques, improving our ability to recognize and rectify millimeter-scale delivery and positioning errors.

Beam optics and isocenter property of SC200 proton therapy gantry

Studies on beam optics and the isocenter property in the gantry system for the SC200 (Superconducting Isochronous Cyclotron of 200 MeV in Hefei, China) are presented in this paper. The physical design of the isocentric gantry system is developed with the software TRANSPORT, which realizes a beam with a circular shape at the isocenter and a full width at half maximum of 4–10 mm. For stereotactic radiosurgery, the isocenter deviation of the gantry system should be less than 1 mm in diameter. In order to explore the property of the isocenter, an electromagnetic structure coupling analysis has been conducted given the self-gravity and the electromagnetic (EM) force of the magnets on the gantry beam line. The correlation between the isocenter property and the EM force has also been carefully studied. This paper puts forward two methods to obtain the isocenter deviation based on the characteristics of the nozzle installation structure and the calculation of the optical path after the nozzle, respectively. The results show that the maximum isocenter deviation is less than 0.33 mm with a safety factor of 1.5, and the deviation caused by the EM force is 0.05 mm. The latter result indicates that the impact of the EM force is negligible. This paper puts forward one possible way to realize real-time acquisition of the isocenter deviation in practical application. The gantry of SC200 is under construction at ASIPP.

Improvements in pencil beam scanning proton therapy dose calculation accuracy in brain tumor cases with a commercial Monte Carlo algorithm

A commercial Monte Carlo (MC) algorithm (RayStation version 6.0.024) for the treatment of brain tumors with pencil beam scanning (PBS) proton therapy is validated and compared via measurements and analytical calculations in clinically realistic scenarios. For the measurements a 2D ion chamber array detector (MatriXX PT) was placed underneath the following targets: (1) an anthropomorphic head phantom (with two different thicknesses) and (2) a biological sample (i.e. half a lamb’s head). In addition, we compared the MC dose engine versus the RayStation pencil beam (PB) algorithm clinically implemented so far, in critical conditions such as superficial targets (i.e. in need of a range shifter (RS)), different air gaps, and gantry angles to simulate both orthogonal and tangential beam arrangements. For every plan the PB and MC dose calculations were compared to measurements using a gamma analysis metrics (3%, 3 mm). For the head phantom the gamma passing rate (GPR) was always >96% and on average >99% for the MC algorithm; the PB algorithm had a GPR of ⩽90% for all the delivery configurations with a single slab (apart 95% GPR from the gantry of 0° and small air gap) and in the case of two slabs of the head phantom the GPR was >95% only in the case of small air gaps for all three (0°, 45°, and 70°) simulated beam gantry angles. Overall the PB algorithm tends to overestimate the dose to the target (up to 25%) and underestimate the dose to the organ at risk (up to 30%). We found similar results (but a bit worse for the PB algorithm) for the two targets of the lamb’s head where only two beam gantry angles were simulated. Our results suggest that in PBS proton therapy a range shifter (RS) needs to be used with caution when planning a treatment with an analytical algorithm due to potentially great discrepancies between the planned dose and the dose delivered to the patient, including in the case of brain tumors where this issue could be underestimated. Our results also suggest that a MC evaluation of the dose has to be performed every time the RS is used and, mostly, when it is used with large air gaps and beam directions tangential to the patient surface.

Vertical dynamics of a single-span beam subjected to moving mass-suspended payload system with variable speeds

This paper presents the vertical dynamics of a simply supported Euler–Bernoulli beam subjected to a moving mass-suspended payload system of variable velocities. A planar theoretical model of the moving mass-suspended payload system of variable speeds is developed based on several assumptions: the rope is massless and rigid, and its length keeps constant; the stiffness of the gantry beam is much greater than the supporting beam, and the gantry beam can be treated as a mass particle traveling along the supporting beam; the supporting beam is assumed as a simply supported Bernoulli-Euler beam. The model can be degenerated to consider two classical cases—the moving mass case and the moving payload case. The proposed model is verified using both numerical and experimental methods. To further investigate the effect of possible influential factors, numerical examples are conducted covering a range of parameters, such as variable speeds (acceleration or deceleration), mass ratios of the payload to the total moving load, and the pendulum lengths. The effect of beam flexibility on swing response of the payload is also investigated. It is shown that the effect of a variable speed is significant for the deflections of the beam. The accelerating movement tends to induce larger beam deflections, while the decelerating movement smaller ones. For accelerating or decelerating movements, the moving mass model may underestimate the deflections of the beam compared with the presented model; while for uniform motion, both the moving mass model and the moving mass-payload model lead to same beam responses. Furthermore, it is observed that the swing response of the payload is not sensitive to the stiffness of the beam for operational cases of a moving crane, thus a simple moving payload model can be employed in the swing control of the payload.

Emittance matching of a slow extracted beam for a rotating gantry

The introduction of a heavy-ion rotating gantry is in progress at the Heavy Ion Medical Accelerator in Chiba (HIMAC) for realizing high-precision cancer therapy using heavy ions. A scanning irradiation method will be applied to this gantry course with 48–430MeV/u beam energy. In the rotating gantry, the horizontal and vertical beam parameters are coupled by its rotation. To maintain a circular spot shape at the isocenter irrespective of the gantry angle, achieving symmetric phase space distribution of the horizontal and vertical beam at the entrance of the rotating gantry is necessary. Therefore, compensating the horizontal and vertical emittance is necessary. We consider using a thin scatterer method to compensate the emittance. After considering the optical design for emittance matching, the scatterer device is located in the high-energy beam transport line. In the beam commissioning, we confirm that the symmetrical spot shape is obtained at the isocenter without depending on the gantry angle.

Evaluation of ceramic marker for the treatment of ocular melanoma with proton therapy

BioMarc SecureTM, a fiducial marker with ceramic core (zirconium oxide) and carbon coating, was evaluated for use in the localization and alignment of ocular melanoma targets using proton pencil beam scanning (PBS). Measurements and Monte Carlo (MC) simulations were performed to estimate the dose attenuation behind the marker. Imaging tests were performed to evaluate the visibility of marker for x-ray based alignment. An anthropomorphic phantom with implanted fiducial marker was scanned to obtain two-dimensional kilo-voltage (kV) x-ray and three-dimensional computed tomographic (CT) images. For dose shadow measurements, an in-house phantom was constructed using solid water, Styrofoam and EBT3 gafchromic film. Dose attenuation behind the marker was measured at multiple depths and for three cardinal orientations: parallel, perpendicular, and transverse. The measurements were performed at the proton pencil beam gantry at the Seattle Cancer Care Alliance Proton Therapy center. A beam model for pencil beam gantry was created in the validated MC simulation toolkit GATE (GEANT4 Applications for Tomographic Emission). MC simulations were set up to mimic the measurement geometry and dose attenuation behind the marker was studied for a single layer PBS beam. Additional simulations were performed in water using the clinical relevant spread out Bragg peak (SOBP) beam. Qualitative analysis of the planar kV images showed excellent visibility of marker. Reconstruction artifacts in the form of light and dark streaks were present on the CT images. The film and MC simulations showed significant dose attenuation behind the marker that worsened when marker was placed close to the distal edge. Dose shadows of up to 45% and range loss of up to 1.1 mm were seen behind the marker. The transverse marker orientation in the clinical SOBP beam showed dose reduction of up to 61% and range pull-back of 2.4 mm. The dose attenuation was found to be a function of marker orientation and location within the SOBP. The analysis showed that small volumes behind the marker can see severe under-dosing. This effect should be taken into account during the planning of treatment. Comparing the results of this study to published results for tantalum marker, the BioMarc Secure marker was found to be inferior due to its greater dose attenuation and extended dose shadow on larger underlying volume. Furthermore, its bigger physical size may mean more discomfort to patient.

Radiation shielding calculation for digital breast tomosynthesis rooms with an updated workload survey

Purpose. To present shielding calculations for clinical digital breast tomosynthesis (DBT) rooms with updated workload data from a comprehensive survey and to provide reference shielding data for DBT rooms. Methods. The workload survey was performed from eight clinical DBT (Hologic Selenia Dimensions) rooms at Massachusetts General Hospital (MGH) for the time period between 10/1/2014 and 10/1/2015. Radiation output related information tags from the DICOM header, including mAs, kVp, beam filter material and gantry angle, were extracted from a total of 310 421 clinical DBT acquisitions from the PACS database. DBT workload distributions were determined from the survey data. In combination with previously measured scatter fraction data, unshielded scatter air kerma for each room was calculated. Experiment measurements with a linear-array detector were also performed on representative locations for verification. Necessary shielding material and thickness were determined for all barriers. For the general purpose of DBT room shielding, a set of workload-distribution-specific transmission data and unshielded scatter air kerma values were calculated using the updated workload distribution. Results. The workload distribution for Hologic DBT systems could be simplified by five different kVp/filter combinations for shielding purpose. The survey data showed the predominance of 45° gantry location for medial-lateral-oblique views at MGH. When taking into consideration the non-isotropic scatter fraction distribution together with the gantry angle distribution, accurate and conservative estimate of the unshielded scatter air kerma levels were determined for all eight DBT rooms. Additional shielding was shown to be necessary for two 4.5 cm wood doors. Conclusions. This study provided a detailed workload survey and updated transmission data and unshielded scatter air kerma values for Hologic DBT rooms. Example shielding calculations were presented for clinical DBT rooms.

Choice of appropriate beam model and gantry rotational angle for low-dose gradient-based craniospinal irradiation using volumetric-modulated arc therapy

AbstractObjectivesWe aimed to assess the impact of advanced multileaf collimator (MLC) models and flattening filter-free (3F) beam in volumetric-modulated arc therapy (VMAT)-based craniospinal irradiation (CSI).MethodsCT scans of five medulloblastoma patients who previously received CSI at our hospital were used for the present study. Patients were planned for a prescription dose of 35 Gy to craniospinal axis. A three-dimensional conformal radiotherapy (3DCRT) plan and a VMAT plan using 1 cm MLC leaf width were generated as the gold standard (reference arm). Test VMAT plans were generated using Agility MLC model (MLC leaf width 5 mm) for various combinations of flattened beam (F) and 3F beam for treating the brain and spine planning target volume (PTV). Organs at risks (OARs) were analysed for dose 5, 50, 75 and 90% volumes, mean dose and maximum dose.ResultsAll 3DCRT plans and VMAT plans were aimed to cover 95% of PTV by at least 95% prescription dose. VMAT demonstrated lesser dose spillage than 3DCRT to body volume minus PTV (NTID: non tumor integral dose) for a dose threshold above 7·5 Gy. For the low-dose range (1–7 Gy), variation between the dose coverage between all VMAT plans (for either spine or brain PTV) was &lt;1%. Intra-VMAT plan dose variation for all OAR’s for all tested parameters was &lt;1 Gy. Average monitor unit (MU) difference among different VMAT plans ranged between 1·52 and 2·13 when normalised to 3DCRT MU. For VMAT plans, flat beam with 1 cm MLC showed the highest MU, whereas Agility MLC with 3F beam had the least MU values for intra-VMAT plans. No statistical significance variation (p) was observed in between reference arm and test arm plans except for mean dose and V107% for PTV spine. When compared between reference arm 3DCRT and test arm VMAT plans. For OAR’s, no statistical difference was observed between reference and test arm VMAT plans.ConclusionsReference arm plans and test arm plans exhibit no statistically significant difference. However, as compared with 3DCRT, VMAT plans are more conformal and produce lesser dose to OAR at the cost of higher delivered MU. 3F beams or finer width MLC’s (width &lt;5 mm) have no advantage over the conventional 1 cm MLC and flat beam except that 3F beams have a shorter beam delivery time. This study demonstrate a significantly lesser spillage dose to NTID/body that of the reported literature, which is attributed to limited rotational arc length used for VMAT plans.

Beam commissioning of a superconducting rotating-gantry for carbon-ion radiotherapy

A superconducting rotating-gantry for carbon-ion radiotherapy was developed. This isocentric gantry can transport carbon ions having kinetic energies of between E=430 and 48MeV/u to an isocenter over an angle of ±180°, and is further capable of performing three-dimensional raster-scanning irradiation. Construction of the entire rotating-gantry system was completed by the end of September 2015. Prior to beam commissioning, phase-space distributions of extracted carbon beams from the synchrotron were deduced by using an empirical method. In this method, phase-space distributions at the extraction channel of the synchrotron were modeled with 8 parameters, and the best parameters were determined so as to minimize a difference between the calculated and measured beam profiles by using a simplex method. Based on the phase-space distributions, beam optics through the beam-transport lines as well as the rotating gantry were designed. Since horizontal and vertical beam emittances, as extracted slowly from the synchrotron, generally differ with each other, a horizontal–vertical beam coupling would occur when the gantry rotates. Thus, the size and shape of beam spots at the isocenter should vary depending on the gantry angle. To compensate for the difference in the emittances, we employed a method to utilize multiple Coulomb scattering of the beam particles by a thin scatterer. Having compensated for the emittances and designed beam optics through the rotating gantry, beam commissioning over various combinations of gantry angles and beam energies was performed. By finely tuning the superconducting quadrupoles of the rotating gantry, we could successfully obtain the designed beam quality, which satisfies the requirements of scanning irradiation.

SU‐F‐T‐177: Impacts of Gantry Angle Dependent Scanning Beam Properties for Proton Treatment

Purpose:In pencil beam scanning (PBS), the delivered spot MU, position and size are slightly different at different gantry angles. We investigated the level of delivery uncertainty at different gantry angles through a log file analysis.Methods:34 PBS fields covering full 360 degrees gantry angle spread were collected retrospectively from 28 patients treated at our institution. All fields were delivered at zero gantry angle and the prescribed gantry angle, and measured at isocenter with the MatriXX 2D array detector at the prescribed gantry angle. The machine log files were analyzed to extract the delivered MU per spot and the beam position from the strip ionization chambers in the treatment nozzle. The beam size was separately measured as a function of gantry angle and beam energy. Using this information, the dose was calculated in a water phantom at both gantry angles and compared to the measurement using the 3D γ‐index at 2mm/2%.Results:The spot‐by‐spot difference between the beam position in the log files from the delivery at the two gantry angles has a mean of 0.3 and 0.4 mm and a standard deviation of 0.6 and 0.7 mm for × and y directions, respectively. Similarly, the spot‐by‐spot difference between the MU in the log files from the delivery at the two gantry angles has a mean 0.01% and a standard deviation of 0.7%. These small deviations lead to an excellent agreement in dose calculations with an average γ pass rate for all fields being approximately 99.7%. When each calculation is compared to the measurement, a high correlation in γ was also found.Conclusion:Using machine logs files, we verified that PBS beam delivery at different gantry angles are sufficiently small and the planned spot position and MU. This study brings us one step closer to simplifying our patient‐specific QA.

SU‐F‐T‐232: Monthly Quality Assurance in External Beam Radiation Therapy Using a Single System

Purpose:Monthly quality assurance (QA) is time consuming for external beam radiation therapy, taking as long as 6–8 hours for each machine. It is due to the use and setup of multiple devices for different QA procedures. We have developed a single system with rotational capability for the measurement of both optical light and radiation which significantly reduces the time spent on Monthly QA.Methods:A single system using mirrors, a phosphor screen and a CCD camera is housed on a cylindrical motor so that it can rotate 360 degrees. For monthly QA, the system is placed on the patient couch of the medical accelerator with the plane of the phosphor screen at isocenter for all measurements. For optical QA such as optical distance indicator, room laser and light field, the optical image is collected directly with the camera. For radiation QA such as beam profile, MLC speed, picket‐fence test, collimator rotation, table rotation and gantry rotation, a brass build‐up plate is attached to the top of the phosphor screen. Two brass plates with islands of different thickness were designed for photon energy and electron energy constancy checks. Flex map, distortion map and uniformity map were developed to calibrate the motor bearing, camera/lens distortion, and the phosphor screen's measured response across the field.Results:Following the TG142 guidelines for monthly QA with our system, the overall run time is reduced from 6–8 hours to 1.5 hours. Our system's rotating design allows for quick testing of the gantry radiation isocenter test that is also independent of the sag of the gantry and the EPID.Conclusion:Our system significantly shortens the time needed for monthly QA by unifying the tests with a single system. Future work will be focused on extending the technology to Brachytherapy, IMRT and proton therapy QAs.This work is funded in part by a sponsor research grant from JPLC who owns the Raven technology. John Wong is a co‐founder of JPLC.

SU‐G‐JeP3‐06: Lower KV Image Dose Are Expected From a Limited‐Angle Intra‐Fractional Verification (LIVE) System for SBRT Treatments

Purpose:In order to track the tumor movement for patient positioning verification during arc treatment delivery or in between 3D/IMRT beams for stereotactic body radiation therapy (SBRT), the limited‐angle kV projections acquisition simultaneously during arc treatment delivery or in‐between static treatment beams as the gantry moves to the next beam angle was proposed. The purpose of this study is to estimate additional imaging dose resulting from multiple tomosynthesis acquisitions in‐between static treatment beams and to compare with that of a conventional kV‐CBCT acquisition.Methods:kV imaging system integrated into Varian TrueBeam accelerators was modeled using EGSnrc Monte Carlo user code, BEAMnrc and DOSXYZnrc code was used in dose calculations. The simulated realistic kV beams from the Varian TrueBeam OBI 1.5 system were used to calculate dose to patient based on CT images. Organ doses were analyzed using DVHs. The imaging dose to patient resulting from realistic multiple tomosynthesis acquisitions with each 25–30 degree kV source rotation between 6 treatment beam gantry angles was studied.Results:For a typical lung SBRT treatment delivery much lower (20–50%) kV imaging doses from the sum of realistic six tomosynthesis acquisitions with each 25–30 degree x‐ray source rotation between six treatment beam gantry angles were observed compared to that from a single CBCT image acquisition.Conclusion:This work indicates that the kV imaging in this proposed Limited‐angle Intra‐fractional Verification (LIVE) System for SBRT Treatments has a negligible imaging dose increase. It is worth to note that the MV imaging dose caused by MV projection acquisition in‐between static beams in LIVE can be minimized by restricting the imaging to the target region and reducing the number of projections acquired. For arc treatments, MV imaging acquisition in LIVE does not add additional imaging dose as the MV images are acquired from treatment beams directly during the treatment.

SU‐F‐J‐191: Dosimetric Evaluation of a Left Chestwall Patient Treated with a Compact Proton Pencil Beam Gantry Utilizing Daily Setup CBCT

Purpose:To evaluate the robustness of the gradient technique for treating a multi‐isocenter left chest wall patient with a compact proton pencil beam gantry. Both CBCT and stereoscopic imaging are used to facilitate daily treatment setup.Methods:To treat the elongated chest wall planning target volume (PTV) with the compact PBS system, a 28 fraction (5040 CcGE) treatment plan was created using two fields with gradient matching technique. Daily table shifts between treatment field isocenters were obtained from the record and verify system for each treatment fraction. Copies of the initial treatment plan were made for each fraction and the field isocenter coordinates for each plan copy were adjusted to reflect daily table shifts. Doses were re‐calculated for each fraction, summed, and compared against the initial plan.Results:The table shifts (average and range) were 2.2 (−5.1–+3.9), 3.0 (−6.0–+4.0) and 3.0 (−10.1–+1.9) millimeters in the anterior‐posterior, superior‐inferior and right‐left directions, respectively. Dose difference to the PTV, heart and ipsilateral lung were evaluated. The percentage of the PTV receiving the prescription dose decreased from 94.6% to 89.1%. The D95 of the PTV increased from 99.6% to 99.9%. The maximum dose in PTV increased from 106.6% to 109.2% and V105 increased from 1.0% to 16.5%. The V20 of the ipsilateral lung increased from 18.5% to 21.0%. The mean heart dose difference was negligible.Conclusion:Observed dose differences to lung and heart tissues due to daily setup variations remained acceptably low while maintaining sufficient dose coverage to the PTV. This initial case study demonstrates the robustness of the gradient technique to treat a large target, multi‐isocenter plan with a compact proton pencil beam gantry equipped with CBCT and stereoscopic imaging modalities.