L Rankine

On the feasibility of optical-CT imaging in media of different refractive index.

PURPOSE Achieving accurate optical-CT 3D dosimetry without the use of viscous refractive index (RI) matching fluids would greatly increase convenience. METHODS Software has been developed to simulate optical-CT 3D dosimetry for a range of scanning configurations including parallel-beam, point, and converging light sources. For each configuration the efficacy of three refractive media was investigated: air, water, a fluid closely matched to PRESAGE(®), and perfect matching (RI = 1.00, 1.33, 1.49, and 1.501 respectively). Reconstructions were performed using both filtered backprojection (FBP) and algebraic reconstruction technique (ART). The efficacy of the three configurations and the two algorithms was evaluated by calculating the usable radius (i.e., the outermost radius where data were accurate to within 2%), and gamma (Γ) analysis. This definition recognizes that for optical-CT imaging, errors are greatest near the edge of the dosimeter, where refraction can be most pronounced. Simulations were performed on three types of dose distribution: uniform, volumetric modulated arc therapy (VMAT), and brachytherapy (Cs-137). RESULTS For a uniformly irradiated dosimeter the usable radius achieved with filtered backprojection was 68% for water-matching and 31% for dry-scanning in air. Algebraic reconstruction gave usable radii of 99% for both water and air (dry-scanning), indicating greater recovery of useful data for the uniform distribution. FBP and ART performed equally well for a VMAT dose distribution where less dose is delivered near the edge of the dosimeter. In this case, the usable radius was 86% and 53% for scanning in water and air, respectively. For brachytherapy, the usable radius was 99% and 98% for scanning in water and air, respectively using FBP, and a major decrease was seen with ART. Point source geometry provided 1%-2% larger usable radii than parallel geometry. Converging geometry recovered less usable dosimetry data (up to 10% reduced usable radii) than point and parallel geometries. A further disadvantage of converging geometry was an increased requirement on detector size by up to 18°. CONCLUSIONS For applications where dose information is not required in the periphery of the dosimeter, some dry and low-viscous matching configurations may be feasible. For all three dose distributions (uniform, VMAT, brachytherapy) the point source geometry produced slightly more favorable results (an extra 1%-2% usable radii) than parallel and converging. When dosimetry is required on the periphery, best results were obtained using close refractive matching and ART. A concern for water or dry-scanning is the increase in required detector size, introducing potential cost penalties for manufacturing.

Characterization of the onboard imaging unit for the first clinical magnetic resonance image guided radiation therapy system.

PURPOSE To characterize the performance of the onboard imaging unit for the first clinical magnetic resonance image guided radiation therapy (MR-IGRT) system. METHODS The imaging performance characterization included four components: ACR (the American College of Radiology) phantom test, spatial integrity, coil signal to noise ratio (SNR) and uniformity, and magnetic field homogeneity. The ACR phantom test was performed in accordance with the ACR phantom test guidance. The spatial integrity test was evaluated using a 40.8 × 40.8 × 40.8 cm(3) spatial integrity phantom. MR and computed tomography (CT) images of the phantom were acquired and coregistered. Objects were identified around the surfaces of 20 and 35 cm diameters of spherical volume (DSVs) on both the MR and CT images. Geometric distortion was quantified using deviation in object location between the MR and CT images. The coil SNR test was performed according to the national electrical manufacturers association (NEMA) standards MS-1 and MS-9. The magnetic field homogeneity test was measured using field camera and spectral peak methods. RESULTS For the ACR tests, the slice position error was less than 0.10 cm, the slice thickness error was less than 0.05 cm, the resolved high-contrast spatial resolution was 0.09 cm, the resolved low-contrast spokes were more than 25, the image intensity uniformity was above 93%, and the percentage ghosting was less than 0.22%. All were within the ACR recommended specifications. The maximum geometric distortions within the 20 and 35 cm DSVs were 0.10 and 0.18 cm for high spatial resolution three-dimensional images and 0.08 and 0.20 cm for high temporal resolution two dimensional cine images based on the distance-to-phantom-center method. The average SNR was 12.0 for the body coil, 42.9 for the combined torso coil, and 44.0 for the combined head and neck coil. Magnetic field homogeneities at gantry angles of 0°, 30°, 60°, 90°, and 120° were 23.55, 20.43, 18.76, 19.11, and 22.22 ppm, respectively, using the field camera method over the 45 cm DSV. CONCLUSIONS The onboard imaging unit of the first commercial MR-IGRT system meets ACR, NEMA, and vendor specifications.

Commissioning and initial experience with the first clinical gantry-mounted proton therapy system

The purpose of this study is to describe the comprehensive commissioning process and initial clinical experience of the Mevion S250 proton therapy system, a gantry‐mounted, single‐room proton therapy platform clinically implemented in the S. Lee Kling Proton Therapy Center at Barnes‐Jewish Hospital in St. Louis, MO, USA. The Mevion S250 system integrates a compact synchrocyclotron with a C‐inner gantry, an image guidance system and a 6D robotic couch into a beam delivery platform. We present our commissioning process and initial clinical experience, including i) CT calibration; ii) beam data acquisition and machine characteristics; iii) dosimetric commissioning of the treatment planning system; iv) validation through the Imaging and Radiation Oncology Core credentialing process, including irradiations on the spine, prostate, brain, and lung phantoms; v) evaluation of localization accuracy of the image guidance system; and vi) initial clinical experience. Clinically, the system operates well and has provided an excellent platform for the treatment of diseases with protons. PACS number(s): 87.55.ne, 87.56.bdThe purpose of this study is to describe the comprehensive commissioning process and initial clinical experience of the Mevion S250 proton therapy system, a gantry-mounted, single-room proton therapy platform clinically implemented in the S. Lee Kling Proton Therapy Center at Barnes-Jewish Hospital in St. Louis, MO, USA. The Mevion S250 system integrates a compact synchrocyclotron with a C-inner gantry, an image guidance system and a 6D robotic couch into a beam delivery platform. We present our commissioning process and initial clinical experience, including i) CT calibration; ii) beam data acquisition and machine characteristics; iii) dosimetric commissioning of the treatment planning system; iv) validation through the Imaging and Radiation Oncology Core credentialing process, including irradiations on the spine, prostate, brain, and lung phantoms; v) evaluation of localization accuracy of the image guidance system; and vi) initial clinical experience. Clinically, the system operates well and has provided an excellent platform for the treatment of diseases with protons. PACS number(s): 87.55.ne, 87.56.bd.

Measured Neutron Spectra and Dose Equivalents From a Mevion Single-Room, Passively Scattered Proton System Used for Craniospinal Irradiation

PURPOSE To measure, in the setting of typical passively scattered proton craniospinal irradiation (CSI) treatment, the secondary neutron spectra, and use these spectra to calculate dose equivalents for both internal and external neutrons delivered via a Mevion single-room compact proton system. METHODS AND MATERIALS Secondary neutron spectra were measured using extended-range Bonner spheres for whole brain, upper spine, and lower spine proton fields. The detector used can discriminate neutrons over the entire range of the energy spectrum encountered in proton therapy. To separately assess internally and externally generated neutrons, each of the fields was delivered with and without a phantom. Average neutron energy, total neutron fluence, and ambient dose equivalent [H* (10)] were calculated for each spectrum. Neutron dose equivalents as a function of depth were estimated by applying published neutron depth-dose data to in-air H* (10) values. RESULTS For CSI fields, neutron spectra were similar, with a high-energy direct neutron peak, an evaporation peak, a thermal peak, and an intermediate continuum between the evaporation and thermal peaks. Neutrons in the evaporation peak made the largest contribution to dose equivalent. Internal neutrons had a very low to negligible contribution to dose equivalent compared with external neutrons, largely attributed to the measurement location being far outside the primary proton beam. Average energies ranged from 8.6 to 14.5 MeV, whereas fluences ranged from 6.91 × 10(6) to 1.04 × 10(7) n/cm(2)/Gy, and H* (10) ranged from 2.27 to 3.92 mSv/Gy. CONCLUSIONS For CSI treatments delivered with a Mevion single-gantry proton therapy system, we found measured neutron dose was consistent with dose equivalents reported for CSI with other proton beamlines.

Investigations into the Feasibility of Optical-CT 3D Dosimetry with Minimal Use of Refractively Matched Fluids

PURPOSE In optical-CT, the use of a refractively matched polyurethane solid-tank in place of a fluid bath has the potential to greatly increase practical convenience, reduce cost, and possibly improve the efficacy of flood corrections. This work investigates the feasibility of solid-tank optical-CT imaging for 3D dosimetry through computer simulation. METHODS A matlab ray-tracing simulation platform, ScanSim, was used to model a parallel-source telecentric optical-CT imaging system through a polyurethane solid-tank containing a central cylindrical hollow into which PRESAGE radiochromic dosimeters can be placed. A small amount of fluid fills the 1-5 mm gap between the dosimeter and the walls of the tank. The use of the solid-tank reduces the required amount of fluid by approximately 97%. To characterize the efficacy of solid-tank, optical-CT scanning simulations investigated sensitivity to refractive index (RI) mismatches between dosimeter, solid-tank, and fluid, for a variety of dosimeter (RI = 1.5-1.47) and fluid (RI = 1.55-1.0) combinations. Efficacy was evaluated through the usable radius (ru) metric, defined as the fraction of the radius of the dosimeter where measured dose is predicted to be within 2% of the ground truth entered into the simulation. Additional simulations examined the effect of increasing gap size (1-5 mm) between the dosimeter and solid-tank well. The effects of changing the lens tolerance (0.5°-5.0°) were also investigated. RESULTS As the RI mismatch between the dosimeter and solid-tank increased from 0 to 0.02, the usable radius decreased from 97.6% to 50.2%. The optimal fluid RI decreased nonlinearly from 1.5 to 1.34 as the mismatch increased and was up to 9% lower than the tank. Media mismatches between the dosimeter and solid-tank also exacerbate the effects of changing the gap size, with no easily quantifiable relationship with usable radius. Generally, the optimal fluid RI value increases as gap size increases and is closely matched to the dosimeter at large gap sizes (> 3 mm). Increasing the telecentric lens tolerance increases the usable radius for all refractive media combinations and improves the maximum usable radius of mismatched media to that of perfectly matched media for tolerances > 5.0°. The maximum usable radius can be improved up to a factor of 2 when lens tolerances are small (< 1.0°). CONCLUSIONS Dry solid-tank optical-CT imaging in a telecentric system is feasible if the dosimeter RI is a close match with the solid-tank (< 0.01 difference), providing accurate dose measurements within ± 2% of true dose to over 80% of the dosimeter volume. In order to achieve accurate measurements over 96% of the dosimeter volume (representing out to 2 mm from the dosimeter edge), the dosimeter-tank RI mismatch must be less than 0.005. Optimal results occur when the RI of the dosimeter and tank is the same, in which case the fluid will have the same RI. If mismatches between the tank and dosimeter RI occur, the RI of the matching fluid needs to be fine tuned to achieve the highest usable radius.

Towards comprehensive characterization of Cs-137 Seeds using PRESAGE® dosimetry with optical tomography.

We describe a method to directly measure the radial dose and anisotropy functions of brachytherapy sources using polyurethane based dosimeters read out with optical CT. We measured the radial dose and anisotropy functions for a Cs-137 source using a PRESAGE® dosimeter (9.5cm diameter, 9.2cm height) with a 0.35cm channel drilled for source placement. The dosimeter was immersed in water and irradiated to 5.3Gy at 1cm. Pre- and post-irradiation optical CT scans were acquired with the Duke Large field of view Optical CT Scanner (DLOS) and dose was reconstructed with 0.5mm isotropic voxel size. The measured radial dose factor matched the published fit to within 3% for radii between 0.5–3.0cm, and the anisotropy function matched to within 4% except for θ near 0° and 180° and radii >3cm. Further improvements in measurement accuracy may be achieved by optimizing dose, using the high dynamic range scanning capability of DLOS, and irradiating multiple dosimeters. Initial simulations indicate an 8 fold increase in dose is possible while still allowing sufficient light transmission during optical CT. A more comprehensive measurement may be achieved by increasing dosimeter size and flipping the source orientation between irradiations.

Functional airway obstruction observed with hyperpolarized 129Xenon-MRI

Hyperpolarized 129Xenon‐MRI (HP 129Xe MRI) is an emerging imaging modality that allows assessment of both ventilation and gas transfer. Most research to date has focused on non‐malignant pulmonary diseases. However, the capability of evaluating the two primary physiological processes of the lung (ventilation and gas transfer) makes HP 129Xe MRI a promising imaging modality in the management of patients with lung cancer.

TH-CD-BRA-11: Implementation and Evaluation of a New 3D Dosimetry Protocol for Validating MRI Guided Radiation Therapy Treatments

PURPOSE To develop, evaluate and apply a novel high-resolution 3D remote dosimetry protocol for validation of MRI guided radiation therapy treatments (MRIdian by ViewRay™). We demonstrate the first application of the protocol (including two small but required new correction terms) utilizing radiochromic 3D plastic PRESAGE™ with optical-CT readout. METHODS A detailed study of PRESAGE™ dosimeters (2kg) was conducted to investigate the temporal and spatial stability of radiation induced optical density change (ΔOD) over 8 days. Temporal stability was investigated on 3 dosimeters irradiated with four equally-spaced square 6MV fields delivering doses between 10cGy and 300cGy. Doses were imaged (read-out) by optical-CT at multiple intervals. Spatial stability of ΔOD response was investigated on 3 other dosimeters irradiated uniformly with 15MV extended-SSD fields with doses of 15cGy, 30cGy and 60cGy. Temporal and spatial (radial) changes were investigated using CERR and MATLABs Curve Fitting Tool-box. A protocol was developed to extrapolate measured ΔOD readings at t=48hr (the typical shipment time in remote dosimetry) to time t=1hr. RESULTS All dosimeters were observed to gradually darken with time (<5% per day). Consistent intra-batch sensitivity (0.0930±0.002 ΔOD/cm/Gy) and linearity (R2=0.9996) was observed at t=1hr. A small radial effect (<3%) was observed, attributed to curing thermodynamics during manufacture. The refined remote dosimetry protocol (including polynomial correction terms for temporal and spatial effects, CT and CR) was then applied to independent dosimeters irradiated with MR-IGRT treatments. Excellent line profile agreement and 3D-gamma results for 3%/3mm, 10% threshold were observed, with an average passing rate 96.5%± 3.43%. CONCLUSION A novel 3D remote dosimetry protocol is presented capable of validation of advanced radiation treatments (including MR-IGRT). The protocol uses 2kg radiochromic plastic dosimeters read-out by optical-CT within a week of treatment. The protocol requires small corrections for temporal and spatially-dependent behaviors observed between irradiation and readout.

SU-F-T-546: A Radiobiological Comparative Study of Robotic and LINAC-Based Lung SBRT

PURPOSE To expand the dosimetric comparison of CyberKnife and Elekta Versa HD regarding their capability to perform SBRT in lung cases by including radiobiological measures, which can estimate the clinical impact of the observed differences. METHODS This study analyzes treatment data of 10 patients, who were treated with SBRT on Elekta Versa HD and replanned using CyberKnife. The original structure sets from the RayStation treatment plans were transferred to MultiPlan and were used for planning, with the exception of the PTV. Special attention was given to the following structures during optimization in MultiPlan: PTV (expanded off GTV instead of ITV), ipsilateral and contralateral lungs, and chest wall. For each patient, the same dosimetric constraints were applied to both plans. The dose volume histograms (DVH) of the targets and organs at risk (OAR) were computed and used to compare the dosimetric differences between the two modalities. Additionally, the tumor control probability (TCP), normal tissue complication probability (NTCP) and complication-free tumor control probability (P+) radiobiological indices were calculated and compared. RESULTS The dosimetric indices show a negligible differences (Versa - CK) for the PTV (mean dose difference of 1.7 Gy and average minimum dose difference of -0.3 Gy). For the OARs, the average differences in mean and maximum doses, which varied between 0.4 to 1.7 Gy and 1.1 to 4.7Gy, respectively. The average differences in the radiobiological indices were also similar TCP = 0.3±0.6%, Composite NTCP = 2.4±2.9%, and P+ = -.1±3.3%, respectively. CONCLUSION Both modalities managed to meet the same dosimetric goals equally well. The similarities between the dosimetric and radiobiological indices indicate that the two treatment modalities are comparable. A larger patient cohort involving patients of different complexities would give a better insight for this comparison.

TH-AB-303-12: Commissioning of Magnetic Resonance Imaging-Based Tumor Tracking and Beam Control

Purpose: To develop a methodology for commissioning the real-time tumor tracking and beam control capabilities of a magnetic resonance imaging radiation therapy (MR-IGRT) system. Methods: The MRIdian (ViewRay, Inc., Oakwood Village, OH) MR-IGRT machine consists of a 0.35-T magnet and a three-head Co-60 delivery system capable of real-time sagittal plane acquisition at 4 frames per second (fps). After obtaining a volumetric MR image, a “target” is chosen by contouring an area of interest with different signal intensity than the surrounding medium. A “boundary” is created by expanding the target by some margin or using a previous contour. After a sagittal plane is chosen, during delivery the system deforms the target contour on each acquired plane, and compares it to the boundary - if the target contour is observed outside the boundary, the beam is turned off. Since the system uses no external or internal surrogates, careful tests of the individual system components had to be created and executed to characterize the accuracy and potential dosimetric errors associated with real-time gating on real anatomy. Results: Tests for spatial, temporal, and dosimetric accuracy were devised; these included latency, spatial integrity of the magnetic field, film, and ionization chamber measurements. A new set of phantoms was created, including a CIRS (Norfolk, VA) motion phantom that was modified to function in the magnetic field and have appropriate signal intensities. This phantom also included optical sensors that allowed for signal readout of its target position, and subsequent comparison to “beam off” signal on the heads. Boundary margins and other parameters were evaluated with film dosimetry by comparing the spatial distribution of the dose for static vs. gated deliveries. Ionization chambers were used to confirm an implemented shutter dose correction. Conclusion: The first commercially-available MR-IGRT system’s gating capabilities were characterized and successfully implemented in the clinic. Drs. Green, Kashani, and Mutic have held non-compensated consultant roles, received honoraria, and had travel expenses paid by ViewRay, Inc.