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Purpose: To develop tools to plan modulated electron radiotherapy (MERT) and to compare the MERT plans to conventional or intensity modulated radiotherapy (IMRT) treatment plans., Methods: Monte Carlo dose calculations of electron fields shaped with the inherent photon multileaf collimators (MLCs) were investigated in this study. Treatment plans for four postmastectomy breast cancer patients were generated using MERT. The distances from the patient skin surfaces to the distal planning target volume surfaces were computed along the beam axis direction to determine the physical depth. Electron beam energies were selected to provide target coverage at these depths and energy bins were generated. A custom built MERT treatment planning graphical user interface (MERTgui) was used to shape the electron bins into deliverable electron segments. Monte Carlo dose distribution simulations were performed using the MLC-defined segments generated from the MERTgui. A custom built superposition gui was used to combine doses for each segment using relative weights and final MERT treatment plans were compared to the conventional or IMRT treatment plans. In addition, a demonstration of combined MERT and IMRT treatment plans was performed., Results: The MERT treatment plans provided acceptable target organ coverage in all cases. Relative to 3D conventional or IMRT treatment plans, the MERT plans predicted lower heart doses in all cases; average of the heart D20 of all plans was reduced from 14.1 to 3.3 Gy. The contralateral breast and contralateral lung doses decreased substantially with MERT planning compared to IMRT (on average, contralateral breast heart D20 was reduced from 8.7 to 0.7 Gy and contralateral lung D20 was reduced from 8.4 to 1.2 Gy with MERT). Ipsilateral lung D20 was lower with MERT than with the conventional plans (44.6 vs 29.2 Gy with MERT), but greater when compared against IMRT treatment plans (25.4 vs 28.9 Gy with MERT). A MERT and IMRT combination plan was generated to benefit from the complementary advantages of MERT and IMRT, resulting in satisfactory target coverage and reduced organ at risk doses., Conclusions: MERT tools can facilitate treatment planning and provide plans for treatment of shallow targets such as the postmastectomy chest wall.

Purpose/objective(s): To evaluate Biology-guided Radiotherapy (BgRT) plans generated using PET data acquired on the first installation of the Reflexion™ X1 system. BgRT treatment planning optimizes for fluence indirectly by calculating firing filters that best translate the X1 acquired PET image to the fluences that deliver the prescribed radiation dose. During a treatment session, the firing filters are applied to limited time sampled (LTS) PET images to calculate delivery fluences.Materials/methods: An FDG fillable phantom insert that consists of a 22 mm diameter spherical target (PTV1, ball) and a C-shape object (PTV2 with 26mm axial length, 255° partial annulus with major and minor radii of 27 mm and 12 mm) was imaged using a CT Simulator to create planning contours. The water filled insert surrounding these objects was filled with FDG to simulate a warm background. Both targets were filled with FDG, simulating a target to background ratio of 8:1. Two different BgRT treatment plans were generated for PTV1 simulating homogenous uptake, and PTV2 with an inhomogeneous FDG uptake due to necrosis (a cylinder encompassing the C shape PET avid object). A structure that is 12.5 mm away from these targets was used to simulate OAR. Two PET/CT imaging acquisitions were performed on the RefleXion X1 masking the signal outside of each target plus 10 mm margin. Two different BgRT plans were generated prescribing 50 Gy delivered in 5 fractions to each target, separately. The quality of each treatment plan was evaluated based on PTV coverage, Conformity Index at 50% and 80% isodose lines (CI50, CI80) and OAR sparing. Both BgRT plans then delivered to the independent quality assurance tool for patient specific QA verification where the 3%/3 mm gamma criteria was used.Results: BgRT plans for both homogenous and heterogenous FDG uptake targets were created successfully. Prescription dose coverages were 94.2% and 96.3% for PTV1 and PTV2, respectively. The maximum OAR doses were 30.9 and 33.0 Gy and average OAR doses were 14.4 and 20.1 Gy for PTV1 and PTV2 plans, respectively. CI50 were 7.7 and 6.8 where CI80 were 2.5 and 2.6 for PTV1 and PTV2 plans. The patient specific QA passed with 100 % for both plans.Conclusion: This is the first report of the BgRT plan creation and QA delivery using a clinical installation of the Reflexion X1 system, opening the potential towards BgRT dose delivery using the real time FDG uptake measurements to biologically guide active radiation beam delivery. The X1 system was able to optimize and deliver the BgRT plans for both homogenous and heterogenous FDG uptake scenarios. BgRT is a new paradigm of dose optimization, where a new set of optimization and evaluation approaches will be needed. [ABSTRACT FROM AUTHOR]

A Biology-guided Radiotherapy (BgRT) based device is designed to use Positron Emission Tomography (PET) signals to achieve tracked dose delivery. The goal of this study is to investigate the dose delivery accuracy in case of interruption during BgRT treatment, and resumption in a separate treatment session for a multi-target delivery, as the PET activity continues to decay. A custom-built large anthropomorphic phantom (LAP) including a 26 mm spherical target with 3D independent motion and two 22 mm spherical targets with 1D sinusoidal motion embedded in water was used. All three targets were filled with FGD in an 8:1 target to background uptake ratio (41.52 kBq/ml in target and 5.19 kBq/ml in background). During BgRT delivery, the treatment was intentionally paused during delivery to the second target and the current treatment session was ended to generate a partial fraction. Then the partial fraction was continued in a new session, where the CT scan localization and PET pre-scan were repeated using the existing PET activity present in the phantom. The newly acquired PET pre-scan, was then used to determine if sufficient PET counts were present to resume treatment delivery. The interruption and recovery algorithm is designed to calculate the fluence that needs to be delivered to the remaining targets as well as the residual fluence to be given to the targets that have already received partial dose prior to the interruption. Once the new fluence is recomputed, the treatment is resumed. The delivered doses were captured using radiochromic film (EBT-XD) inserted in the target as well as post-treatment dose calculations based on the delivered beamlet sequence to evaluate the results in terms of dosimetric coverage and margin loss. The margin loss is calculated as the maximum difference between the distance from the Clinical Target Volume (CTV) contour to the 97% isodose contour in the treatment plan and the on the film. The dosimetric coverage is defined as the percentage of voxels within the CTV that lies within 97% and 130% of the prescribed dose. As shown in the table below, a margin loss of less than 3 mm for all targets and 100% CTV coverage was achieved. After treatment interruptions, the PET safety evaluation based on the PET pre-scan helped to determine whether the treatment could be continued on the same day using the same injected PET activity (an NTS value ≧ 2 and AC value ≧ 5 kBq/ml). This study demonstrated that the BgRT system is able to deliver the prescribed dose to all targets with independent motion, even when an interruption and resumption occurs during treatment. In case such an interruption if the remaining PET activity satisfies the BgRT safety evaluation, the treatment can continue to deliver the remainder of the BgRT doses. [ABSTRACT FROM AUTHOR]

We present the first dosimetric measurements of single session, multi-target BgRT deliveries using a clinically realistic motion phantom on a research-only version of the RefleXion X1 system. A custom-made anthropomorphic phantom of a human torso with embedded fillable targets mimicking 18F-FDG-avid lesions was used. From the three embedded spherical targets, Target 1 was 26 mm in diameter coupled with a 3D independent respiratory motion with 22 mm range, whereas Target 2 and 3 were 22 mm in diameter and moved with a 1D 5 mm maximum sinusoidal motion. The 18F-FDG concentration in the background cavity of the phantom was 5 kBq/ml, and the targets were loaded with 10:1, 8:1 and 6:1 contrast relative to the background for Targets 1, 2, 3, respectively. Spherical structures were contoured as GTVs (CTV = GTV) and a 5 mm margin was added to create PTVs. Motion extent of the tumors were captured to create biological tracking zones for each target. Treatment plans were generated using a research version of the Reflexion treatment planning software to deliver 8 Gy/fx to the PTVs. The treatment delivery was repeated 2 times, and each time the phantom was refilled according to the plan. PET image evaluation metrics for each of the three targets were also recorded. Target dosimetry was measured using a combination of radiographic film and ion chamber. The maximum distance between the 97% prescription isodose line from the plan and the film measurements was used to characterize the dosimetric accuracy of the tracked deliveries. CTV and PTV min, max, and mean doses measured on film were also recorded for each target. Treatment plans were successfully created with 100% prescription dose coverage to each target loaded with different FDG ratios. Total treatment times for the single-plan, three-target deliveries were less than 80 minutes. PET evaluation metrics at imaging-only and pre-scan, and planning and film dosimetry to the GTV and PTV for each of the three targets is shown in table below (mean ± standard deviation of both deliveries). The CTV dose coverage was maintained for all targets. The shrinkage distance of the 97% prescription dose isodose line on the film plane for all three targets was less than 3 mm for both tests, and ranged from -0.4 to -2.34 mm. These results demonstrate that high tracking accuracy and dosimetric accuracy can be achieved in single session, multi-target deliveries over a range of target-to-background 18F-FDG concentrations and target motion patterns. [ABSTRACT FROM AUTHOR]

The first clinical biology-guided radiation therapy (BgRT) system – RefleXionTM X1 - was installed and commissioned for clinical use at our institution. This study aimed at evaluating the treatment plan quality and delivery efficiency for IMRT/SBRT cases without PET-guidance. Forty-two patient plans across six cancer sites (conventionally-fractionated lung, head and neck, anus, prostate, brain, and lung SBRT) planned with the EclipseTM treatment planning system (TPS) and treated with either a TrueBeam® or Trilogy® were selected for this retrospective study. For each Eclipse VMAT plan, two corresponding plans were generated on the X1 TPS with 10 mm jaws (X1 10mm) and 20 mm jaws (X1 20mm) using our institutional planning constraints. All clinically relevant metrics in this study, including PTV D95%, PTV D2%, Conformity Index (CI), R50, organs- at-risk (OAR) constraints, and beam-on time were analyzed and compared between 126 VMAT and RefleXion plans using paired t-tests. All but three planning metrics were either equivalent or superior for the X1 10mm plans as compared to the Eclipse VMAT plans across all planning sites investigated. The Eclipse VMAT and X1 10mm plans generally achieved superior plan quality and sharper dose fall-off superior/inferior to targets as compared to the X1 20mm plans, however, the X1 20mm plans were still considered acceptable for treatment. Strong correlation was observed between the TPS-calculated beam-on time and PTV length in the superior-inferior direction for the X1 10mm (R 2 = 0. 87) and X1 20mm (R 2 = 0. 85) plans. On average, the required beam-on time increased by a factor of 1.6 across all sites for X1 10mm compared to X1 20mm plans. Clinically acceptable IMRT/SBRT treatment plans were generated with the X1 TPS for both the 10 mm and 20 mm jaw settings. The X1 10mm plans were equivalent or superior to the Eclipse VMAT plans, however the beam-on times were longer. [ABSTRACT FROM AUTHOR]

Studies show ultra-high dose rate (UHDR) radiotherapy (RT), at rates of ≥40 Gy per second, can produce the FLASH effect: reduced toxicity with equal tumor control when compared to conventional RT. Leveraging these benefits for clinical translation can profoundly impact RT. Early clinical trials exhibited safe delivery of UHDR-RT using specialized RT machines. Our prior study showed that a reversible configuration of a standard linear accelerator (LINAC) can be optimized for UHDR output. Here, we explored the components necessary for clinically practical delivery of LINAC-based electron FLASH RT. Using a standard, clinically operational system for image-guided radiation, we configured the program board for a decommissioned electron beam energy for UHDR delivery. No LINAC hardware manipulation or exchange was performed. Key changes included adjustment of the dose per pulse and pulse delivery control using the gating interface. Of note, the scattering foil remained in place within the beam so that field sizes and depth doses could approximate those of standard clinical operations. A commercial electron arc applicator was installed at the face of the gantry using the standard applicator mount to allow for short SSD electron collimation. Copper electron blocks were fabricated with apertures ranging from 3.0 to 20.0 cm. Dose rates were measured at dmax for 100 to 59 cm source-to-surface distances (SSD) (level of machine head) and beam profile measurements were captured at 80 cm SSD using an IC profiler and radiographic film. UHDR rates ranging from 36.8 to 112.8 Gy/s were measured at 100 to 59 cm SSD, respectively. An 80 cm SSD was selected for treatment arrangement measurements to maintain UHDR and clinical utility, allowing for 13.5 cm of clearance. UHDR ≥60 Gy/s was observed at 80 cm SSD with all field sizes, using a standard, scattered electron beam, with an R80 depth of 6.1 cm, approximating to a 16-18 MeV energy. Average dose per pulse was measured at 0.33 Gy/pulse. Circular block apertures of 3, 10, and 20 cm exhibited beam flatness <5.0 % variability and symmetry of 2.2 to 6.2%. Penumbra width between the 80 and 20% isodose lines (IDL) measured at 0.7, 1.1, and 1.4 cm, respectively. Optimizing a reversible setup for UHDR irradiation on a standard LINAC, electron delivery at dose rates expected to produce the FLASH effect was achieved. Flat, homogenous field sizes, ranging from 3 to 20 cm of coverage, were obtained at a clinically practical SSD of 80 cm with a treatment depth of 6.1 cm to the 80% IDL. This setup would enable UHDR irradiation at useful depths for a number of different clinical indications, bringing us a step closer to practical UHDR RT using a standard LINAC in future clinical trials. [ABSTRACT FROM AUTHOR]

Ultra-high dose rate (UHDR) radiotherapy (RT) at ≥40 Gy/s has been shown preclinically to reduce toxicity with similar tumor control compared to conventional RT, a phenomenon known as the FLASH effect. We previously described a configuration of a clinical-use LINAC that can output ∼17 MeV electron UHDR RT at 45-80 Gy/s with clinically feasible beam profiles, field sizes (3-20 cm), and SSDs (70-90 cm). Safe and effective clinical trials are needed to advance translation of FLASH treatment in humans, such as one proposed at our institution for cutaneous lymphomas. This study characterizes the beam output of our electron UHDR RT set-up under realistic clinical conditions for common anatomic sites using an anthropomorphic phantom, to assess its feasibility for clinical translation. Our electron UHDR RT set-up utilizes a treatment delivery system with a configured program board of a decommissioned electron energy to adjust the beam output and control, without hardware modification. The electron cone is replaced with a flat applicator with custom cut-outs that lies in the accessory mount. We tested this set-up under realistic clinical conditions on a RANDO phantom for treatment of the chest/breast, leg, and scalp. We used 3 cm and 10 cm circular field sizes at 80 cm SSD with the gantry angled en-face to the targets. Tissue-equivalent bolus and flexible tungsten-silicone skin collimators (made from silicone sheets with tungsten powder cut into donut shapes of varying diameters) were used to mimic clinical treatment standards. Measurements were done using radiographic film and we evaluated the beam width, percent depth dose, and penumbra width. All results reported include 1 cm bolus and skin collimators unless otherwise specified. The values for R80 (depth at 80% dose), beam width of 80% dose (at R95), and penumbra width (at R85) for the 10 cm chest/breast field are 5.2, 8.7, and 0.9 cm, respectively. Without skin collimators, the values are 5.4, 9.0, and 1.2 cm, respectively. The values for the 3 cm chest/breast field are 2.9, 1.9, and 0.6 cm, respectively. Without skin collimators, the values are 3.2, 1.9, and 0.7 cm, respectively. The values for the 10 cm leg field are 4.0, 6.6, and 0.9 cm, respectively. The values for the 3 cm scalp field (with 4 cm bolus to minimize dose penetration to the brain) are 1.1, 1.6, and 0.9 cm, respectively. The steep curvature of the leg and scalp sites led to expected decrease in beam flatness but retained a treatment area that allows for a reasonable range of tumor sizes and depths. Our configuration is able to deliver electron UHDR RT with clinically applicable beam profiles to various anatomic sites under realistic clinical conditions. Tungsten-silicone skin collimators are an easy-to-use tool to narrow the penumbra. Overall, these results show potential for treatment of superficial indications, including cutaneous lymphoma, extremity sarcoma, and partial breast irradiation, and place us one step closer toward the clinical translation of FLASH. [ABSTRACT FROM AUTHOR]

Purpose/objective(s): The first clinical biology-guided radiation therapy (BgRT) system - RefleXion X1 - is installed and is being commissioned for clinical use at our institution. The system delivers 6MV-FFF beam via 50 firing positions at 850 cGy/min dose rate on 60 rpm rotating gantry with the couch advancing in 2.1 mm increments. Collimation is achieved through a binary multi-leaf collimator system composed of 64 (6.25 mm width at 850 cm SAD) leaves and 10mm or 20mm jaws. The purpose of this study was to compare the treatment plan quality and delivery efficiency for SBRT cases without PET-guidance.Materials/methods: Five lung cancer patients (50 Gy/4fx) with motion-inclusive PTV previously treated with VMAT SBRT were selected for this retrospective study. For each VMAT plan a corresponding plan was generated on the RefleXion treatment planning system (TPS) using our institutional planning constraints. VMAT SBRT plans used 1-2 partial arcs with 10MV-FFF energy and 2400 MU/min dose rate. RefleXion SBRT plans used all 50 firing positions and 1 cm jaw collimation. As plan normalization option does not yet exist in the RefleXion TPS, the plans were optimized to achieve PTV D95% coverage with 100% of prescription dose to the best of planner's ability. All clinically relevant metrics in this study, including plan PTV D95%, PTV D1%, 50% isodose volume (I50%), Conformity Index (CI), organs at risk constraints, and treatment time were analyzed and compared between VMAT and RefleXion plans using paired t-tests.Results: Clinically acceptable plans were obtained with both techniques. The average PTV volume was 14.1 ± 6.4cc. For VMAT and RefleXion plans, no statistically significant difference was observed in PTV D95%, PTV D1%, CI or I50%. Ipsilateral lung volume spared at 12.4 Gy (VS12.4) was slightly greater (31.6cc increase, P < 0.035) and skin Dmax was slightly lower (1.4 Gy decrease, P < 0.038) for RefleXion plans. Treatment beam-on time was significantly longer for RefleXion plans (23.4min increase, P < 0.0002).Conclusion: RefleXion TPS provided comparable plan quality to lung SBRT VMAT plans. Treatment beam-on time is significantly longer for the RefleXion system. [ABSTRACT FROM AUTHOR]

Purpose/objective(s): A novel therapeutic machine that combines PET, fan beam kVCT and linear accelerator to support real-time PET-guided treatment, termed biology-guided radiotherapy (BgRT). This study reports the positioning accuracy, image quality, and dose commissioning of the kVCT imaging system of the first clinical BgRT machine at our institute.Materials/methods: The helical fan-beam kVCT subsystem consists of an X-ray tube with a voltage of 120kV and a 16-row GOS Ceramic Scintillator detector. The gantry rotation speed is 60 rpm. The tube currents are 45, 67, 133mA for 0.5 pitch; 100, 150, and 300mA for 0.5 pitch; 267 and 400mA for 1.333 pitch. The image slice thickness is 1.25mm with a field of view of 500mm and a resolution of 512 × 512. A ball-cube phantom was used to test the positioning accuracy of the kVCT subsystem with respect to the setup laser and MV beam. Catphan504 phantom was imaged with 0.5 pitch and 150mAs to test the kVCT image quality of the BgRT system, including the uniformity, noise, HU constancy, spatial resolution, geometric distortion, slice thickness, and low contrast visibility and detectability using the PIPSproTM software. The RTI MAS-2 and Piranha detectors were used to verify the accuracy of the mA and voltage. The CTDI dose was measured using a CTDI phantom and a Standard Imaging A101 pencil chamber.Results: The localization error of the ball cube phantom was -0.8, 0.9, and 0.3mm in IEC x, y, and z directions, respectively. For the Catphan504 tests, the vertical and horizontal uniformities were 0.926 and 0.857, respectively. The HU constancy tests of the air, water, polystyrene, acrylic, Delrin, Teflon were all within 40HU to the expected values. The modulation transfer function test results were 0.467, 0.528, and 0.595 lp/mm for F50, F40, and F30, respectively, which showed a comparable spatial resolution to regular CT simulators. The geometric distortion was 0.12mm and slice thickness inaccuracy was 0.13mm. The low contrast detectability was 0.734. The measured mA and voltage were within 5% of the nominal setting of 120 kV and 150 mA. The CTDIw was less than 1.4 cGy for body and 3 cGy for head with a protocol of 120 kVp, 150 mAs, and 1.25 mm slice thickness.Conclusion: This evaluation represents the kVCT characteristics of the novel BgRT system with architecture designed to accommodate CT, PET and a LINAC. The image quality and HU constancy are comparable to our CT simulators and is a useful tool for online adaptive radiotherapy. The commissioning data will serve as a standard for units installed at future clinical sites. [ABSTRACT FROM AUTHOR]

Purpose/objective(s): A biologically guided radiotherapy (BgRT) system was developed and equipped with a novel O-ring gantry Linac, positron emission tomography (PET) and computed tomography technologies. This architectural design is to support real-time PET-guided treatment, termed biology-guided radiotherapy. To reduce latency between PET data acquisition and radiation delivery, the BgRT system incorporates a fast-transitioning binary multi-leaf collimator (MLC) leaves sandwiched between a split-jaw. This study reports the dosimetric commissioning of the first clinical BgRT machines at our institution, and compared with the reference data provided by the manufacturer.Materials/methods: The dosimetric commissioning of the 6 MV flattening filter-free (FFF) photon beam was characterized using a 3D water phantom and diode detectors at a source to surface distance of 85 cm. In-air profiles of 40cm x 2cm field, 40cm x 1cm field, and various leaf combinations plus in water PDD data of 12 field sizes (1.25, 2.5, 5, 10, 20 and 40 cm MLC opened leaves with 1 and 2cm jaw settings) were acquired first for beam modeling. Additional longitudinal and transverse profile scans were performed for these 12 field sizes at the depth of 1.5, 5, 10, 15 and 20 cm to verify the beam model accuracy. The relative outputs factors were measured with the W2 1mmx1mm scintillator at the depth of 10cm for the field sizes from the smallest 0.6cm x 1cm to the maximum 40cm x 2cm.Results: For the PDDs of reference field size of 10cm x 2cm measured at Stanford, the maximum doses (dmax) were 12.5mm within the manufacturer's reference range of 13 ± 0.75 mm. The percentage doses at 10cm depth (d10) at reference field size were 56.9% and 57.6% for Stanford and RefleXion measurements, respectively. The longitudinal field sizes for all tested fields were agreed within 0.5mm and the transverse off-axis ratio is within 1% in the beam core (80% of nominal field size). The output factors measured at our institute were ranging from 0.706 to 1.012 relative to the reference 10cm x 2cm field.Conclusion: This evaluation represents the first commissioning evaluation of the dosimetric characteristics of the BgRT system with architecture designed to accommodate both PET detector and LINAC subsystems. This data will serve as a clinical reference for the BgRT treatment planning system's beam model and can act as a standard for comparison and beam model improvement for units installed at future clinical sites. [ABSTRACT FROM AUTHOR]

Purpose/objective(s): Biology-guided radiotherapy (BgRT) integrates PET detectors with a LINAC and delivers a real-time tracked dose of radiation to moving targets using a continuous stream of limited time-sampled (LTS) PET images as a biological fiducial. These LTS PET images are converted into fluences and delivered to the target that cumulatively sum to deliver the intended prescription dose to the planning target volume (PTV). Physical validation of BgRT directed at a moving target has not previously been reported.Materials/methods: An 18F-Fluorodeoxyglucose (FDG) fillable insert that fits inside the independent quality assurance tool cavity was used in these experiments. The insert included a 22mm diameter target enclosed within a Styrofoam mesh compartment, simulating a lung-like background. The insert was filled with FDG with a target-to-background ratio of 8:1, placed in the independent quality assurance tool cavity which was then mounted on a custom motion stage that could move in the axial direction (IEC-Y). Motion was simulated using a breathing waveform that had a large amplitude motion (from -14.3 mm to +14.4 mm with respect to the mean position), with random amplitude and period with a baseline shift over time, simulating breathing baseline shifts. A BgRT plan (prescription of 8Gy) was created using motion averaged PET images, acquired on a pre-commercial version of the RefleXion system using a 32 × 32 × 32mm (IEC-X, Y, Z) PTV - 5mm expansion over the clinical target volume (CTV). An internal tumor volume (ITV) based SBRT plan (prescription of 10Gy) with a 32 × 32 × 62mm PTV covering the motion extent was also created. Radiochromic EBT-XD film was inserted into a slot in the spherical target to measure the delivered dose. Using the same motion waveform, BgRT and SBRT plans were delivered to the moving target, with an additional 8Gy BgRT delivery (32 × 32 × 32mm PTV) to the stationary target serving as control. Dosimetric accuracy for moving targets was measured using the concept of coverage where 100% of the CTV receives at least 97% of the prescription dose and maximum dose in the CTV was = < 130% of the maximum planned dose.Results: Under motion, both SBRT (32 × 32 × 62mm PTV) and BgRT (32 × 32 × 32mm PTV) met CTV dose coverage. CTV dose ranges were a) Motion SBRT: measured = [10.4Gy,12.4Gy], plan = [9.7Gy, 17.2Gy], b) Motion BgRT: measured = [9.7Gy,11.2Gy], plan = [7.8Gy, 15.3Gy], c) Static BgRT: measured = [8.7Gy,10.1Gy], plan = [7.8Gy), 10.4Gy]. Motion coverage results indicate that BgRT with a PTV that is 48.4% smaller than the SBRT PTV, delivering a tracked dose to the moving target., indicating a more conformal delivery.Conclusion: This is the first report of BgRT delivery achieving a tracked dose distribution directed at a moving target. Because tracked distributions have better conformality and normal tissue sparing than free-breathing internal tumor volume approaches, BgRT may improve the toxic-therapeutic ratio for applications such as early-stage lung cancer. [ABSTRACT FROM AUTHOR]