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Stereotactic ablative radiation therapy (SABR) leads to excellent local control in patients with primary or metastatic lung tumors. While there are multiple strategies for motion management in thoracic SABR, there is limited data to compare the efficacy of different approaches. We compared local tumor control by the extent of tumor motion and the motion management strategy for patients treated on the iSABR clinical trial. This is a secondary analysis of patients treated on the multicenter phase II iSABR clinical trial that utilized an individualized dosing strategy for patients with primary or metastatic lung tumors. We evaluated the extent of tumor motion on 4D CT and the motion management approach used during treatment (free breathing, breath hold [BH], or expiratory gating [EG]). For patients treated with free breathing or EG approaches, an internal target volume (ITV) was contoured based on tumor movement on the 4D CT. Patients were treated on a linear accelerator using Real-time Position Management (RPM) triggering. In-field local control rates were compared using the Kaplan-Meier method. In total, 62 patients with 63 tumors had available data on motion management. Fifteen patients (24.2%) underwent SABR with free breathing, 22 (35.5%) had BH (20 inspiratory, and 2 expiratory), and 26 (41.9%) had EG (24 amplitude based, and 2 phase based). 1-, and 2-year local control rates were 100% and 100% with free breathing, 94.7% and 83.2% with BH, and 90.7% and 78.6% with EG (p = 0.2), respectively. Overall, median maximal tumor motion was 10mm (3.5mm for tumors treated with free breathing vs 14.5mm for tumors treated with BH or EG). There was no association between local control and maximum extent of motion on 4D CT (HR = 1.01, p = 0.75). 1- and 2-year local control rates were 92.9% and 89.1% for tumors with maximal motion <10mm, and 96.3% and 82.5% for tumors with maximal motion >10mm (HR = 0.6, p = 0.53), respectively. Patients on the iSABR trial were treated with a range of motion management strategies including motion inclusive free breathing, BH and EG with no observed association between local control by treatment approach. The similar local control rates between mobile and non-mobile tumors suggests that the RPM approach used on the iSABR trial was effective. Our results will be further analyzed pending motion management data collection for the entire iSABR cohort. [ABSTRACT FROM AUTHOR]

Biology-guided radiotherapy (BgRT) is a novel tracked dose delivery modality using real-time positron emission tomography (PET) to guide radiotherapy beamlets. The present study was performed with sequential cohorts of participants to evaluate the performance and safety of BgRT. Primary endpoints were previously reported. We hereby report on one of the secondary endpoints assessing a novel treatment planning machine with integrated dual kVCT/PET imaging ("novel device") performance in comparison to a third-party diagnostic PET/CT scan. This single-arm, open-label, prospective study included participants with at least 1 FDG-avid targetable primary or metastatic tumor (≥2cm and ≤5cm) in the lung or bone. PET imaging data were collected on the novel device and on a third-party diagnostic PET/CT performed in sequence once at the planning timepoint in Cohort I, and immediately before the last fraction among patients undergoing stereotactic radiotherapy in Cohort II. Three central read radiation oncologists (CRRO) provided an interpretation of the novel device PET scans which were compared to an agreement standard based on 3 central radiologists' review of the paired diagnostic PET/CT scan. Positive percent agreement for localization of the target tumor within the biology-tracking zone (BTZ) was the key metric because it reflects whether advancing patients to subsequent steps in the BgRT workflow based on the novel device's imaging was ultimately appropriate. In Cohort 1, 6 image comparisons were performed. The positive (%) agreement for the aggregate radiation oncologist review was 100% (5/5), reflecting that in all 5 cases where the aggregate radiation oncologists deemed the tumor to fall within the BTZ based upon the novel device PET images, the central radiologists came to the same conclusion upon review of the paired diagnostic PET/CT images. The overall (%) agreement for the aggregate radiation oncologist review was 83.3% (5/6): localization was not established on the novel device in 1 case, even though it was established on the diagnostic PET/CT. This would not pose risk in real world practice as BgRT candidacy would be aborted for tumors not visible on the novel device. In Cohort II, among the 7 image comparisons, there was 100% positive percent agreement between the aggregate CRRO and the agreement standard as the localization criteria was met in both scans for all 7 patients. This was concordant with a 100% overall percent agreement. This investigation demonstrated a 100% positive percent agreement between central review of this novel device images by radiation oncologists and central review of the accompanying third-party PET/CT images by radiologists. There were no cases where a positive localization by the aggregate CRRO was not confirmed by the third-party PET/CT standard, providing evidence against the likelihood of falsely positive localizations on the novel device that would inappropriately advance patients in the workflow. [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): The X1 system ("X1") integrates a fully functional Positron Emission Tomography (PET) combined with a fan-beam kVCT system into the architecture of a ring-gantry linear accelerator (LINAC) for biology-guided radiotherapy (BgRT) delivery. The integration of PET into a LINAC framework required design features that deviate from the diagnostic setting. For instance, the X1 houses two opposed 90° PET arcs that rotate at 60 RPM to generate sinogram reconstructed images rather than a static, circumferential array of detectors. To better understand X1 PET performance, we conducted a side-by-side comparison of images of a PET phantom obtained sequentially on the X1 PET subsystem and a traditional PET/CT scanner.Materials/methods: An independent quality assurance phantom utilized a custom insert with two-insert targets filled with 18F-fluorodeoxyglucose (FDG) diluted water. Spherical target had a diameter of 22 mm and C-shape geometry consisted of an active volume of 26mm axial length and 255-degree partial annulus with major and minor radii of 27 mm x 12 mm, respectively. The targets were filled with an activity concentration of 9.3 kBq/ml to achieve a target to background ratio of 8:1 simulating the target uptake with hot background in a patient. The phantom was imaged within 30 minutes of preparation on the X1 PET and then immediately transferred to a PET/CT Biograph scanner for a second image. This experiment was repeated three times in separate days to assess reproducibility of PET acquisition. The PET images were visually assessed by the radiation oncologists to ensure the PET signal is within PTV. The PET signals of the targets were also evaluated using the full-width-half-max (FWHM) calculated on left-right (RL), anterior-posterior (AP) and superior-inferior (SI) directions for both targets and compared to the physical dimensions of the cavities.Results: The imaging acquisition tests were successfully completed on all three days for both systems, as per the prescribed workflow. In all experiments, radiation oncologists verified that the PET avid area were within the PTV in both systems. Average FWHM results of the 22 mm spherical target on X1 PET were 24.5, 21.7 and 17.0 mm as compared to 20.0, 20.0 and 18.2 mm for PET/CT in LR, AP and SI directions, respectively. For the C-shape object X1 PET resulted in 18.8, 19.5, 21.8 and 25.0 mm whereas the PET/CT resulted in 12.7, 13.2, 13.2 and 24.7 mm FWHMs for the two RL C-shape legs, AP and SI directions, respectively as compared to 15-, 15-, 15- and 26-mm physical dimensions.Conclusion: The X1 PET subsystem - which utilizes dual PET arcs and a sinogram image reconstruction algorithm in order to integrate into a LINAC architecture - generates images that are qualitatively comparable to a traditional PET scanner. In general, X1 FWHM results were slightly larger and PET/CT ones were smaller than the physical dimensions in axial plane. Both systems slightly underestimated the target size in SI direction. [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]