ACR benchmark testing of a novel high-speed ring-gantry linac kV-CBCT system.

A new generation cone-beam computed tomography (CBCT) system with new hardware design and advanced image reconstruction algorithms is available for radiation treatment simulation or adaptive radiotherapy (HyperSight CBCT imaging solution, Varian Medical Systems-a Siemens Healthineers company). This study assesses the CBCT image quality metrics using the criteria routinely used for diagnostic CT scanner accreditation as a first step towards the future use of HyperSight CBCT images for treatment planning and target/organ delineations. Image performance was evaluated using American College of Radiology (ACR) Program accreditation phantom tests for diagnostic computed tomography systems (CTs) and compared HyperSight images with a standard treatment planning diagnostic CT scanner (Siemens SOMATOM Edge) and with existing CBCT systems (Varian TrueBeam version 2.7 and Varian Halcyon version 2.0).  Image quality performance for all Varian HyperSight CBCT vendor-provided imaging protocols were assessed using ACR head and body ring CT phantoms, then compared to existing imaging modalities. Image quality analysis metrics included contrast-to-noise (CNR), spatial resolution, Hounsfield number (HU) accuracy, image scaling, and uniformity. All image quality assessments were made following the recommendations and passing criteria provided by the ACR. The Varian HyperSight CBCT imaging system demonstrated excellent image quality, with the majority of vendor-provided imaging protocols capable of passing all ACR CT accreditation standards. Nearly all (8/11) vendor-provided protocols passed ACR criteria using the ACR head phantom, with the Abdomen Large, Pelvis Large, and H&N vendor-provided protocols produced HU uniformity values slightly exceeding passing criteria but remained within the allowable minor deviation levels (5-7 HU maximum differences). Compared to other existing CT and CBCT imaging modalities, both HyperSight Head and Pelvis imaging protocols matched the performance of the SOMATOM CT scanner, and both the HyperSight and SOMATOM CT substantially surpassed the performance of the Halcyon 2.0 and TrueBeam version 2.7 systems. Varian HyperSight CBCT imaging system could pass almost all tests for all vendor-provided protocols using ACR accreditation criteria, with image quality similar to those produced by diagnostic CT scanners and significantly better than existing linac-based CBCT imaging systems.

Impact of the MLC leaf-tip model in a commercial TPS: Dose calculation limitations and IROC-H phantom failures.

PURPOSE: Treatment planning system (TPS) dose calculation is sensitive to multileaf collimator (MLC) modeling, especially when treating with intensity-modulated radiation therapy (IMRT) or VMAT. This study investigates the dosimetric impact of the MLC leaf-tip model in a commercial TPS (RayStation v.6.1). The detectability of modeling errors was assessed through both measurements with an anthropomorphic head-and-neck phantom and patient-specific IMRT QA using a 3D diode array. METHODS AND MATERIALS: An Agility MLC (Elekta Inc.) was commissioned in RayStation. Nine IMRT and VMAT plans were optimized to treat the head-and-neck phantom from the Imaging and Radiation Oncology Core Houston branch (IROC-H). Dose distributions for each plan were re-calculated on 27 beam models, varying leaf-tip width (2.0, 4.5, and 6.5 mm) and leaf-tip offset (-2.0 to +2.0 mm) values. Doses were compared to phantom TLD measurements. Patient-specific IMRT QA was performed, and receiver-operating characteristic (ROC) analysis was performed to determine the detectability of modeling errors. RESULTS: Dose calculations were very sensitive to leaf-tip offset values. Offsets of ±1.0 mm resulted in dose differences up to 10% and 15% in the PTV and spinal cord TLDs respectively. Offsets of ±2.0 mm caused dose deviations up to 50% in the spinal cord TLD. Patient-specific IMRT QA could not reliably detect these deviations, with an ROC area under the curve (AUC) value of 0.537 for a ±1.0 mm change in leaf-tip offset, corresponding to >7% dose deviation. Leaf-tip width had a modest dosimetric impact with <2% and 5.6% differences in the PTV and spinal cord TLDs respectively. CONCLUSIONS: Small changes in the MLC leaf-tip offset in this TPS model can cause large changes in the calculated dose for IMRT and VMAT plans that are difficult to identify through either dose curves or standard patient-specific IMRT QA. These results may, in part, explain the reported high failure rate of IROC-H phantom tests.

Predicted Inferior Outcomes for Lung SBRT With Treatment Planning Systems That Fail Independent Phantom-Based Audits.

PURPOSE: More than 15% of radiation therapy clinics fail external audits with anthropomorphic phantoms conducted by Imaging and Radiation Oncology Core-Houston (IROC-H) while passing other industry-standard quality assurance (QA) tests. We seek to evaluate the predicted effect of such failed plans on outcomes for patients treated with stereotactic body radiation therapy (SBRT) for lung tumors. METHODS AND MATERIALS: We conducted a retrospective study of 55 patients treated with SBRT for lung tumors with a prescription biologically equivalent dose (BED)10 ≥100 Gy using a treatment planning system (TPS) that passed IROC-H phantom audits. Sample linear accelerator beam models with introduced errors were commissioned by varying the multileaf collimator leaf-tip offset parameter (ie, dosimetric leaf gap) over the range ±1.0 mm relative to the validated model. These models mimic TPS that pass internal QA measures but fail IROC-H tests. Patient plans were recalculated on sample beam models. The predicted tumor control probability (TCP) and normal tissue complication probability (NTCP) were calculated based on published dose-response models. RESULTS: A leaf-tip offset value of -1.0 mm decreased the fraction of plans receiving a planning treatment volume of BED10 ≥100 Gy from 95% to 27%. This translated to a significant decrease in 2-year TCP of 4.8% (95% CI: 2.0%-5.5%) with a decrease in TCP up to 21%. Conversely, a leaf-tip offset of +1.0 mm resulted in 36% of patients exceeding previously met organs at risk (OAR) constraints, including 2 instances of spinal cord and brachial plexus overdoses and a small increase in chest wall NTCP of 0.7%, (95% CI: 0.5%-0.8%). CONCLUSIONS: Simulated treatment plans with modest MLC leaf offsets result in lung SBRT plans that significantly underdose tumor or exceed OAR constraints. These dosimetric endpoints translate to significant detriments in TCP. These simulated plans mimic planning systems that pass internal QA measures but fail independent phantom-based tests, underscoring the need for enhanced quality assurance including external audits of TPS.

Radiation shielding and safety implications following linac conversion to an electron FLASH-RT unit.

PURPOSE: Due to their finite range, electrons are typically ignored when calculating shielding requirements in megavoltage energy linear accelerator vaults. However, the assumption that 16 MeV electrons need not be considered does not hold when operated at FLASH-RT dose rates (~200× clinical dose rate), where dose rate from bremsstrahlung photons is an order of magnitude higher than that from an 18 MV beam for which shielding was designed. We investigate the shielding and radiation protection impact of converting a Varian 21EX linac to FLASH-RT dose rates. METHODS: We performed a radiation survey in all occupied areas using a Fluke Biomedical Inovision 451P survey meter and a Wide Energy Neutron Detection Instrument (Wendi)-2 FHT 762 neutron detector. The dose rate from activated linac components following a 1.8-min FLASH-RT delivery was also measured. RESULTS: When operated at a gantry angle of 180° such as during biology experiments, the 16 MeV FLASH-RT electrons deliver ~10 µSv/h in the controlled areas and 780 µSv/h in the uncontrolled areas, which is above the 20 µSv in any 1-h USNRC limit. However, to exceed 20 µSv, the unit must be operated continuously for 92 s, which corresponds in this bunker and FLASH-RT beam to a 3180 Gy workload at isocenter, which would be unfeasible to deliver within that timeframe due to experimental logistics. While beam steering and dosimetry activities can require workloads of that magnitude, during these activities, the gantry is positioned at 0° and the dose rate in the uncontrolled area becomes undetectable. Likewise, neutron activation of linac components can reach 25 µSv/h near the isocenter following FLASH-RT delivery, but dissipates within minutes, and total doses within an hour are below 20 µSv. CONCLUSION: Bremsstrahlung photons created by a 16 MeV FLASH-RT electron beam resulted in consequential dose rates in controlled and uncontrolled areas, and from activated linac components in the vault. While our linac vault shielding proved sufficient, other investigators would be prudent to confirm the adequacy of their radiation safety program, particularly if operating in vaults designed for 6 MV.

A Dose of Reality: How 20 Years of Incomplete Physics and Dosimetry Reporting in Radiobiology Studies May Have Contributed to the Reproducibility Crisis.

PURPOSE: A large proportion of preclinical or translational studies using radiation have poor replicability. For a study involving radiation exposure to be replicable, interpretable, and comparable, its experimental methodology must be well reported, particularly in terms of irradiation protocol, including the amount, rate, quality, and geometry of radiation delivery. Here we perform the first large-scale literature review of the current state of reporting of essential experimental physics and dosimetry details in the scientific literature. METHODS AND MATERIALS: For 1758 peer-reviewed articles from 469 journals, we evaluated the reporting of basic experimental physics and dosimetry details recommended by the authoritative National Institute of Standards and Technology symposium. RESULTS: We demonstrate that although some physics and dosimetry parameters, such as dose, source type, and energy, are well reported, the majority are not. Furthermore, highly cited journals and articles are systematically more likely to be lacking experimental details related to the irradiation protocol. CONCLUSIONS: These findings show a crucial deficiency in the reporting of basic experimental details and severely affect the reproducibility and translatability of a large proportion of radiation biology studies.

Characterizing a deformable registration algorithm for surface-guided breast radiotherapy.

PURPOSE: Surface-guided radiation therapy (SGRT) is a nonionizing imaging approach for patient setup guidance, intra-fraction monitoring, and automated breath-hold gating of radiation treatments. SGRT employs the premise that the external patient surface correlates to the internal anatomy, to infer the treatment isocenter position at time of treatment delivery. Deformations and posture variations are known to impact the correlation between external and internal anatomy. However, the degree, magnitude, and algorithm dependence of this impact are not intuitive and currently no methods exist to assess this relationship. The primary aim of this work was to develop a framework to investigate and understand how a commercial optical surface imaging system (C-RAD, Uppsala, Sweden), which uses a nonrigid registration algorithm, handles rotations and surface deformations. METHODS: A workflow consisting of a female torso phantom and software-introduced transformations to the corresponding digital reference surface was developed. To benchmark and validate the approach, known rigid translations and rotations were first applied. Relevant breast radiotherapy deformations related to breast size, hunching/arching back, distended/deflated abdomen, and an irregular surface to mimic a cover sheet over the lower part of the torso were investigated. The difference between rigid and deformed surfaces was evaluated as a function of isocenter location. RESULTS: For all introduced rigid body transformations, C-RAD computed isocenter shifts were determined within 1 mm and 1˚. Additional translational shifts to correct for rotations as a function of isocenter location were determined with the same accuracy. For yaw setup errors, the difference in shift corrections between a plan with an isocenter placed in the center of the breast (BrstIso) and one located 12 cm superiorly (SCFIso) was 2.3 mm/1˚ in lateral direction. Pitch setup errors resulted in a difference of 2.1 mm/1˚ in vertical direction. For some of the deformation scenarios, much larger differences up to 16 mm and 7˚ in the calculated shifts between BrstIso and SCFIso were observed that could lead to large unintended gaps or overlap between adjacent matched fields if uncorrected. CONCLUSIONS: The methodology developed lends itself well for quality assurance (QA) of SGRT systems. The deformable C-RAD algorithm determined accurate shifts for rigid transformations, and this was independent of isocenter location. For surface deformations, the position of the isocenter had considerable impact on the registration result. It is recommended to avoid off-axis isocenters during treatment planning to optimally utilize the capabilities of the deformable image registration algorithm, especially when multiple isocenters are used with fields that share a field edge.

Dosimetric consequences of gold nanoparticle clustering during photon irradiation.

PURPOSE: The radiation dose enhancement caused by introducing gold nanoparticles (GNP) into cells can increase the dose locally absorbed. A disconnect between experimentally determined survival and dose enhancements predicted by Monte Carlo simulations on macroscopic scales, suggests small-scale energy deposition patterns play an important role in GNP dose enhancement. Clustering of the GNPs could potentially alter small-scale energy deposition patterns. Here we use Monte Carlo simulations to quantify energy deposition patterns in the presence of clustered GNPs and address the question of whether clustering of the nanoparticles affects the energy deposition patterns and ultimately cellular response. METHODS: Using the PENELOPE Monte Carlo code, we examine the absorption of energy in the environment of a single irradiated GNP following its interaction with a set of primary monoenergetic photon beams. We introduce successive GNPs to form a cluster about the particle in which the primary photon interactions occur and report on the energy deposited locally (within a 500 nm radius) and nonlocally (beyond 500 nm) in the surrounding water-equivalent medium as a function of the number of additional GNPs and the packing geometry they assume. RESULTS: When additional GNPs cluster in tightly packed formations about a GNP in which an incident photon interacts, both the energy deposited locally and released nonlocally are reduced relative to the case where other GNPs are not present. The degree of the reduction depends on incident photon energy, the number of GNPs added to the cluster, and the packing geometry. With 90 additional GNPs in a hexagonal close packing (HCP) cluster about a directly irradiated test particle, the local energy deposition was reduced to 29% (of the value in the absence of neighbors) in the most extreme monoenergetic case. Energy released into the nonlocal volume was most affected by the cluster for low-incident photon energies (< 40 keV), where reductions to 26% of the value in the absence of a cluster were shown. The packing geometry mitigated these results. When the irradiated GNP was on the periphery of the HCP cluster, or when the cluster was confined to a plane, the observed effects were weaker and when an equal number of GNPs were uniformly distributed in the local volume, the changes were trivial (less than 2%). CONCLUSIONS: The findings provide grounds for reconciling the observations of cell survival with Monte Carlo predictions of GNP dose enhancement. This work is significant because it demonstrates that GNP clustering needs to be understood and accounted to optimize local dose enhancement.

The relative biological effectiveness of out-of-field dose.

PURPOSE: using simulations and models derived from existing literature, this work investigates relative biological effectiveness (RBE) for out-of-field radiation and attempts to quantify the relative magnitudes of different contributing phenomena (spectral, bystander, and low dose hypersensitivity effects). Specific attention is paid to external beam radiotherapy treatments for prostate cancer. MATERIALS AND METHODS: using different biological models that account for spectral, bystander, and low dose hypersensitivity effects, the RBE was calculated for different points moving radially out from isocentre for a typical single arc VMAT prostate case. The RBE was found by taking the ratio of the equivalent dose with the physical dose. Equivalent doses were calculated by determining what physical dose would be necessary to produce the same overall biological effect as that predicted using the different biological models. RESULTS: spectral effects changed the RBE out-of-field less than 2%, whereas response models incorporating low dose hypersensitivity and bystander effects resulted in a much more profound change of the RBE for out-of-field doses. The bystander effect had the largest RBE for points located just outside the edge of the primary radiation beam in the cranial caudal (z-direction) compared to low dose hypersensitivity and spectral effects. In the coplanar direction, bystander effect played the largest role in enhancing the RBE for points up to 8.75 cm from isocentre. CONCLUSIONS: spectral, bystander, and low dose hypersensitivity effects can all increase the RBE for out-of-field radiation doses. In most cases, bystander effects seem to play the largest role followed by low dose hypersensitivity. Spectral effects were unlikely to be of any clinical significance. Bystander, low dose hypersensitivity, and spectral effect increased the RBE much more in the cranial caudal direction (z-direction) compared with the coplanar directions.

Experimental validation of a kilovoltage x-ray source model for computing imaging dose.

PURPOSE: To introduce and validate a kilovoltage (kV) x-ray source model and characterization method to compute absorbed dose accrued from kV x-rays. METHODS: The authors propose a simplified virtual point source model and characterization method for a kV x-ray source. The source is modeled by: (1) characterizing the spatial spectral and fluence distributions of the photons at a plane at the isocenter, and (2) creating a virtual point source from which photons are generated to yield the derived spatial spectral and fluence distribution at isocenter of an imaging system. The spatial photon distribution is determined by in-air relative dose measurements along the transverse (x) and radial (y) directions. The spectrum is characterized using transverse axis half-value layer measurements and the nominal peak potential (kVp). This source modeling approach is used to characterize a Varian(®) on-board-imager (OBI(®)) for four default cone-beam CT beam qualities: beams using a half bowtie filter (HBT) with 110 and 125 kVp, and a full bowtie filter (FBT) with 100 and 125 kVp. The source model and characterization method was validated by comparing dose computed by the authors' inhouse software (kVDoseCalc) to relative dose measurements in a homogeneous and a heterogeneous block phantom comprised of tissue, bone, and lung-equivalent materials. RESULTS: The characterized beam qualities and spatial photon distributions are comparable to reported values in the literature. Agreement between computed and measured percent depth-dose curves is ⩽ 2% in the homogeneous block phantom and ⩽ 2.5% in the heterogeneous block phantom. Transverse axis profiles taken at depths of 2 and 6 cm in the homogeneous block phantom show an agreement within 4%. All transverse axis dose profiles in water, in bone, and lung-equivalent materials for beams using a HBT, have an agreement within 5%. Measured profiles of FBT beams in bone and lung-equivalent materials were higher than their computed counterparts resulting in an agreement within 2.5%, 5%, and 8% within solid water, bone, and lung, respectively. CONCLUSIONS: The proposed virtual point source model and characterization method can be used to compute absorbed dose in both the homogeneous and heterogeneous block phantoms within of 2%-8% of measured values, depending on the phantom and the beam quality. The authors' results also provide experimental validation for their kV dose computation software, kVDoseCalc.