TrueBeam STx Research Articles

Analyzing RPM and RGSC Infrared Camera Sensitivity: A Comparative Study of Breathing Cycle Replication Verifications and Limitations.

This study addresses challenges in delivering high radiation doses and managing organ motion in Stereotactic Body Radiation Therapy (SBRT) for thoracic and abdominal cancer. It evaluates Varian's Real Time Position Management (RPM) system's infrared camera sensitivity during crucial Four-Dimensional computed tomography (4D-CT) scans for planning and treatment. The analysis includes CT simulator, LINAC (Novalis Tx and TrueBeam STx). This research enhances SBRT precision by offering insights into RPM and RGSC system performance across machines, impacting treatment planning and delivery optimization. The QUASAR™ Respiratory Motion Assembly phantom is aligned with precision using lasers. It is configured with either six-dot reflective or four-dot lens marker blocks featuring a retroreflective marker placed on the phantom's surface. Motion is induced by adjusting the amplitude, and the camera position is finely tuned to monitor the marker's movements. This investigation entails variations in seconds per breath (SPB) within the Quasar breath platform, specifically at intervals of 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0 seconds while maintaining a 1cm amplitude camera setting. For TrueBeam-STx: Ensure SPB values are kept above 1.8 seconds for accurate replication. For Novalis-Tx: Stay within an SPB range of up to 2.0 seconds for reliable reproducibility. For CT Simulator: Optimal replication up to an SPB of 2.2 seconds; avoid SPB values below 1.8 seconds for reliable detection. Data for TrueBeam-STx, Novalis-Tx, and the CT simulator shows discrepancies in replicating the breathing cycle as Seconds Per Breath (SPB) decreases. Effective Infrared (IR) sensitivity is observed until SPB thresholds: 1.8s (TrueBeam-STx), 2.2s (Novalis-Tx), and 2.2s (CT simulator). We should consider values equal to or greater than the mentioned breathing periods. Variations in replicating breathing cycles signal challenges in planning and delivering treatments, especially with lower SPB values. These insights guide clinicians to adapt treatments based on machine-specific capabilities for accurate and reproducible outcomes.

Evaluation and Comparison of AAA and AXB Dose Calculation Algorithms for Lung SBRT on TrueBeam STx with Eclipse 13.6

Evaluation and Comparison of AAA and AXB Dose Calculation Algorithms for Lung SBRT on TrueBeam STx with Eclipse 13.6

Performance of cone-beam computed tomography imaging during megavoltage beam irradiation under phase-gated conditions

PurposeTarget positions should be acquired during beam delivery for accurate lung stereotactic body radiotherapy. We aimed to perform kilovoltage (kV) imaging during beam irradiation (intra-irradiation imaging) under phase-gated conditions and evaluate its performance. MethodsCatphan 504 and QUASAR respiratory motion phantoms were used to evaluate image quality and target detectability, respectively. TrueBeam STx linac and the Developer Mode was used. The imaging parameters were 125 kVp and 1.2 mAs/projection. Flattened megavoltage (MV) X-ray beam energies 6, 10 and 15 MV and un-flattened beam energies 6 and 10 MV were used with field sizes of 5 × 5 and 15 × 15 cm2 and various frame rates for intra-irradiation imaging. In addition, using a QUASAR phantom, intra-irradiation imaging was performed during intensity-modulated plan delivery. The root-mean-square error (RMSE) of the CT-number for the inserted rods, image noise, visual assessment, and contrast-to-noise ratio (CNR) were evaluated. ResultsThe RMSEs of intra-irradiation cone-beam computed tomography (CBCT) images under gated conditions were 50–230 Hounsfield Unit (HU) (static < 30 HU). The noise of the intra-irradiation CBCT images under gated conditions was 15–35 HU, whereas that of the standard CBCT images was 8.8–27.2 HU. Lower frame rates exhibited large RMSEs and noise; however, the iterative reconstruction algorithm (IR) was effective at improving these values. Approximately 7 fps with the IR showed an equivalent CNR of 15 fps without the IR. The target was visible on all the gated intra-irradiation CBCT images. ConclusionSeveral image quality improvements are required; however, intra-irradiated CBCT images showed good visual target detection.

Dosimetric characterization for GRID collimator-based spatially fractionated radiation therapy: Dosimetric parameter acquisition and machine interchangeability investigation.

The purpose of this study is to characterize the dosimetric properties of a commercial brass GRID collimator for high energy photon beams including 15 and 10 MV. Then, the difference in dosimetric parameters of GRID beams among different energies and linacs was evaluated. A water tank scanning system was used to acquire the dosimetric parameters, including the percentage depth dose (PDD), beam profiles, peak to valley dose ratios (PVDRs), and output factors (OFs). The profiles at various depths were measured at 100cm source to surface distance (SSD), and field sizes of 10×10 cm2 and 20×20 cm2 on three linacs. The PVDRs and OFs were measured and compared with the treatment planning system (TPS) calculations. Compared with the open beam data, there were noticeable changes in PDDs of GRID fields across all the energies. The GRID fields demonstrated a maximal of 3mm shift in dmax (Truebeam STX, 15MV, 10×10 cm2). The PVDR decreased as beam energy increases. The difference in PVDRs between Trilogy and Truebeam STx using 6MV and 15MV was 1.5%±4.0% and 2.1%±4.3%, respectively. However, two Truebeam linacs demonstrated less than 2% difference in PVDRs. The OF of the GRID field was dependent on the energy and field size. The measured PDDs, PVDRs, and OFs agreed with the TPS calculations within 3% difference. The TPS calculations agreed with the measurements when using 1mm calculation resolution. The dosimetric characteristics of high-energy GRID fields, especially PVDR, significantly differ from those of low-energy GRID fields. Two Truebeam machines are interchangeable for GRID therapy, while a pronounced difference was observed between Truebeam and Trilogy. A series of empirical equations and reference look-up tables for GRID therapy can be generated to facilitate clinical applications.

GATE Monte Carlo approach to heterogeneity dose distribution in small fields used in radiation therapy

The Accurate dosage prediction in Radiation Therapy is challenging, prompting a need for precision beyond conventional clinical Treatment Planning Systems (TPS). Monte Carlo-based methods are sought for their superior accuracy. The aim of this study is to compare dose distributions between the ACUROS algorithm and the GATE platform in various tissue densities and field sizes, focusing on smaller fields. This study was initiated with a homogeneous validation of the TrueBeam STX system, using measurements obtained from the Centre Hospitalier Interregional Edith Cavell (CHIREC) in Brussels. The validation compared dosimetric functions (Percentage Depth Dose (PDD), Dose profile (DP) and Collimator scatter fraction (CSF)) employing the GAMMA index with a 2% / 2 mm criterion tolerance. Following this, heterogeneous studies examined dose distributions between the ACUROS algorithm and the GATE platform in various tissue densities and field sizes, with a specific focus on smaller fields. Simulations were conducted using both platforms on chest phantoms with heterogeneous slabs representing bone, lung, and heart, each housing a central tumor. The impact of electronic equilibrium on tumors for different small field sizes was evaluated. Results showed a remarkable 99% agreement between measurements and GATE calculations in the homogeneous validation of the TrueBeam STX system. However, in heterogeneous studies, ACUROS consistently overestimated lung doses by up to 8% compared to GATE simulation, especially evident with a flattening filter and smaller beam sizes at density interfaces. This highlights significant dose estimation discrepancies between ACUROS and GATE, emphasizing the need for precise calculations. The findings support exploring Monte Carlo-based methods for enhanced accuracy in Radiation Therapy treatment planning.

Comparative analysis of dosimetric uncertainty using Gafchromic™ EBT4 and EBT3 films in radiochromic film dosimetry

Advancements in radiotherapy dosimetry aim to enhance precision and accuracy, with Gafchromic™ EBT4 film emerging as a promising tool. However, its performance compared to EBT3 remains underexplored, necessitating a comprehensive analysis of dosimetric uncertainties. This study evaluates the efficacy of EBT4 in radiochromic dosimetry, comparing it to EBT3. Employing a 6 MV beam quality, absorbed doses ranging from 0 to 10 Gy were investigated using different measurement quantities and fitting functions. Gafchromic™ EBT3 and EBT4 films (8′′ × 10″) were irradiated in a water-equivalent phantom, with calibration doses delivered using a Varian Truebeam Stx linear accelerator. Two measurement quantities, pixel values (PV) and net change transmittance (netΔT), were analyzed, along with calibration functions including a rational function for PV and a logarithmic function for netΔT. Uncertainty analyses considered experimental, fitting, and calibration dose uncertainties. Dose-response curves for EBT3 and EBT4 films demonstrated robust fits, emphasizing the reliability of both. EBT4 consistently exhibited lower uncertainties, particularly below 2 Gy, showcasing superior precision in low-dose scenarios. The choice of response quantity and calibration function influenced results, with PV-rational function excelling for doses 2–10 Gy, and netΔT-logarithmic function showing enhanced precision in the 0–2 Gy range. The uncertainty analysis emphasized EBT4's superior performance, especially in low-dose ranges, with lower relative uncertainties and absolute errors, indicating enhanced reliability in dose measurements. The study underscores EBT4's potential to contribute to improved accuracy in various clinical scenarios. However, users should tailor the choice of response quantities and calibration functions to specific dose calibration ranges, suggesting a nuanced approach.

Integrating cine EPID, dynamic delivery, and the off-axis Winston-Lutz test to enhance quality control in multiple brain metastasis stereotactic radiotherapy

PurposeStereotactic radiotherapy (SRT) has transformed cancer treatment, especially for brain metastases. Ensuring accurate SRT delivery is crucial, with the Winston-Lutz test being an important quality control tool. Off-axis Winston-Lutz (OAWL) tests are designed for accuracy assessment, but most are limited to fixed angles and hampered by local-field shifts caused by suboptimal Multi-Leaf Collimator (MLC) positioning. This study introduces a new OAWL approach for quality control in multi-brain-metastasis SRT. Utilizing cine Electronic Portal Imaging Device (EPID) images, it can be used with dynamic conformal arc (DCA) therapy. However, dynamic OAWL (DOAWL) is prone to more local-field shifts due to dynamic MLC movements. A two-step DOAWL is proposed: step 1 calculates local-field shifts using dynamic MLC movements in the beam-eye view data from the Treatment Planning System (TPS), while step 2 processes cine EPID images with an OAWL algorithm to isolate true deviations. MethodsValidation involved an anthropomorphic head phantom with metallic ball-bearings, Varian TrueBeam STx accelerator delivering six coplanar/non-coplanar DCA beams, cine EPID, and ImageJ's OAWL analysis algorithm. ResultsInherent local-field shifts ranged from 0.11 to 0.49 mm; corrected mean/max EPID-measured displacement was 0.34/1.03 mm. Few points exceeded 0.75/1.0-mm thresholds. ConclusionsThis two-step DOAWL test merges cine-EPID acquisitions, DCA, OAWL, and advanced analysis and offers effective quality control for multi-brain-metastasis SRT. Its routine implementation may also improve physicist knowledge of the treatment precision of their machines.

New optically stimulated luminescence dosimetry film optimized for energy dependence guided by Monte Carlo simulations

Optically stimulated luminescence (OSL) film dosimeters, based on BaFBr:Eu2+ phosphor material, have major dosimetric advantages such as dose linearity, high spatial resolution, film re-usability, and immediate film readout. However, they exhibit an energy-dependent over-response at low photon energies because they are not made of tissue-equivalent materials. In this work, the OSL energy-dependent response was optimized by lowering the phosphor grain size and seeking an optimal choice of phosphor concentration and film thickness to achieve sufficient signal sensitivity. This optimization process combines measurement-based assessments of energy response in narrow x-ray beams with various energy response calculation methods applied to different film metrics. Theoretical approaches and MC dose simulations were used for homogeneous phosphor distributions and for isolated phosphor grains of different dimensions, where the dose in the phosphor grain was calculated. In total 8 OSL films were manufactured with different BaFBr:Eu2+ median particle diameters (D50): 3.2 μm, 1.5 μm and 230 nm and different phosphor concentrations (1.6%, 5.3% and 21.3 %) and thicknesses (from 5.2 to 49 μm). Films were irradiated in narrow x-ray spectra (N60, N80, N-150 and N-300) and the signal intensity relative to the nominal dose-to-water value was normalized to Co-60. Finally, we experimentally tested the response of several films in Varian 6MV TrueBeam STx linear accelerator using the following settings: 10 × 10 cm2 field, 0deggantry angle, 90 cm SSD, 10 cm depth. The x-ray irradiation experiment reported a reduced energy response for the smallest grain size with an inverse correlation between response and grain size. The N-60 irradiation showed a 43% reduction in the energy over-response when going from 3 μm to 230 nm grain size for the 5% phosphor concentration. Energy response calculation using a homogeneous dispersion of the phosphor underestimated the experimental response and was not able to obtain the experimental correlation between grain size and energy response. Isolated grain size modeling combined with MC dose simulations allowed to establish a good agreement with experimental data, and enabled steering the production of optimized OSL-films. The clinical 6 MV beam test confirmed a reduction in energy dependence, which is visible in small-grain films where a decrease in out-of-field over-response was observed.

A novel approach to quality assurance of radiation treatments with a visual interface of GEANT4 for Varian TrueBeam STx

A novel approach to quality assurance of radiation treatments with a visual interface of GEANT4 for Varian TrueBeam STx

MR-guided stereotactic radiation therapy for head and neck cancers

PurposeMR-guided radiotherapy (MRgRT) has the advantage of utilizing high soft tissue contrast imaging to track daily changes in target and critical organs throughout the entire radiation treatment course. Head and neck (HN) stereotactic body radiation therapy (SBRT) has been increasingly used to treat localized lesions within a shorter timeframe. The purpose of this study is to examine the dosimetric difference between the step-and-shot intensity modulated radiation therapy (IMRT) plans on Elekta Unity and our clinical volumetric modulated arc therapy (VMAT) plans on Varian TrueBeam for HN SBRT. MethodFourteen patients treated on TrueBeam sTx with VMAT treatment plans were re-planned in the Monaco treatment planning system for Elekta Unity MR-Linac (MRL). The plan qualities, including target coverage, conformity, homogeneity, nearby critical organ doses, gradient index and low dose bath volume, were compared between VMAT and Monaco IMRT plans. Additionally, we evaluated the Unity adaptive plans of adapt-to-position (ATP) and adapt-to-shape (ATS) workflows using simulated setup errors for five patients and assessed the outcomes of our treated patients. ResultsMonaco IMRT plans achieved comparable results to VMAT plans in terms of target coverage, uniformity and homogeneity, with slightly higher target maximum and mean doses. The critical organ doses in Monaco IMRT plans all met clinical goals; however, the mean doses and low dose bath volumes were higher than in VMAT plans. The adaptive plans demonstrated that the ATP workflow may result in degraded target coverage and OAR doses for HN SBRT, while the ATS workflow can maintain the plan quality. ConclusionThe use of Monaco treatment planning and online adaptation can achieve dosimetric results comparable to VMAT plans, with the additional benefits of real-time tracking of target volume and nearby critical structures. This offers the potential to treat aggressive and variable tumors in HN SBRT and improve local control and treatment toxicity.

Influence of dose calculation algorithms on the helical diode array using volumetric-modulated arc therapy for small targets.

For patient-specific quality assurance (PSQA) for small targets, the dose resolution can change depending on the characteristics of the dose calculation algorithms. This study aimed to evaluate the influence of the dose calculation algorithms Acuros XB (AXB), anisotropic analytical algorithm (AAA), photon Monte Carlo (pMC), and collapsed cone (CC) on a helical diode array using volumetric-modulated arc therapy (VMAT) for small targets. ArcCHECK detectors were inserted with a physical depth of 2.9cm from the surface. To evaluate the influence of the dose calculation algorithms for small targets, rectangular fields of 2×100, 5×100, 10×100, 20×100, 50×100, and 100×100 mm2 were irradiated and measured using ArcCHECK with TrueBeam STx. A total of 20 VMAT plans for small targets, including the clinical sites of 19 brain metastases and one spine, were also evaluated. The gamma passing rates (GPRs) were evaluated for the rectangular fields and the 20 VMAT plans using AXB, AAA, pMC, and CC. For rectangular fields of 2×100 and 5×100 mm2, the GPR at 3%/2mm of AXB was<50% because AXB resulted in a coarser dose resolution with narrow beams. For field sizes>10×100 mm2, the GPR at 3%/2mm was>88.1% and comparable for all dose calculation algorithms. For the 20 VMAT plans, the GPRs at 3%/2mm were 79.1±15.7%, 93.2±5.8%, 94.9±4.1%, and 94.5±4.1% for AXB, AAA, pMC, and CC, respectively. The behavior of the dose distribution on the helical diode array differed depending on the dose calculation algorithm for small targets. Measurements using ArcCHECK for VMAT with small targets can have lower GPRs owing to the coarse dose resolution of AXB around the detector area.

Comprehensive characterisation of the IBA myQA SRS for SRS and SBRT patient specific quality assurance.

The myQA SRS (IBA) is a new to market 2D complementary metal oxide semiconductor detector array with an active area 140 × 120mm2 and 0.4mm resolution, making it a potential real-time dosimetry alternative to radiochromic film for stereotactic plan verification. Characterisation of the device was completed to assess performance. The dosimetric properties of the device were assessed for 6FF and 6FFF beams from a Varian TrueBeam STx with high definition multileaf collimator. Clinical suitability of the device for Patient Specific Quality Assurance was verified using ten SRS/SBRT plans, compared against other detectors, as well as multi leaf collimator (MLC) tests including picket fence and chair. Gamma analysis was performed using myQA software with criteria of 4%/1mm. The device demonstrated compliance with recommended specifications for basic tests. After the required warm-up period, the maximum deviation in detector signal from initial readings was 0.2%. Short-term and long-term reproducibility was 0.1% (6FF) and 1.0% (6FFF), respectively. Dose linearity was within 0.3% (6FF) and 0.7% (6FFF) and dose-rate dependence within 1.7% (6FF) and 2.9% (6FFF) and were verified with a Farmer type ionization chamber (PTW 30013). Angular dependence was quantified for coplanar and non-coplanar situations. Output factors and beam profiles measured on the device showed agreement within 1% of baseline RAZOR diode (IBA) and CC04 ionisation chamber (IBA) measurements for field sizes 1 × 1 to 10 × 10cm2. The minimum gamma (4%/1mm) pass rates for MLC-pattern tests were 96.5% and 98.1% for the myQA SRS and film, respectively. The average gamma (4%/1mm) pass rates for SBRT and SRS plans were 98.8% and 99.8% respectively. This work represents one of the first studies performed on the commissioning and performance characterisation of this novel device, demonstrating its accuracy and reliability, making it highly useful as a film alternative in stereotactic treatment plan verification.

Photon beam modeling: A comparative study of primo and gate simulation toolkits for the TrueBeam STx Linac

This study compares the PRIMO and GATE Monte Carlo simulation toolkits for modeling photon beams from a TrueBeam STx Linac used in radiation therapy. Various beam configurations were evaluated against Varian's Golden Beam Data using the Gamma Index method. Both toolkits demonstrated good agreement overall, with GATE generally achieving higher gamma pass rates for percent depth dose curves than PRIMO.

Stereotactic high-dose irradiation for intramedullary metastases

To study the results of stereotactic high-dose irradiation for IMM considering local control of metastases, neurological status and life expectancy. Nineteen patients with 29 IMMs underwent high-dose irradiation (Cyber Knife G4 and TrueBeam STx devices) between 2016 and 2022. There were 1-10 fractions, radiation dose 14.0-35.0 Gy with isodose 70-100%. Local control and new spinal cord metastases were assessed using contrast-enhanced MRI. Clinical status and neurological functions were assessed using the Karnofsky and McCormick scales. Local control rates were 91% and 78% after 6 and 12 months, respectively. Overall survival after treatment was 7.3 months, 6- and 12-month overall survival - 58% and 37%, respectively. In addition, 8 patients were available for follow-up. Of these, 6 ones demonstrated improvement or stabilization of neurological function after treatment. The main cause of death was progression of the underlying disease. No significant treatment-related toxic effects were observed. High-dose irradiation is a safe, effective and time-saving treatment for IMM. Considering unadvisable surgical methods and chemotherapy for this pathology, we can recommend high-dose irradiation for patients with spinal cord metastases.

Particular issues to be considered in small field dosimetry for TrueBeam STx commissioning

This study aims to report the relevant issues concerning small fields in the commissioning of a TrueBeam STx for photon energies of 6MV, 10MV, 6FFF, and 10FFF. Percent depth doses, profiles, and field output factors were measured according to the beam model configuration of the treatment planning system. Multiple detectors were used based on the IAEA TRS-483 protocol as well as EBT3 radiochromic film. Analytical Anisotropic and Acuros XB algorithms, were configured and validated through basic dosimetry comparisons and end-to-end clinical tests.

Application of a Novel Methodology for Efficient Optimization of Hippocampal Avoidance Whole-Brain Radiotherapy

A growing awareness of the long-term neurocognitive toxicity associated with whole-brain radiotherapy (WBRT) has led to the development of newer delivery methods to achieve relative hippocampal sparing. In order to accomplish the required target coverage and dose constraints, however, hippocampal-avoidance WBRT (HA-WBRT) can prove labor intensive and time-consuming. We therefore developed a simplified and efficient planning model for generating optimized HA-WBRT treatment. Target volumes and organs at risk (OARs) were contoured per the NRG-CC001 protocol, including the whole-brain parenchyma, bilateral hippocampi, and optic apparatus. Treatment was planned using Eclipse 16.1 for a TrueBeam STx 2.7 LINAC, applying customizable optimization and clinical goals templates to rapidly generate uniform plans. An isocenter was placed at the point corresponding to the center of mass between the bilateral contoured 3-dimensional hippocampi. Volumetric modulated arc therapy (VMAT) using 6MV photons was designed using two sets of bi-directional full arcs, with collimator rotations of 85° and 95°. The two paired sets independently covered the superior and inferior portions of the treatment volume, respectively. Arcs shared a total 2cm overlap in the superior-inferior (X Jaws) direction. A total of 10 consecutive cases of HA-WBRT were retrospectively re-optimized using the described planning model. All plans were normalized to cover 95% of the treatment volume with prescription dose (V30Gy = 95%). The model achieved favorable dosimetry relative to NRG-CC001 per protocol constraints. Average maximal dose to the hottest 2% and minimal dose to 98% of the volume were 32.22 ± 0.26 Gy and 29.82 ± 1.46 Gy, well within protocol limits of 37.5 Gy and 25 Gy, respectively. The average maximal dose to 100% of the hippocampi (D100%) and to its hottest 0.03cc (Dmax) were 8.50 ± 0.34 Gy and 12.85 ± 0.70 Gy, compared to protocol limits of 9Gy and 16Gy, respectively. Mean Dmax to the total optic apparatus was 29.67 ± 0.52 Gy, with a maximum value within the cohort of 30 Gy, matching protocol constraints. The above presented methodology for optimizing HA-WBRT can consistently and efficiently generate plans with excellent target coverage and strict hippocampal avoidance per current collaborative group constraints. This method can be broadly applied to decrease the burden and heterogeneity across HA-WBRT planning.

Generation, validation, and benchmarking of a commercial independent Monte Carlo calculation beam model for multi-target SRS

BackgroundDosimetric validation of single isocenter multi-target radiosurgery plans is difficult due to conditions of electronic disequilibrium and the simultaneous irradiation of multiple off-axis lesions dispersed throughout the volume. Here we report the benchmarking of a customizable Monte Carlo secondary dose calculation algorithm specific for multi-target radiosurgery which future users may use to guide their commissioning and clinical implementation. PurposeTo report the generation, validation, and clinical benchmarking of a volumetric Monte Carlo (MC) dose calculation beam model for single isocenter radiosurgery of intracranial multi-focal disease. MethodsThe beam model was prepared within SciMoCa (ScientificRT, Munich Germany), a commercial independent dose calculation software, with the aim of broad availability via the commercial software for use with single isocenter radiosurgery. The process included (1) definition & acquisition of measurement data required for beam modeling, (2) tuning model parameters to match measurements, (3) validation of the beam model via independent measurements and end-to-end testing, and finally, (4) clinical benchmarking and validation of beam model utility in a patient specific QA setting. We utilized a 6X Flattening-Filter-Free photon beam from a TrueBeam STX linear accelerator (Siemens Healthineers, Munich Germany). ResultsIn addition to the measured data required for standard IMRT/VMAT (depth dose, central axis profiles & output factors, leaf gap), beam modeling and validation for single-isocenter SRS required central axis and off axis (5 cm & 9 cm) small field output factors and comparison between measurement and simulation of backscatter with aperture for jaw much greater than MLCs. Validation end-to-end measurements included SRS MapCHECK in StereoPHAN geometry (2%/1 mm Gamma = 99.2% ± 2.2%), and OSL & scintillator measurements in anthropomorphic STEEV phantom (6 targets, volume = 0.1–4.1cc, distance from isocenter = 1.2–7.9 cm) for which mean difference was −1.9% ± 2.2%. For 10 patient cases, MC for individual PTVs was −0.8% ± 1.5%, −1.3% ± 1.7%, and −0.5% ± 1.8% for mean dose, D95%, and D1%, respectively. This corresponded to custom passing rates action limits per AAPM TG-218 guidelines of ±5.2%, ±6.4%, and ±6.3%, respectively. ConclusionsThe beam modeling, validation, and clinical action criteria outlined here serves as a benchmark for future users of the customized beam model within SciMoCa for single isocenter radiosurgery of multi-focal disease.

Applicability evaluation of the TRS-483 protocol for the determination of small-field output factors using different multi-leaf collimator and field-shaping types

PurposeTo evaluate the applicability of TRS-483 output correction factors (CFs) for small-field output factors (OFs) using different multi-leaf collimators (MLC) and field-shaping types. MethodsAll measurements were performed on TrueBeam, TrueBeam STx, and Halcyon using 6 MV flattening filter-free energy. Four detectors, including CC01, CC04, microDiamond, and EDGE, were used. Nominal field sizes ranging from 1 × 1 to 4 × 4, and 10 × 10 cm2 were used to measure small-field OFs at source-to-axis distance of 100 cm with a 0° gantry angle in a 3D water phantom. Further, the field-shaping types were defined using jaw collimator or MLC (five different configurations). A field size of 10 × 10 cm2 was used as the reference for calculation of OFs obtained as ratio of detector readings (OFdet). The percentage difference and coefficient of variation of OFdet and OFdet corrected by applying CF were compared for each field size and configuration. ResultsFor OFdet corrected by applying CF, the ranges of percentage difference and coefficient of variation in all configurations for ≥ 2 × 2 cm2 fields were reduced from 1.2–2.2 to 0.8–1.3 percentage points (%pt) and from 0.5–1.0 to 0.4–0.7%, respectively. For 1 × 1 cm2 field, the ranges of percentage difference and coefficient of variation were reduced from 3.3–5.7 to 1.2–2.2 %pt and from 2.2–3.7 to 0.8–1.1%, respectively. ConclusionsThe CFs described in TRS-483 dosimetry protocol have broad applicability in reducing OF variations between detectors under different MLC and field-shaping types.

Skin/mucosa avoidance radiotherapy (SMART) versus conventional volumetric arc-based radiotherapy (VMAT) for the treatment of head and neck cancer: Dosimetric feasibility study.

Intensity modulated radiotherapy (IMRT) for head and neck cancer has led to a reduction in radiotherapy doses to normal tissues, like the salivary glands, while maintaining high rates of local control. Oral mucosal and skin toxicity is still a major source of treatment-related morbidity, occurring in most patients. We conducted a dosimetric feasibility study with the goal of creating a methodology that could theoretically reduce the dose of radiation to skin and oral mucosa, while maintaining comparable avoidance of other organs at risk, and planning target volume (PTV) coverage. The clinical plans of patients treated previously were replanned using coplanar VMAT arcs on a TrueBeam STx using the photon optimizer (PO) version 15.6 and the Acuros XB dose calculation algorithm. Comparisons were made between three methodologies: "Conventional," "Skin Sparing" and a skin/mucosa avoiding ("SMART") technique, with dose metrics being compared using analysis of variance, with a Bonferroni correction to account for multiple pairwise comparisons. The maximum grade of mucositis and radiation dermatitis during treatment was correlated to different dose-volume metrics to predict what could be clinically meaningful. Sixteen patients met the study criteria and were replanned using the skin sparing and SMART techniques. Maximum doses to the skin sparing structure were reduced from 64.2Gy to 56.6 and 55.9Gy, in the skin sparing and SMART plans (p<0.0001), and mean doses reduced from 26.7Gy to 20.0 and 20.2Gy, respectively (p<0.0001). Maximum doses to the oral cavity structure were not reduced by either technique, however mean dose to the oral cavity structure was reduced from 39.03Gy to 33.5Gy by the SMART technique (p<0.0001). There was a slight reduction in PTV_High coverage by the V95% in the SMART plans (99.52%vs. 98.79%, p=0.0073), and a similar slight reduction in PTV_Low coverage by the V95% by both the skin sparing and SMART plans (99.74%vs. 97.89%vs. 97.42%, p<0.0001). Maximum doses to organs at risk were not statistically different between techniques. Dose to oral cavity and maximum grade experienced during radiotherapy correlated. The Spearman correlation coefficient for dose to 20%, 50%, and 80% of the volume of oral cavity was 0.5 (p=0.048), 0.64 (p=0.007), and 0.62 (p=0.010), respectively. Skin toxicity grade was only found to be correlated with the D20% of the skin sparing structure (Spearman correlation coefficient of 0.58, p=0.0177). The SMART technique appears to be able to reduce maximum and mean skin dose, as well as mean oral cavity doses, while only slightly reducing PTV coverage, with acceptable OAR doses. We feel the improvements warrant investigation in a clinical trial.

Evaluation of plan quality and treatment efficiency in cranial stereotactic radiosurgery treatment plans with a variable source-to-axis distance.

Radiotherapy deliveries with dynamic couch motions that shorten the source-to-axis distance (SAD) on a C-arm linac have the potential to increase treatment efficiency through the increase of the effective dose rate. In this investigation, we convert clinically deliverable volumetric modulated arc therapy (VMAT) and dynamic conformal arc (DCA) plans for cranial radiosurgery into virtual isocenter plans through implementation of couch trajectories that maintain the target at a shortened but variable SAD throughout treatment. A randomly sampled population of patients treated with cranial radiosurgery from within the last three years were separated into groups with one, two, and three lesions. All plans had a single isocenter (regardless of number of targets), and a single prescription dose. Patient treatment plans were converted from their original delivery at a standard isocenter to a dynamic virtual isocenter in MATLAB. The virtual isocenter plan featured a variable isocenter position based upon the closest achievable source-to-target distance (referred to herein as a virtual source-to-axis distance [vSAD]) which avoided collision zones on a TrueBeam STx platform. Apertures were magnified according to the vSAD and monitor units at a given control point were scaled based on the inverse square law. Doses were calculated for the plans with a virtual isocenter in the Eclipse (v13.6.23) treatment planning system (TPS) and were compared with the clinical plans. Plan metrics (MU, Paddick conformity index, gradient index, and the volume receiving 12 Gy or more), normal brain dose-volume differences, as well as maximum doses received by organs at risk (OARs) were assessed. The values were compared between standard and virtual isocenter plans with Wilcoxon Sign Ranked tests to determine significance. A subset of the plans were mapped to the MAX-HD anthropomorphic phantom which contained an insert housing EBT3 GafChromic film and a PTW 31010 microion chamber for dose verification on a linac. Delivering plans at a virtual isocenter resulted in an average reduction of 20.9% (p = 3×10-6 ) and 20.6% (p = 3.0×10-6 ) of MUs across all VMAT and all DCA plans, respectively. There was no significant change in OAR max doses received by plans delivered at a virtual isocenter. The low dose wash (1.0-2.0 Gy or 5-11% of the prescription dose) was increased (by approximately 20 cc) for plans with three lesions. This was equivalent to a 2.7%-3.8% volumetric increase in normal tissue receiving the respective dose level when comparing the plan with a virtual isocenter to a plan with a standard isocenter. Gamma pass rates with a 5%/1mm analysis criteria were 96.40% ± 2.90% and 95.07% ± 3.10% for deliveries at standard and virtual isocenter, respectively. Absolute point dose agreements were within -0.36% ± 3.45% and -0.55% ± 3.39% for deliveries at a standard and virtual isocenter, respectively. Potential time savings per arc were found to have linear relationship with the monitor units delivered per arc (savings of 0.009 s/MU with an r2 = 0.866 when fit to plans with a single lesion). Converting clinical plans at standard isocenter to a virtual isocenter design did not show any losses to plan quality while simultaneously improving treatment efficiency through MU reductions.