BACKGROUND: New radiotherapy machines such as Halcyon are capable of delivering dose-rate of 600 monitor-units per minute, allowing large numbers of patients treated per day. However, patient-specific quality assurance (QA) is still required, which dramatically decrease machine availability. Innovative artificial intelligence (AI) algorithms could predict QA result based on complexity metrics. However, no AI solution exists for Halcyon machines and the complexity metrics to be used have not been definitively determined. The aim of this study was to develop an AI solution capable of firstly determining the complexity indices to be obtained and secondly predicting patient-specific QA in a routine clinical setting.
METHODS: Three hundred and eighteen beams from 56 patients with breast cancer were used. The seven complexity indices named Modulation-Complexity-Score (MCS), Small-Aperture-Score (SAS10), Beam-Area (BA), Beam-Irregularity (BI), Beam-Modulation (BM), Gantry and Collimator angles were used as input to the AI model. Machine learning (ML) and deep learning (DL) models using tensorflow were set up to predict DreamDose QA conformance.
RESULTS: MCS, BI, gantry and collimator angle are not correlated with QA compliance. Therefore, ML and DL models were trained using SAS10, BA and BM complexity indices. ROC analyses enabled to find best predicted probability threshold to increase specificity and sensitivity. ML models did not show satisfactory performance with an area under-the-curve (AUC) of 0.75 and specificity and sensitivity of 0.88 and 0.86. However, optimised DL model showed better performance with an AUC of 0.95 and specificity and sensitivity of 0.98 and 0.97.
CONCLUSIONS: The DL model demonstrated a high degree of accuracy in its predictions of the quality assurance (QA) results. Our online predictive QA-platform offers significant time savings in terms of accelerator occupancy and working time.

Radiotherapy planning for nasopharyngeal carcinoma is a complex process due to the proximity of critical structures. Volumetric modulated arc therapy (VMAT) can improve the therapeutic ratio. However, multiple treatment delivery systems offer VMAT with varying technical specifications. This study compares the dosimetric plan quality of 2 systems, Clinac-iX and Halcyon in nasopharyngeal carcinoma. We utilized contrast-enhanced computed tomography (CECT) simulation and magnetic resonance (MR) image datasets from thirty patients with nasopharyngeal carcinoma to contour target volumes and organs at risk (OARs). Two medical physicists independently performed dosimetric planning for Clinac-iX and Halcyon machines, following standard international dosimetric constraints for OARs. We compared plan quality for dosimetric profiles, indices, and plan complexity parameters from both machines. Dosimetric coverage for target volumes and plan quality indices, such as homogeneity, conformity, and coverage, showed no significant differences between Clinac-iX and Halcyon. However, Halcyon demonstrated significantly better OAR sparing, particularly for the spinal cord, optic chiasm, lenses, eyeballs and lower brain volume integral dose (BVID) (p < 0.05). Complexity parameters showed that both systems used a similar number of arcs, but Halcyon had higher monitor units and lower treatment time per fraction owing to higher dose rate. Our study results favor Halcyon for better plan quality regarding critical organ sparing, low brain volume integral dose, and fast treatment delivery. This study can be used as a reference for selecting an optimal treatment delivery system for nasopharyngeal carcinoma patients in centres equipped with multiple linear accelerators.

Purpose We sought to explore the feasibility of using the current co-planar Halcyon ring delivery system (RDS) with a novel multileaf collimator (MLC) aperture shape controller in delivering a single high dose of 30 Gy to solitary lung lesions via stereotactic body radiotherapy (SBRT). Materials and methods Thirteen non-small-cell lung cancer (NSCLC) patients previously treated with a single dose of 30 Gy to lung lesions via SBRT on the TrueBeam (6MV-FFF) using non-coplanar volumetric modulated arc therapy (VMAT) arcs were anonymized and replanned onto the Halcyon RDS (6MV-FFF) following RTOG-0915 single-fraction criteria. The Halcyon plans utilized a novel dynamic conformal arc (DCA)-based MLC-fitting approach before VMAT optimization with a user-defined aperture shape controller option. The clinical TrueBeam and Halcyon plans were compared via their protocol compliance, target conformity, gradient index, and dose to organs-at-risk (OAR). Treatment delivery efficacy and accuracy were assessed through end-to-end quality assurance (QA) tests on Halcyon and independent dose verification via in-house Monte Carlo (MC) second-check validation. Results All Halcyon lung SBRT plans met RTOG-0915 protocol\'s requirements for target coverage, conformity, and gradient indices, and maximum dose 2 cm away from the target (D2cm) while being statistically insignificant (p > 0.05) when compared to clinical TrueBeam plans. Additionally, Halcyon provided a similar dose to OAR except for the ribs, where Halcyon demonstrated a lower maximum dose (15.22 Gy vs 17.01 Gy, p < 0.001). However, Halcyon plans required a higher total monitor unit (8892 MU vs 7413 MU, p < 0.001), resulting in a higher beam modulation factor (2.96 MU/cGy vs 2.47 MU/cGy, p < 0.001) and an increase in beam-on time by a factor of 2.1 (11.11 min vs 5.3 min, p < 0.005). End-to-end QA measurements demonstrate that Halcyon plans were clinically acceptable with an average gamma passing rate of 99.8% for 2%/2mm criteria and independent MC 2nd checks within ±2.86%. Conclusion Our end-to-end testing and validation study demonstrates that by utilizing a DCA-based MLC aperture shape controller before VMAT optimization, Halcyon can be used for delivering a single dose of lung SBRT treatment. However, future improvements of Halcyon RDS are recommended to allow higher output rates, rotational couch corrections, and an integrated intrafraction motion management system that will further enhance Halcyon\'s capability for site-specific single dosage of SBRT.

BACKGROUND: With the ever-increasing requirements of accuracy and personalization of radiotherapy treatments, stereotactic radiotherapy (SRT) with volumetric modulated arc therapy (VMAT) on O-ring Halcyon radiotherapy system could potentially provide a fast, safe, and feasible treatment option.
OBJECTIVE: The purpose of this study was to assess the delivery of Halcyon VMAT plans for small targets.
METHODS: Well-defined VMAT-SRT plans were created on Halcyon radiotherapy system with the stacked and staggered dual-layer MLC design for the film measurement set-up and the target sizes and shapes designed to emulate the targets of the stereotactic treatments. The planar dose distributions were acquired with film measurements and compared to a current clinical reference dose calculation with AcurosXB (v18.0, Varian Medical Systems) and to Monte Carlo simulations. With the collapsed arc versions of the VMAT-SRT plans, the uncertainty in dose delivery due to the multileaf collimator (MLC) without the gantry rotation could be separated and analyzed.
RESULTS: The target size was mainly limited by the resolution originated from the design of the MLC leaves. The results of the collapsed arc versions of the plans show good consistency among measured, calculated, and simulated dose distributions. With the full VMAT plans, the agreement between calculated and simulated dose distributions was consistent with the collapsed arc versions. The measured dose distribution agreed with the calculated and simulated dose distributions within the target regions, but considerable local differences were observed in the margins of the target. The largest differences located in the steep gradient regions presumably originating from the deviation of the isocenter.
CONCLUSIONS: The potential of the Halcyon radiotherapy system for VMAT-SRT delivery was evaluated and the study revealed valuable insights on the machine characteristics with the delivery.

The shielding parameters can vary depending on the geometrical structure of the linear accelerators (LINAC), treatment techniques, and beam energies. Recently, the introduction of O-ring type linear accelerators is increasing. The objective of this study is to evaluate the shielding parameters of new type of linac using a dedicated program developed by us named ORSE (O-ring type Radiation therapy equipment Shielding Evaluation). The shielding evaluation was conducted for a total of four treatment rooms including Elekta Unity, Varian Halcyon, and Accuray Tomotherapy. The developed program possesses the capability to calculate transmitted dose, maximum treatable patient capacity, and shielding wall thickness based on patient data. The doses were measured for five days using glass dosimeters to compare with the results of program. The IMRT factors and use factors obtained from patient data showed differences of up to 65.0% and 33.8%, respectively, compared to safety management report. The shielding evaluation conducted in each treatment room showed that the transmitted dose at every location was below 1% of the dose limit. The results of program and measurements showed a maximum difference of 0.003 mSv/week in transmitted dose. The ORSE program allows for the shielding evaluation results to the clinical environment of each institution based on patient data.

OBJECTIVE: Field size limitations on Halcyon and Ethos treatment machines largely preclude use of the conventional monoisocentric three-field technique for breast/chest wall and regional lymph nodes. We present an alternative, IMRT-based planning approach that facilitates treatment on Halcyon and Ethos while preserving plan quality.
METHODS: Eight breast and regional node cases (four left-sided, four right-sided) were planned for an Ethos machine using a 15-17 field IMRT technique. Institutional plan quality metrics for CTV and PTV coverage and OAR sparing were assessed. Five plans (four right-sided, one left-sided) were also planned using a hybrid 3D multisocenter technique. CTV coverage and OAR sparing were compared to the IMRT plans. Eclipse scripting tools were developed to aid in beam placement and plan evaluation through a set of dosimetric scorecards, and both are shared publicly.
RESULTS: On average, the IMRT plans achieved breast CTV and PTV coverage at 50 Gy of 97.9% and 95.7%, respectively. Supraclavicular CTV and PTV coverages at 45 Gy were 100% and 95.5%. Axillary lymph node CTV and PTV coverages at 45 Gy were 100% and 97.1%, and IMN CTV coverage at 45 Gy was 99.2%. Mean ipsilateral lung V20 Gy was 19.3%, and average mean heart dose was 1.6 Gy for right-sided cases and 3.0 Gy for left-sided. In comparison to the hybrid 3D plans, IMRT plans achieved higher breast and supraclavicular CTV coverage (99.9% vs. 98.6% and 99.9% vs. 93.4%), higher IMN coverage (99.6% vs. 78.2%), and lower ipsilateral lung V20 Gy (19.6% vs. 28.2%).
CONCLUSIONS: Institutional plan quality benchmarks were achieved for all eight cases using the IMRT-based planning approach. The IMRT-based planning approach offered superior conformity and OAR sparing than a competing hybrid 3D approach.

With the increasing use of flattening filter free (FFF) beams, it is important to evaluate the impact on the skin dose and target coverage of breast cancer treatments. This study aimed to compare skin doses of treatments using FFF and flattening filter (FF) beams for breast cancer. The study established treatment plans for left breast of an anthropomorphic phantom using Halcyon\'s 6-MV FFF beam and TrueBeam\'s 6-MV FF beam. Volumetric modulated arc therapy (VMAT) with varying numbers of arcs and intensity modulated radiation therapy (IMRT) were employed, and skin doses were measured at five points using Gafchromic EBT3 film. Each measurement was repeated three times, and averaged to reduce uncertainty. All plans were compared in terms of plan quality to ensure homogeneous target coverage. The study found that when using VMAT with two, four, and six arcs, in-field doses were 19%, 15%, and 6% higher, respectively, when using Halcyon compared to TrueBeam. Additionally, when using two arcs for VMAT, in-field doses were 10% and 15% higher compared to four and six arcs when using Halcyon. Finally, in-field dose from Halcyon using IMRT was about 1% higher than when using TrueBeam. Our research confirmed that when treating breast cancer with FFF beams, skin dose is higher than with traditional FF beams. Moreover, number of arcs used in VMAT treatment with FFF beams affects skin dose to the patient. To maintain a skin dose similar to that of FF beams when using Halcyon, it may be worth considering increasing the number of arcs.

Currently, there is a lack of methods and tools that efficiently evaluate the auto-feathering junctions created by multileaf collimator (MLCs) for supine volumetric modulated arc therapy (VMAT) craniospinal irradiation (CSI) plans. We have investigated the feasibility of stitching together multi-isocenter fluence maps to then analyze the feathered junctions for patient-specific quality assurance (QA). Furthermore, we investigated the capability of Halcyon for the treatment of CSI patients. Three patients, who previously underwent VMAT CSI treatment on TrueBeam (6-MV flattening filter-free (FFF)) for 36 Gy in 20 fractions were replanned for Halcyon. A multi-isocenter approach with only translational superior-inferior shifts was used for both platforms. Each isocenter consists of two full arcs with anterior avoidance sectors, ±5° collimator rotations between arcs, and 5-8 cm of overlapping MLC auto-feathering junctions. All plans were QA\'d via electronic portal imaging device (EPID) portal dosimetry and analyzed with a gamma criteria of 3%/3 mm. A variety of plan quality metrics were analyzed to evaluate dose distributions to the target, doses to organs at risk (OARs), and integral dose to the patient. A MATLAB script was developed to stitch the calculated and measured fluence maps in order to perform patient-specific QA for the composite fluence. The Halcyon plans provided highly conformal and homogenous dose distributions to the entire CSI target, superior to the clinical TrueBeam plans, while sparing critical organs with significantly lower values of V10Gy and V18Gy by up to 2% and 2.5%, respectively. Qualitative depictions of vertical dose profiles from the stitched DICOM of the entire CSI target for both planned and delivered fluence maps demonstrated equivalency, with slightly lower average pass rates with Halcyon (97%) compared to TrueBeam (99.9%). This approach to stitch multiple measured versus calculated EPID fluence maps has shown to be a feasible and accurate method and will be helpful for comprehensive VMAT CSI QA on both platforms. Further implementation of this script will be used in examining dosimetric impacts of daily patient positioning errors at MLC auto-feathering junctions.

UNASSIGNED: The objective of this research is to compare the efficacy of conventional and hypofractionated radiotherapy treatment plans for breast cancer patients, with a specific focus on the unique features of the Halcyon system.
UNASSIGNED: The study collected and analyzed dose volume histogram (DVH) data for two groups of treatment plans implemented using the Halcyon system. The first group consisted of 19 patients who received conventional fractionated (CF) treatment with a total dose of 50 Gy in 25 fractions, while the second group comprised 9 patients who received hypofractionated (HF) treatment with a total dose of 42.56 Gy in 16 fractions. The DVH data was used to calculate various parameters, including tumor control probability (TCP), normal tissue complication probability (NTCP), and equivalent uniform dose (EUD), using radiobiological models.
UNASSIGNED: The results indicated that the CF plan resulted in higher TCP but lower NTCP for the lungs compared to the HF plan. The EUD for the HF plan was approximately 49 Gy (114% of its total dose) while that for the CF plan was around 53 Gy (107% of its total dose).
UNASSIGNED: The analysis suggests that while the CF plan is better at controlling tumors, it is not as effective as the HF plan in minimizing side effects. Additionally, it is suggested that there may be an optimal configuration for the HF plan that can provide the same or higher EUD than the CF plan.

Objective. With the introduction of Cherenkov imaging technology on the Halcyon O-ring linear accelerator platform, we seek to demonstrate the imaging feasibility and optimize camera placement.Approach. Imaging parameters were probed by acquiring triggering data Cherenkov image frames for simplistic beams on the Halcyon and comparing the analyzed metrics with those from the TrueBeam platform. Camera position was analyzed by performing 3D rendering of patient treatment plans for various sites and iterating over camera positions to assess treatment area visibility.Main results. Commercial Cherenkov imaging systems are compatible with the pulse timing of the Halcyon, and this platform design favorably impacts signal to noise in Cherenkov image frames. Additionally, ideal camera placement is treatment site dependent and is always within a biconical zone of visibility centered on the isocenter. Visibility data is provided for four treatment sites, with suggestions for camera placement based on room dimensions. Median visibility values were highest for right breast plans, with values of 80.33% and 68.49% for the front and rear views respectively. Head and neck plans presented with the lowest values at 26.44% and 38.18% respectively.Significance. This work presents the first formal camera positional analysis for Cherenkov imaging on any platform and serves as a template for performing similar work for other irradiation platforms. Additionally, this study confirms the Cherenkov imaging parameters do not need to be changed for optimal imaging on the Halcyon. Lastly, the presented methodology provides a framework which could be further expanded to other optical imaging systems which rely on line of sight visibility to the patient.