Halcyon System Research Articles

A tangent based solution for Radiotherapy to the Breast using a Varian Halcyon system limited to 6MV flattening-filter-free (6FFF)

A tangent based solution for Radiotherapy to the Breast using a Varian Halcyon system limited to 6MV flattening-filter-free (6FFF)

Dosimetric and radiobiological comparison between conventional and hypofractionated breast treatment plans using the Halcyon system.

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. 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. 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). 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.

Evaluation of initial patient setup methods for breast cancer between surface-guided radiation therapy and laser alignment based on skin marking in the Halcyon system

BackgroundThis study was conducted to evaluate the efficiency and accuracy of the daily patient setup for breast cancer patients by applying surface-guided radiation therapy (SGRT) using the Halcyon system instead of conventional laser alignment based on the skin marking method.Methods and materialsWe retrospectively investigated 228 treatment fractions using two different initial patient setup methods. The accuracy of the residual rotational error of the SGRT system was evaluated by using an in-house breast phantom. The residual translational error was analyzed using the couch position difference in the vertical, longitudinal, and lateral directions between the reference computed tomography and daily kilo-voltage cone beam computed tomography acquired from the record and verification system. The residual rotational error (pitch, yaw, and roll) was also calculated using an auto rigid registration between the two images based on Velocity. The total setup time, which combined the initial setup time and imaging time, was analyzed to evaluate the efficiency of the daily patient setup for SGRT.ResultsThe average residual rotational errors using the in-house fabricated breast phantom for pitch, roll, and yaw were 0.14°, 0.13°, and 0.29°, respectively. The average differences in the couch positions for laser alignment based on the skin marking method were 2.7 ± 1.6 mm, 2.0 ± 1.2 mm, and 2.1 ± 1.0 mm for the vertical, longitudinal, and lateral directions, respectively. For SGRT, the average differences in the couch positions were 1.9 ± 1.2 mm, 2.9 ± 2.1 mm, and 1.9 ± 0.7 mm for the vertical, longitudinal, and lateral directions, respectively. The rotational errors for pitch, yaw, and roll without the surface-guided radiation therapy approach were 0.32 ± 0.30°, 0.51 ± 0.24°, and 0.29 ± 0.22°, respectively. For SGRT, the rotational errors were 0.30 ± 0.22°, 0.51 ± 0.26°, and 0.19 ± 0.13°, respectively. The average total setup times considering both the initial setup time and imaging time were 314 s and 331 s, respectively, with and without SGRT.ConclusionWe demonstrated that using SGRT improves the accuracy and efficiency of initial patient setups in breast cancer patients using the Halcyon system, which has limitations in correcting the rotational offset.

Experience in the Treatment of Breast Cancer Using Tunnel Type Linear Accelerator

Purpose: Describes the features of dosimetric planning for breast cancer and conducts statistics analysis of the radiation treatment planning results, while taking into account 2.5 years of work experience on the Halcyon linear accelerator at the Irkutsk Regional Cancer Center.
 Materials and methods: Varian Halcyon tunnel type linear accelerator and Varian TrueBeam C-arm linear accelerator were used for breast irradiation. Treatment planning was performed in the Eclipse planning system, Acuros dose calculation algorithm. The experience of using the specified equipment and developments in the field of radiation treatment planning using the Halcyon system are presented.
 Results: Developed an approach to planning breast radiation at the Halcyon linear accelerator. A retrospective analysis of 84 radiation treatment plans on linear accelerators of various types was carried out. Breast treatment plans for Halcyon linear accelerator have equal dosimetric parameters in comparison to the TrueBeam treatment plans.
 Conclusion: Linear accelerator Halcyon can be used for treating breast cancer if staff understands specifics of treatment planning and treatment delivery.

Treatment time of image-guided radiotherapy with a Halcyon 2.0 system

IntroductionThe treatment fraction time is a key indicator of the external beam radiotherapy process. The Halcyon system was designed to improve the clinical workflow, according to the manufacturer (Varian Medical Systems). Few works studied the actual delivery efficiency of the Halcyon system. This work analyzed the treatment time on a Halcyon 2.0 unit for a variety of sites along a period of 9 months. Materials and MethodsTreatment time included patient setup, image acquisition, image-guided online couch correction, and radiation delivery time. Data were extracted from the ARIA oncology information system and were studied as a function of the treatment site, the delivery modality, and the time from the first day of treatments with the Halcyon 2.0 system in our institution. ResultsA total of 8599 fractions were delivered during the analyzed period (69.5% from VMAT plans, and 30.5% from IMRT plans). The number of fractions by site ranged from 30 for anal canal to 1933 for prostate. Five sites (prostate, lung, pelvis with prostate, breast, and gynecological sites) accounted for the 84% of the fractions. After a 2-week adaptation period of the staff, the daily mean treatment time was reduced to less than 12 min. The mean treatment time of all the fractions amounted to 10.5 ± 3.8 min. ConclusionsThe Halcyon 2.0 allowed delivering online image-guided radiation therapy in all fractions with total treatment time consistently below the 12-min standard time slot, for most of the analyzed treatment sites.

Dosimetric comparison between VMAT plans using the fast-rotating O-ring linac with dual-layer stacked MLC and helical tomotherapy for nasopharyngeal carcinoma

BackgroundTo compare the dosimetric profiles of volumetric modulated arc therapy (VMAT) plans using the fast-rotating O-ring linac (the Halcyon system) based on a dual-layer stacked multi-leaf collimator and helical tomotherapy (HT) for nasopharyngeal cancer (NPCa).MethodsFor 30 NPCa patients, three sets of RT plans were generated, under the same policy of contouring and dose constraints: HT plan; Halcyon VMAT plan with two arcs (HL2arc); and Halcyon VMAT plan with four arcs (HL4arc), respectively. The intended dose schedule was to deliver 67.2 Gy to the planning gross target volume (P-GTV) and 56.0 Gy to the planning clinical target volume (P-CTV) in 28 fractions using the simultaneously integrated boost concept. Target volumes and organ at risks dose metrics were evaluated for all plans. Normal tissue complication probabilities (NTCP) for esophagus, parotid glands, spinal cord, and brain stem were compared.ResultsThe HT plan achieved the best dose homogeneity index for both P_GTV and P_CTV, followed by the HL4arc and L2arc plans. No significant difference in the dose conformity index (CI) for P_GTV was observed between the HT plan (0.80) and either the HL2arc plan (0.79) or the HL4arc plan (0.83). The HL4arc plan showed the best CI for P_CTV (0.88), followed by the HL2arc plan (0.83) and the HT plan (0.80). The HL4arc plan (median, interquartile rage (Q1, Q3): 25.36 (22.22, 26.89) Gy) showed the lowest Dmean in the parotid glands, followed by the HT (25.88 (23.87, 27.87) Gy) and HL2arc plans (28.00 (23.24, 33.99) Gy). In the oral cavity (OC) dose comparison, the HT (22.03 (19.79, 24.85) Gy) plan showed the lowest Dmean compared to the HL2arc (23.96 (20.84, 28.02) Gy) and HL4arc (24.14 (20.17, 27.53) Gy) plans. Intermediate and low dose regions (40–65% of the prescribed dose) were well fit to the target volume in HL4arc, compared to the HT and HL2arc plans. All plans met the dose constraints for the other OARs with sufficient dose margins. The between-group differences in the median NTCP values for the parotid glands and OC were < 3.47% and < 1.7% points, respectively.ConclusionsThe dosimetric profiles of Halcyon VMAT plans were comparable to that of HT, and HL4arc showed better dosimetric profiles than HL2arc for NPCa.

Clinical performance evaluation of O-Ring Halcyon Linac: A real-world study.

BACKGROUNDRadiation therapy, especially the development of linear accelerators, plays a key role in cancer management. The fast-rotating coplanar O-ring Halcyon Linac has demonstrated many advantages. The previous literature has mainly focused on the machine parameters and plan quality of Halcyon, with a lack of relevant research on its clinical application.AIMTo evaluate the clinical performance of the O-ring Halcyon treatment system in a real-world application setting.METHODSData from sixty-one patients who were treated with the Halcyon system throughout the entire radiotherapy process in Peking Union Medical College Hospital between August 2019 and September 2020 were retrospectively reviewed. We evaluated the target tumour response to radiotherapy and irradiation toxicity from 1 to 3 mo after treatment. Dosimetric verification of Halcyon plans was performed using a quality assurance procedure, including portal dosimetry, ArcCHECK and point dose measurements for verification of the system delivery accuracy.RESULTSOf the 61 patients in the five groups, 16, 12, 7 and 26 patients had complete response, partial response, progressive disease and stable disease, respectively. No increase in the irradiated target tumour volume was observed when separately evaluating the local response. Regarding irradiation toxicity, no radiation-induced deaths were observed. Thirty-eight percent (23/61 patients) had no radiation toxicity after radiotherapy, 56% (34/61 patients) experienced radiation toxicity that resolved after treatment, and 6% (4/61 patients) had irreversible adverse reactions. The average gamma passing rates with a 2% dose difference and 2-mm distance to agreement for IMRT/VMAT/SRT plans were ArcCHECK at 96.4% and portal dosimetry at 96.7%, respectively. All of the validated clinical plans were within 3% for point dose measurements, and Halcyon’s ArcCHECK demonstrated a high pass rate of 99.1% ± 1.1% for clinical gamma passing criteria of 3%/3 mm.CONCLUSIONThe O-ring Halcyon Linac could achieve a better therapeutic effect on the target volume by providing accurate treatment delivery plans with tolerable irradiation toxicity.

Comparison of treatment plans for hypofractionated high-dose prostate cancer radiotherapy using the Varian Halcyon and the Elekta Synergy platforms.

PurposeTo compare radiotherapy plans between an O‐ring and a conventional C‐arm linac for hypofractionated high‐dose prostate radiotherapy in terms of plan quality, dose distribution, and quality assurance in a multi‐vendor environment.MethodsTwenty prostate cancer treatment plans were irradiated on the O‐ring Varian Halcyon linac and were re‐optimized for the C‐arm Elekta Synergy Agility linac. Dose‐volume histogram metrics for target coverage and organ at risk dose, quality assurance, and monitor units were retrospectively compared. Patient‐specific quality assurance with ion chamber measurements, gamma index analysis, and portal dosimetry was performed using the Varian Portal Dosimetry system and the ArcCHECK® phantom (Sun Nuclear Corporation). Prostate‐only radiotherapy was delivered with simultaneous integrated boost (SIB) volumetric modulated arc therapy (VMAT) in 20 fractions of 2.5/3.0 Gy each.ResultsFor both linacs, target coverage was excellent and plan quality comparable. Homogeneity in PTVBoost was high for Synergy as well as Halcyon with a mean homogeneity index of 0.07 ± 0.01 and 0.05 ± 0.01, respectively. Mean dose for the organs at risk rectum and bladder differed not significantly between the linacs but were higher for the femoral heads and penile bulb for Halcyon. Quality assurance showed no significant differences in terms of ArcCHECK gamma pass rates. Median pass rate for 3%/2 mm was 99.3% (96.7 to 99.8%) for Synergy and 99.8% (95.6 to 100%) for Halcyon. Agreement between calculated and measured dose was high with a median deviation of −0.6% (−1.7 to 0.8%) for Synergy and 0.2% (−0.6 to 2.3%) for Halcyon. Monitor units were higher for the Halcyon by approximately 20% (p < 0.001).ConclusionHypofractionated high‐dose prostate cancer SIB VMAT on the Halcyon system is feasible with comparable plan quality in reference to a standard C‐arm Elekta Synergy linac.

Out-of-field doses from radiotherapy using photon beams: A comparative study for a pediatric renal treatment.

PurposeFirst, this experimental study aims at comparing out‐of‐field doses delivered by three radiotherapy techniques (3DCRT, VMAT (two different accelerators), and tomotherapy) for a pediatric renal treatment. Secondly, the accuracy of treatment planning systems (TPS) for out‐of‐field calculation is evaluated.MethodsEBT3 films were positioned in pediatric phantoms (5 and 10 yr old). They were irradiated according to four plans: 3DCRT (Clinac 2100CS, Varian), VMAT (Clinac 2100CS and Halcyon, Varian), and tomotherapy for a same target volume. 3D dose determination was performed with an in‐house Matlab tool using linear interpolation of film measurements. 1D and 3D comparisons were made between techniques. Finally, measurements were compared to the Eclipse (Varian) and Tomotherapy (Accuray) TPS calculations.ResultsAdvanced radiotherapy techniques (VMATs and tomotherapy) deliver higher out‐of‐field doses compared to 3DCRT due to increased beam‐on time triggered by intensity modulation. Differences increase with distance to target and reach a factor of 3 between VMAT and 3DCRT. Besides, tomotherapy delivers lower doses than VMAT: although tomotherapy beam‐on time is higher than in VMAT, the additional shielding of the Hi‐Art system reduces out‐of‐field doses. The latest generation Halcyon system proves to deliver lower peripheral doses than conventional accelerators. Regarding TPS calculation, tomotherapy proves to be suitable for out‐of‐field dose determination up to 30 cm from field edge whereas Eclipse (AAA and AXB) largely underestimates those doses.ConclusionThis study shows that the high dose conformation allowed by advanced radiotherapy is done at the cost of higher peripheral doses. In the context of treatment‐related risk estimation, the consequence of this increase might be significative. Modern systems require adapted head shielding and a particular attention has to be taken regarding on‐board imaging dose. Finally, TPS advanced dose calculation algorithms do not certify dose accuracy beyond field edges, and thus, those doses are not suitable for risk assessment.

A Simulated Dosimetric Study of Contribution to Radiotherapy Accuracy by Fractional Image Guidance Protocol of Halcyon System.

PurposeFrequency of conventional kV-image guidance is sometimes sacrificed to reduce concomitant risk, leaving deviations of unguided fractions unknown. MV-imaging and treatment dose can be collectively optimized on Halcyon, where fractional MVCBCT provides complete anatomic records for course-wide dose reconstruction. By retrospective dose accumulation, this work simulated the impact of imaging frequency on patient treatment dose on the platform of Halcyon.MethodsFour hundred and sixteen MVCBCT image sets from 16 patients of various tumor sites treated with radiotherapy on Halcyon were retrospectively selected. After applying the image-guided couch shifts of the clinical records, deformable image registration was performed using Velocity software, to deform the planning CTs to the corresponding MVCBCTs, generating pseudo CTs representing the actual anatomies on the treatment day. Fractional treatment dose was reconstructed on pseudo CTs for accumulation, representing the actual patient dose (Ddaily). To simulate weekly image guidance, fractional dose was reconstructed and accumulated by incorporating 1 CBCT-guided corrections and 4 laser-guided setups of each week (Dweekly). Limited by partially imaged volumes and different organs-at-risk of various sites, only target dose-volume parameters were evaluated across all patients.ResultsGTV_D98%, CTV_D98%, PTV_D90%, PTV_D95%, PGTV_D90%, and PGTV_D95% were evaluated, where Dx% means the minimal dose received by x% volume. Pairwise comparisons were made between plan dose and Ddaily, Ddaily and Dweekly respectively. PGTV_D95% of accumulated Dweekly were significantly lower than those of accumulated Ddaily by up to 32.90% of prescription dose, suggesting that weekly-guidance may result in unacceptable under dose to the target. The broad distribution of fractional differences between Ddaily and Dweekly suggested unreliable patient positioning based on aligning surface markers to laser beams, as a popular approach broadly used on conventional Linac systems. Slight target under-dose was observed on daily reconstructed results compared with planned dose, which provided quantitative data to guide clinical decisions such as the necessity of adaptive radiotherapy.ConclusionFractional image guided radiotherapy on Halcyon provides more reliable treatment accuracy than using sacrificed imaging frequency, which also provides complete anatomic records for deformable dose reconstruction supporting more informed clinical decisions.

Simplified sigmoidal curve fitting for a 6 MV FFF photon beam of the Halcyon to determine the field size for beam commissioning and quality assurance

BackgroundAn O-ring gantry-type linear accelerator (LINAC) with a 6-MV flattening filter-free (FFF) photon beam, Halcyon, includes a reference beam that contains representative information such as the percent depth dose, profile and output factor for commissioning and quality assurance. However, because it does not provide information about the field size, we proposed a method to determine all field sizes according to all depths for radiation therapy using simplified sigmoidal curve fitting (SCF).MethodsAfter mathematical definition of the SCF using four coefficients, the defined curves were fitted to both the reference data (RD) and the measured data (MD). For good agreement between the fitting curve and the profiles in each data set, the field sizes were determined by identifying the maximum point along the third derivative of the fitting curve. The curve fitting included the field sizes for beam profiles of 2 × 2, 4 × 4, 6 × 6, 8 × 8, 10 × 10, 20 × 20 and 28 × 28 cm2 as a function of depth (at 1.3, 5, 10 and 20 cm). The field size results from the RD were compared with the results from the MD using the same condition.ResultsAll fitting curves show goodness of fit, R2, values that are greater than 0.99. The differences in field size between the RD and the MD were within the range of 0 to 0.2 cm. The smallest difference in the field sizes at a depth of 10 cm, which is a surface-to-axis distance, was reported.ConclusionApplication of the SCF method has been proven to accurately capture the field size of the preconfigured RD and the measured FFF photon beam data for the Halcyon system. The current work can be useful for beam commissioning as a countercheck methodology to determine the field size from RD in the treatment planning system of a newly installed Halcyon system and for routine quality assurance to ascertain the correctness of field sizes for clinical use of the Halcyon system.

A Preliminary Simulation Study of Dose-Guided Adaptive Radiotherapy Based on Halcyon MV Cone-Beam CT Images With Retrospective Data From a Phase II Clinical Trial.

Background and purpose: To evaluate the feasibility of dose-guided adaptive radiotherapy (ART) based on deformable image registration (DIR) using fractional megavoltage cone-beam CT (MVCBCT) images from Halcyon system that uses identical beams for treatment and imaging and to retrospectively investigate the influence of anatomic changes on target coverage and organ-at-risk (OAR) sparing across various tumor sites.Materials and Methods: Four hundred twenty-two MVCBCT images from 16 patients (three head and neck, seven thoracic, three abdominal, and three pelvic cases) treated in a phase II clinical trial for Halcyon were selected. DIR between the planning CT and daily MVCBCT image was implemented by Velocity software to create pseudo CT. To investigate the accuracy of dose calculation on pseudo CT, three evaluation patients with rescanned CT and adaptive plans were selected. Dose distribution of adaptive plans calculated on pseudo CT was compared with that calculated on the rescanned planning CT on the three evaluation patients. To investigate the impact of inter-fractional anatomic changes on target dose coverage and dose to OARs of the 16 patients, fractional dose was calculated and accumulated incrementally based on deformable registration between planning CT and daily MVCBCT images.Results: Passing rates using 3 mm/3%/10% threshold local gamma analysis were 93.04, 96.00, and 91.68%, respectively, for the three evaluation patients between the reconstructed dose on pseudo CT (MVCBCT) and rescanned CT, where accumulated dose deviations of over 97% voxels were smaller than 0.5 Gy. Planning target volume (PTV) D95% and D90% (the minimum dose received by at least 95/90% of the volume) of the accumulated dose could be as low as 93.8 and 94.5% of the planned dose, respectively. OAR overdose of various degrees were observed in the 16 patients relative to the planned dose. In most cases, OARs' dose volume histogram (DVH) lines of accumulated and planned dose were very close to each other if not overlapping. Among cases with visible deviations, the differences were bilateral without apparent patterns specific to tumor sites or organs.Conclusion: As a confidence building measure, this simulation study suggested the possibility of ART for Halcyon based on DIR between planning CT and MVCBCT. Preliminary clinical data suggested the benefit of patient-specific dose reconstruction and ART to avoid unacceptable target underdosage and OAR overdosage.

Treatment planning comparison of volumetric modulated arc therapy employing a dual-layer stacked multi-leaf collimator and helical tomotherapy for cervix uteri

PurposeTo ascertain the dosimetric performance of a new delivery system (the Halcyon system, H) equipped with dual-layer stacked multi-leaf collimator (MLC) for risk-adapted targets in cervix uteri cancer patients compared to another ring-based system in clinical operation (Helical Tomotherapy, HT).MethodsTwenty patients were retrospectively included in a treatment planning study (10 with positive lymph nodes and 10 without). The dose prescription (45Gy to the primary tumour volume and a simultaneously integrated boost up to 55Gy for the positive patients) and the clinical planning objectives were defined consistently as recommended by an ongoing multicentric clinical trial. Halcyon plans were optimised for the volumetric modulated arc therapy. The plan comparison was performed employing the quantitative analysis of the dose-volume histograms.ResultsThe coverage of the primary and nodal target volumes was comparable for both techniques and both subsets of patients. The primary planning target volume (PTV) receiving at least 95% of the prescription isodose ranged from 97.2 ± 1.1% (node-negative) to 99.1 ± 1.2% (node-positive) for H and from 96.5 ± 1.9% (node-negative) to 98.3 ± 0.9% (node-positive) for HT. The uncertainty is expressed at one standard deviation from the cohort of patient per each group. For the nodal clinical target volumes, the dose received by 98% of the planning target volume ranged 55.5 ± 0.1 to 56.0 ± 0.8Gy for H and HT, respectively. The only significant and potentially relevant differences were observed for the bowels. In this case, V40Gy resulted 226.3 ± 35.9 and 186.9 ± 115.9 cm3 for the node-positive and node-negative patients respectively for Halcyon. The corresponding findings for HT were: 258.9 ± 60.5 and 224.9 ± 102.2 cm3. On the contrary, V15Gy resulted 1279.7 ± 296.5 and 1557.2 ± 359.9 cm3 for HT and H respectively for node-positive and 1010.8 ± 320.9 versus 1203.8 ± 332.8 cm3 for node-negative.ConclusionThis retrospective treatment planning study, based on the dose constraints derived from the Embrace II study protocol, suggested the essential equivalence between Halcyon based and Helical Tomotherapy based plans for the intensity-modulated rotational treatment of cervix uteri cancer. Different levels of sparing were observed for the bowels with H better protecting in the high-dose region and HT in the mid-low dose regions. The clinical impact of these differences should be further addressed.

Clinical safety assessment of the Halcyon system.

The Halcyon consists of precommissioned linear accelerator and treatment planning algorithms that were designed to simplify the acceptance, commissioning, and clinical workflow for image-guided intensity-modulated radiotherapy. The purpose of this work was to perform a comprehensive safety assessment for the clinical use of the Halcyon. Systems-Theoretic Process Analysis was used as the safety assessment tool. As part of the analysis, a number of control loops and control actions are created to describe system function. Safety is assessed by determining unsafe control actions and a corresponding list of causal scenarios that leads to accidents. The scope of the analysis was from the acceptance of the Halcyon to routine patient treatments. All aspects of treating patients were considered including the roles of physicians, physicists, dosimetrists, and therapists. The analysis was completed by four physicists with input from other members of the radiation therapy team. The causal scenarios were summarized using the causality categories from the consensus recommendations for incident learning database structures in radiation oncology (Med Phys, Vol. 39, No. 12, Dec 2012). Twenty-three (23) control loops containing 52 control actions were created for the clinical use of the Halcyon. One hundred forty-four (144) unsafe control actions were identified with 385 associated causal scenarios. Twenty-seven percent (27%) of the causal scenarios were related to equipment technical issues, while 73% of the causal scenarios were predominantly related to procedural issues, human behavior, and organizational management. For routine clinical use of closed or largely automated radiation therapy equipment, the majority of safety concerns is related to nontechnical issues. The Halcyon and other similar systems may present opportunities to streamline, reduce, or eliminate some traditional equipment commissioning and routine quality assurance activities in exchange for an increased focus on issues related to organizational management, procedures, and human behavior.

Characterization of a prototype rapid kilovoltage x-ray image guidance system designed for a ring shape radiation therapy unit.

This study aims to characterize the performance of a prototype rapid kilovoltage (kV) x-ray image guidance system onboard the newly released Halcyon 2.0 linear accelerator (Varian Medical Systems, Palo Alto, CA) by use of conventional and innovatively designed testing procedures. Basic imaging system performance tests and radiation dose measurements were performed for all eleven kV-cone beam computed tomography (CBCT) imaging protocols available on a preclinical Halcyon 2.0 LINAC. Both conventional CBCT reconstruction using the Feldkamp-Davis-Kress (FDK) algorithm and a novel, advanced iterative reconstruction (iCBCT) available on this platform were evaluated. Standard image quality metrics, including slice thickness accuracy, high-contrast resolution, low-contrast resolution, regional uniformity and noise, and CT Hounsfield unit (HU) number accuracy and linearity were evaluated using a manufacturer-supplied QUART phantom (GmbH, Zorneding, Germany) and an independent image quality phantom (Catphan 500, The Phantom Laboratory, New York, NY). Due to the simplified design of the QUART phantom, we developed surrogate and clinically feasible strategies for measuring slice thickness and high- and low-contrast resolution. Imaging dose delivered by these eleven protocols was measured using a computed tomography dose index phantom and pencil chamber with commonly accepted methods and procedures. A subset of measurements were repeated on a conventional C-arm LINAC (TrueBeam and Trilogy, Varian Medical System) for comparison. Clinical patient images of pelvic and abdominal regions are also presented for qualitative assessment as part of a feasibility study for clinical implementation. Image acquisition time was 17-42s on the Halcyon system compared with 60s on the C-arm LINAC systems. The kV imager projection offset, imaging and treatment isocenter coincidence and the couch three-dimensional match movement all achieved less than1mm mechanical accuracy. All major image quality metrics were within either the national guideline or vendor-recommended tolerances. The designed surrogate approach with the QUART phantom showed a range of 0.24-0.35cycles/mm for spatial resolution, a contrast-to-noise ratio (CNR) of 2-20 for FDK reconstruction and a tolerance of 0.5mm for slice thickness. Other metrics derived from the Catphan images obtained on the Halcyon and C-arm LINACs showed comparable values for the FDK reconstruction. The iterative reconstruction tended to reduce noise, as evidenced by a higher CNR ratio. The fast scan pelvis protocols for Halcyon resulted in 50% lower dose compared to the standard scans, and the thorax fast protocol similarly delivered 10% lower dose than the standard thoracic scan. Preliminary patient images indicated that rapid kV CBCT with breath-hold is feasible, with improved imaging quality compared to free-breathing scans. Independent and comprehensive characterization of the kV imaging guidance system on the Halcyon 2.0 system demonstrated acceptable image quality for clinical use. The imaging unit onboard the Halcyon meets vendor specifications and satisfies requirements for routine clinical use. The fast kV imaging system enables the potential for volumetric CBCT acquisition during a single breath-hold and the iterative reconstruction tends to reduce the noise therefore has the potential to improve the CNR for normal size patient.

Plan quality comparison for cervical carcinoma treated with Halcyon and Trilogy intensity-modulated radiotherapy.

Purpose: Varian Halcyon is a novel machine with dual-layer leaves, single flattening filter free (FFF) energy and an enclosed bore. The purpose of this study was to compare the differences in dosimetry and plan parameters of intensity-modulated radiation therapy (IMRT) plans between the Halcyon and Trilogy accelerators.Methods and Materials: A total of 30 IMRT plans from cervical carcinoma patients were retrospectively analyzed on the Trilogy and Eclipse v13.5 treatment planning systems (TPSs). For each patient, a new plan based on Halcyon was created with the same planning parameters and optimization constraints using the Eclipse Version 15.1 TPS. To compare plan qualities, dosimetry parameters regarding planning target volume (PTV), organs at risk (OARs), monitor unit (MU) efficiency, segment size and treatment time were evaluated. Evaluation of the helical diode array system was performed with gamma-index analysis.Results: The dose distribution of the target volume of the Halcyon and Trilogy plans showed no significant difference (p > 0.05). The mean doses of rectum and both femoral heads for Halcyon plans were significantly reduced compared to those for Trilogy plans (p < 0.05). Compared to Trilogy, Halcyon increased the number of MUs from 1542.9±248.3 MU to 2514.9±328.2 MU (p = 0.00) and decreased the delivery time from 11.28±1.36 min to 3.26±0.26 min (p = 0.00). The average segment areas of Halcyon plans for proximal and distal multileaf collimators (MLCs) were 42.1 ± 31.2 cm2 and 28.4 ± 23.7 cm2, respectively, and that of Trilogy plans was 27.3 ± 16.9 cm2. The mean gamma index (3 mm/3%) results for the Halcyon and Trilogy plans were 99.41±0.26 and 99.76±0.32 (p > 0.05), respectively.Conclusions: All Halcyon treatment plans were recognized as clinically acceptable and had statistically better OAR sparing with higher delivery efficiency. The Halcyon system exhibited fast treatment delivery of IMRT with good dosimetric agreement using ArcCHECK.

33 Commissioning and dosimetric characteristics of new halcyon system

Introduction A new Halcyon linear accelerator had been installed at clinique Charcot in Sainte-Foy-Les-Lyon. It’s the first one in France. The accelerator head is mounted on a ring gantry. There is no jaws and, for each bank, two MLC (proximal and distal) of 29 and 28 leaves of 1 cm width at isocenter with an offset of 5 mm to minimize the transmission. The maximum speed of the leaves is 5 cm/s. The maximum field size is 28 × 28 cm2. Halcyon system has one unique energy of 6XFFF and a maximum dose rate of 800 UM/min. The MV imaging panel size is 43 × 43 cm2, fxed at a distance source to imager of 154 cm and its resolution is 0.22 mm at isocenter. Currently, Halcyon system has only MV imaging (MV/MV or MV CBCT). Imaging dose is calculated by TPS Eclipse (Varian) and taken into account during optimization. Beam model of Halcyon is already preinstalled and fully modelled in Eclipse. Methods All the preinstalled data in Eclipse were measured in a water tank Blue Phantom2 (IBA) using CC13 (IBA), EDGE (SunNuclear) et Farmer (PTW) detectors in accordance with Varian measurements. PDD, profiles, FOC, transmission factor and DLG measurements were compared with preinstalled beam model. Calculated dose distribution with AAA algorithm for PDD and profiles were compared with our measurements using tolerances from SFPM report n°27. End-to-end and RapidArc commissioning were performed. Imaging dose from clinical radiotherapy plans were calculated from several anatomical regions and measured in an anthropomorphic phantom CIRS with a semiflex (PTW) ionisation chamber. Results A very good superposition of preinstalled data with our measurements was observed for every data. Largest variations were observed for small field profiles 2 × 2 cm2. Mean and maximum variations for FOC measurements were −0.03% and 0.71%. Leaf transmission measured through both banks is 0.005%. Beam quality index (PDD(10) with SSD = 100 cm) is 63.5% (Varian specification 63% ± 1). Maximum depth dose for a 10x10cm2 field size at SSD = 100 cm is 1.28 cm (Varian specification 1.3 cm ± 0.2). For every field sizes 100% of the points for PDD and profiles pass the tolerances from SFPM report n°27. Gantry isocentricity measured with Drum phantom (Varian) is 0.65 mm. Absolute dose difference at isocenter for a VMAT plan measured during an End-to-End test performed in the IMRT Thorax Phantom (CIRS) is 1%. Conclusions An excellent agreement between Varian preinstalled beam model and our measures were observed and therefore no changes to the preinstalled beam model have been made.

Feasibility Study for 3-Dimensional Treatment Planning with Flattening Filter Free (FFF) Beam and Jawless Dual-Level MLC Collimation for Breast Irradiation

Varian Halcyon system with 6MV flattening filter free (FFF) beam and jawless dual-level MLC collimation is mainly intended for faster IMRT/VMAT delivery. We examined if three- dimensional (3D) treatment planning is feasible for breast irradiation with Varian Halcyon system. Ten left sided breast cancer patients treated with conventional 6MV and 3D field in field (FIF) technique (group1; breast volume < 1000 cc) and 5 patients treated with mixed energy of 6/18 MV and 3D FIF technique (group2; breast volume ≥ 1000 cc) were selected to re-plan with Varian Halcyon system using 3D FIF technique. Treatment plans for both groups with Varian Halcyon system using 3D FIF were generated using a treatment planning system based on reference plan parameters previously used for the treatment. FIF planning process with Halcyon system for each case was similar to conventional 3D FIF technique, and the plan criteria was to achieve the same (or similar) D95% for breast planning target volume (PTV) as the reference plan. Plan quality was assessed using 1) V105% (PTV covered by 105% of prescription dose; dose homogeneity), 2) mean heart dose, 3) maximum heart dose, 4) V20Gy (ipsilateral lung volume covered by 20Gy) and compared to the reference plan for each case. In addition, three tangential beam IMRT breast plans treated with 6/18 MV mixed energies (group3) were re-planned with Halcyon system using IMRT technique. The mean breast volume for group 1 and group 2 were 548 cc (ranges 312-912 cc) and 1172 cc (ranges 1000- 1360 cc), respectively. D95% for PTV was almost identical between reference plan and Halcyon plan for both groups. Other mean dosimetric values with group1 were as following; 1) V105: 5.8 % vs. 8.3 % (p = 0.006), 2) Mean heart dose: 0.88 Gy vs. 1.13 Gy (p = 0.01), 3) Max heart dose: 13.1 Gy vs. 13.5 Gy (p = 0.8), 4) Ipsilateral lung V20 Gy: 8.7 % vs. 10.4 % (p = 0.26) for reference plan and Halcyon plan, respectively. Similarly for group 2; 1) V105: 5.9 % vs.22.2 % (p = 0.03), 2) Mean heart dose: 0.73 Gy vs. 1.18 Gy (p < 0.001), 3) Max heart dose: 7.7 Gy vs. 11.6 Gy (p = 0.2), 4) Ipsilateral lung V20 Gy: 7.0 % vs. 7.4 % (p = 0.85) for reference plan and Halcyon plan, respectively. For the IMRT plan comparison (group 3; breast volume ranges 1255-1687 cc), all 3 reference plans showed better dosimetric results compared to Halcyon plans; 1) V105: 5.2 % vs.18.1 %, 2) Mean heart dose: 0.85 Gy vs. 3.0 Gy, 3) Max heart dose: 10.6 Gy vs. 40.3 Gy, 4) Ipsilateral lung V20 Gy: 5.3 % vs. 14.9 % for reference plan and Halcyon plan, respectively. The 3D treatment planning with Varian Halcyon system is feasible for breast irradiation with comparable dose homogeneity and critical organs dosimetry for breast volume < 1000 cc. However, for large breast volumes the dose homogeneity is significantly worse with both 3D plan and IMRT probably because of having only low energy with non-flattened beam.

Independent validation of machine performance check for the Halcyon and TrueBeam linacs for daily quality assurance.

PurposeTo evaluate the ability of the machine performance check (MPC) on the Halcyon to detect errors, with comparison with the TrueBeam.Methods MPC is an automated set of quality assurance (QA) tests that use a phantom placed on the couch and the linac's imaging system(s) to verify the beam constancy and mechanical performance of the Halcyon and TrueBeam linacs. In order to evaluate the beam constancy tests, we inserted solid water slabs between the beam source and the megavoltage imager to simulate changes in beam output, flatness, and symmetry. The MPC results were compared with measurements, using two‐dimensional array under the same conditions. We then studied the accuracy of MPC geometric tests. The accuracies of the relative gantry offset and couch shift tests were evaluated by intentionally inserting phantom shifts, using a rotating or linear motion stage. The MLC offset and absolute gantry offset tests were assessed by miscalibrating these motions on a Halcyon linac.ResultsFor the Halcyon system, the average difference in the measured beam output between the IC Profiler and MPC, after intentional changes, was 1.3 ± 0.5% (for changes ≤5%). For Halcyon, the MPC test failed (i.e., prevented treatment) when the beam symmetry change was over 1.9%. The accuracy of the MLC offset test was within 0.05 mm. The absolute gantry offset test was able to detect an offset as small as 0.02°. The accuracy of the absolute couch shift test was 0.03 mm. The accuracy of relative couch shift test of Halcyon was measured as 0.16 mm.ConclusionWe intentionally inserted errors to evaluate the ability of the MPC to identify errors in dosimetric and geometric parameters. These results showed that the MPC is sufficiently accurate to be effectively used for daily QA of the Halcyon and TrueBeam treatment devices.

PO-0993: Evaluation of MV imaging dose for the first clinical Halcyon system

PO-0993: Evaluation of MV imaging dose for the first clinical Halcyon system