MV Flattening Filter Free Beams Research Articles

Semi-automated vertex placement for lattice radiotherapy and dosimetric verification using 3D polymer gel dosimetry.

To evaluate the feasibility of an open-source, semi-automated, and reproducible vertex placement tool to improve the efficiency of lattice radiotherapy (LRT) planning. We used polymer gel dosimetry with a Cone Beam CT (CBCT) readout to commission this LRT technique. We generated a volumetric modulated arc therapy (VMAT)-based LRT plan on a 2 L NIPAM polymer gel dosimeter using our Eclipse Acuros version 15.6 AcurosXB beam model, and also recalculated the plan with a pre-clinical Acuros v18.0 dose calculation algorithm with the enhanced leaf modelling (ELM). With the assistance of the MAAS-SFRThelper software, a lattice vertex diameter of 1.5cm and center-to-center spacing of 3cm were used to place the spheres in a hexagonal, closed packed structure. The verification plan included four gantry arcs with 15°, 345°, 75°, 105° collimator angles. The spheres were prescribed 20Gy to 50% of their combined volume. The 6 MV Flattening Filter Free beam energy was used to deliver the verification plan. The dosimetric accuracy of the LRT delivery was evaluated with 1D dose profiles, 2D isodose maps, and a 3D global gamma analysis. Qualitative comparisons between the 1D dose profiles of the Eclipse plan and measured gel showed good consistency at the prescription dose mark. The average diameter measured 13.3±0.2mm (gel for v15.6), 12.6mm (v15.6 plan), 13.1±0.2mm (gel for v18.0), and 12.3mm (v18.0 plan). 3D gamma analysis showed that all gamma pass percent were>95% except at 1% and 2% at the 1mm distance to agreement criteria. This study presents a novel application of gel dosimetry in verifying the dosimetric accuracy of LRT, achieving excellent 3D gamma results. The treatment planning was facilitated by publicly available software that automatically placed the vertices for consistency and efficiency.

A comprehensive Study of the Out of Field Non Target Dose Associated with 6 and 10 MV Flattened and Flattening Filter Free X Ray Beam in a True Beam Linear Accelerator.

To evaluate the out-of-field dose associated with flattened (FF) and flattening filter-free (FFF) 6 and 10 MV X-ray beams in a TrueBeam linear accelerator (Linac). Measurements were taken in a slab phantom using the metal oxide semiconductor field effect transistor (MOSFET) detector at varying depths (dmax, 5 cm, and 10 cm) for clinically relevant field sizes and up to 30 cm from the field edges for 6 and 10 MV FF and FFF beams in TrueBeam Linac. Dose calculation accuracy of the analytic anisotropic algorithm (AAA) and Acuros algorithm was investigated in the out-of-field region. Similarly, the out-of-field dose associated with volumetric modulated arc therapy (VMAT) head-and-neck plan delivered to a body phantom was evaluated. The out-of-field dose for both FF and FFF photon beams (6 and 10 MV) decreased with increasing distance from the field boundary and size. Furthermore, regardless of FF in the field, higher-energy photon beams were associated with lower out-of-field dose. Both algorithms underestimated the dose in the out-of-field region, with AAA failing to calculate the out-of-field dose at 15 cm from the field edge and Acuros failing to calculate out-of-field radiation at 20 cm. At 5 cm from the field edge, an average of 50% underestimation was observed, and at 10 cm, an average of 60% underestimation was observed for both FF and FFF (6 and 10 MV) beams. The VMAT head-and-neck plan performed with the FFF beam resulted in a lower out-of-field dose than the FF beam for a comparable dose distribution. Compared with flattened beams, the FFF modes on TrueBeam Linac exhibited a clinically relevant reduction in the out-of-field dose. Further dosimetric studies are warranted to determine the significant benefit of FFF beams across different cancer sites.

Suitability of novel 3D-printed and coated vessels for water calorimetry.

In water calorimetry, absolute dose to water is determined by measuring radiation-induced temperature rises. In conventional water calorimeters, temperature detectors are housed in handmade glass vessels that are filled with high-purity water, thus mitigating radiation-induced exo/endothermic chemical reactions of impurities that would otherwise introduce additional heat gain/loss, known as heat defect. Being hand-crafted, these glass vessels may suffer from imperfections, have shape and design constraints, are often backordered, and can be prohibitively expensive. The purpose of this work is to determine suitability of 3D-printed plastic vessels that are further coated for use in water calorimetry applications, and to study their stability and characterize their associated heat defect correction factor ( . This novel vessel production technique would allow for cost-effective rapid construction of vessels that can be produced with high accuracy and designs that are simply not practical with current glass vessel construction techniques. This in turn enables water calorimetry applications in many novel radiation delivery modalities, which may include spherical vessels in GammaKnife ICON water calorimetry as an example. Eight vessels were 3D-printed using Accura ClearVue in an SLA 3D-printer. Two vessels were coated with Parylene C and four were coated with Parylene N. The water calorimetry preparation procedures followed for these vessels was identical to that of our traditional glass-vessels (i.e., same cleaning procedures, same high purity water, and same saturation procedures with high purity hydrogen gas). The performance of each vessel was characterized using our in-house built water calorimeter in an Elekta Versa using both 6MV flattening filter-free (FFF) and 18MV beams. The stability of the coating as function of time and accumulated dose was evaluated through repeated measurements. of each vessel was determined through cross-comparisons against an Exradin A1SL ionization chamber with direction calibration link to Canada's primary standard laboratory. of the two uncoated vessels differed by 2.8% under a 6MV FFF beam. Vessels coated with Parylenes resulted in a stable and reproducible heat defect for both energies. An overall of 1.001±0.010 and 1.005±0.010 were obtained for Parylene N and Parylene C coated vessels respectively. All Parylene coated vessels showed agreement, within the established uncertainties, to the zero-heat defect observed in a hydrogen-saturated glass vessel system. An additional long-term study (17 days) of a Parylene N vessel showed no change in response with accumulated dose and time. Electron microscopy images of a Parylene N coated vessel showed a uniform intact coating after repeated irradiations. An uncoated 3D-printed vessel is not viable for water calorimetry because it exhibits an unstable vessel-dependent heat defect. However, applying a Parylene coating stabilizes the heat defect, suggesting that coated 3D-printed vessels may be suitable for use in water calorimetry. This method facilitates the creation of intricate vessel shapes, which can be efficiently manufactured using 3D printing.

Technical note: Point-by-point ion-recombination correction for accurate dose profile measurement in high dose-per-pulse irradiation field.

Although flattening filter free (FFF) beams are commonly used in clinical treatment, the accuracy of dose measurements in FFF beams has been questioned. Higher dose per pulse (DPP) such as FFF beams from a linear accelerator may cause problems in dose profile measurements using an ionization chamber due to the change of the charge collection efficiency. Ionization chambers are commonly used for percent depth dose (PDD) measurements. Changes of DPP due to chamber movement during PDD measurement can vary the ion collection efficiency of ionization chambers. In the case of FF beams, the DPP fluctuation is negligible, but in the case of the FFF beams, the DPP is 2.5 ∼ 4 times larger than that of the FF beam, and the change in ion collection efficiency is larger than that of the FF beam. PDD profile normalized by maximum dose depth, 10 cm depth for example, may therefore be affected by the ion collection efficiency. In this study, we investigate the characteristics of the ion collection efficiency change depending on the DPP of each ionization chamber in the FFF beam. We furthermore propose a method to obtain the chamber- independent PDD by applying a DPP-dependent ion recombination correction. Prior to investigating the relationship between DPP and charge collection efficiency, Jaffe-plots were generated with different DPP settings to investigate the linearity between the applied voltage and collected charge. The absolute dose measurement using eight ionization chambers under the irradiation settings of 0.148, 0.087, and 0.008 cGy/pulse were performed. Applied voltages for the Jaffe-plots were 100, 125, 150, 200, 250, and 300V. The ion recombination correction factor Pion was calculated by the two-voltage analysis (TVA) method at the applied voltages of 300 and 100V. The DPP dependency of the charge collection efficiency for each ionization chamber were evaluated from the DPP- Pion plot. PDD profiles for the 10 MV FFF beam were measured using Farmer type chambers (TN30013, FC65-P, and FC65-G) and mini-type chambers (TN31010, TN31021, CC13, CC04, and FC23-C). The PDD profiles were corrected with ion recombination correction at negative and positive polar applied voltages of 100 and 300V. From the DPP-Pion relation for each ionization chamber with DPP ranging from 0.008 cGy/pulse to 0.148 cGy/pulse, all Farmer and mini-type chambers satisfied the requirements described in AAPM TG-51 addendum. However, Pion for the CC13 was most affected by DPP among tested chambers. The maximum deviation among PDDs using eight ionization chambers for 10 MV FFF was about 1%, but the deviation was suppressed to about 0.5% by applying ion recombination correction at each depth. In this study, the deviation of PDD profile among the ionization chambers was reduced by the ion recombination coefficient including the DPP dependency, especially for high DPP beams such as FFF beams. The present method is particularly effective for CC13, where the ion collection efficiency is highly DPP dependent.

Benchmarking a New Circular Cone-based Radiosurgery System against Clinically Tested Radiosurgery System on the same Novel Digital Linear Accelerator Platform.

To examine the dosimetric characteristics of circular cones, the accuracy of dose modeling and overall treatment delivery of two radiosurgery systems integrated on a linear accelerator (Linac). The dosimetric characteristics of circular cones (4-17.5 mm) from Varian (VC) and BrainLAB (BLC) were measured for 6 MV flattening filter free beam from Edge linac using stereotactic field diode and 0.65 cc ionization chamber following established protocols. The Eclipse and iPlan modeled dose distribution for VCs and BLCs were validated with EBT3-film measurement. End-to-end tests were performed using stereotactic phantom having PTW 60008 diode connected to a Dose-1 electrometer. The depth at dose maximum, TRP2010 and dose at 10cm depth of the same size VC and BLC agree within ± 0.7 mm, ± 0.71% and ± 0.81% respectively. Full width at half maximum (FWHM) of any cone beyond 15 mm depth increases at 1% of nominal cone size per 10 mm depth. The penumbra of 4mm and 17.5mm VC at 15 mm depth was 1.1 mm and 1.50 mm. At 300 mm depth, penumbra increased by around 0.4 mm for 4 mm cone and up to 1 mm for cone size ≥12.5 mm. The VCs penumbra values were within ±1mm of the corresponding BLCs. Scatter factors for VCs varies from 0.609 to 0.841 and were within ± 1.0% of corresponding values of BLCs. Agreement between the Eclipse and iPlan computed dose fluence and the EBT3-film measured dose fluence was >98% (γ: 1%@1 mm), and the absolute dose difference was ≤ 2.2%, except for the 4 mm cone in which it was >96% and ≤4.83%. Target localization using cone-beam computed tomography was accurate within ± 0.8 mm and ± 0.3° in translation and rotation. The end-to-end dose delivery accuracy for both radiosurgery systems was within ± 3.62%. The dosimetric characteristics of Varian and BLC cones of same diameter was comparable. Both Eclipse and iPlan cone planning system modeled dose fluences agree well with the EBT3 film measurement. The end-to-end tests revealed an excellent target localization accuracy of Edge linac with satisfactory and comparable absolute dose agreement between Varian and BLC radiosurgery systems and hence these can be interchanged on edge linac.

Dosimetric Parameters in Hypofractionated Stereotactic Radiotherapy for Brain Metastases: Do Flattening Filter-Free Beams Bring Benefits? A Preliminary Study.

This study aimed to compare the dosimetric results of flattening filter-free (FFF) vs. flattened (FF) treatment plans for fractionated stereotactic radiotherapy (fSRT), with the goal to highlight potential advantages of FFF beams. A group of 18 patients with brain metastases treated with fSRT (30 Gy delivered in 5 fractions) were included. The dosimetric parameters evaluated were: (1) physical dosimetric parameters (number of monitor units (MUs), conformity index (CI), dose gradient index (DGI), beam on time (BOT)); (2) clinical dosimetric parameters pertaining to target volume (PTV) and organs at risk (OARs). Two treatment plans were performed for all patients: one used 6 MV FFF beams and the other used 6 MV flattened beams. A slight increase in MUs was observed for the FFF mode (+23.3 MUs). The CI showed a difference of -2.7% for the FF plans (p = 0.28), correlated with a poorer coverage of the PTV. DGI values reported in terms of PTV are in line with international recommendations and showed a +1.9% difference for FFF plans. An average BOT of 90.3 s was reported for FFF plans, which was 2.3 times shorter than that required for FF plans delivery (p ≤ 0.001). A slight decrease of PTV coverage (-1.26%, p = 0.036) for FF plans can be considered relevant, but no other significant differences were observed between the two optimizations. No statistically significant benefit of using FFF beams to reduce V20 for normal brain could be demonstrated. These dosimetric results encourage the implementation of fSRT with standard flattened beams in centers where FFF linacs are not available.

Validating TG119 VMAT Commissioning on a Volumetric Phantom

<h3>Purpose/Objective(s)</h3> The present work adapts AAPM TG119 on a volumetric detector array and evaluates its feasibility for VMAT commissioning for two different Treatment Planning Systems (TPS). <h3>Materials/Methods</h3> TG119 describes a group of test plans for standard structure sets following prescription and dose constraints objectives. These plans were calculated and evaluated using both Elekta Monaco V5.0 (Montecarlo) and Varian Eclipse v15.6 (AAA algorithm) TPS. TG119 structure sets were inserted on virtual CT images for volumetric Quality Assurance (QA) phantom. A set of plans calculated on both TPS were delivered on a 6MV photon beam on an Elekta Synergy Linac. A set of plans was generated only on Eclipse for a Varian Halcyon Linac with 6MV Flattening Filter Free beam. Generated plans were delivered, dose distribution was concurrently measured with Ion Chamber (IC) point doses using a cavity plug holder inserted on the QA phantom. Dose constraints and goals comparison between TPS was executed. Gamma analysis was performed for 3%3mm and 3%2mm global gamma index and Confidence Limits (CL) according to the TG119 definition of Normal quantile were calculated. Measured and planned point doses on relevant positions were compared. <h3>Results</h3> TG119 goals were mostly satisfied except for some critical situations: PTV D99 goal on head & neck plan calculated on Eclipse TPS and Superior Target D10 goal on Multitarget Plan for Monaco TPS calculation. Gamma Analysis for 3%3mm range of passing rate was 96.8% to 100% while CLs range between 4.1% and 4.7% for a Normal quantile for Monaco TPS. For Eclipse TPS range of passing rate achieved was among 97.2% and 99.9%, CLs range was between 3.2% and 3.1%. For Eclipse plans delivered on Halcyon, the passing rate range was among 98.6% and 99.8%, CLs between 1.6% and 1.7%. 3%2mm global gamma index analysis shows a range of passing rate of 93.5% and 100% for Monaco TPS while CLs range between 4.8% and 8.3%. For Elekta plans on Eclipse TPS range of passing rate was 95.6% and 98.8%, CLs range was between 5.1% and 4.5%. The gamma passing rate range for Halcyon plans was between 94.5% and 99.3 and CLs between 3.4% and 6.0%. IC measurements for Synergy Linac showed an average dose difference of 0.84% between measured and calculated plans on Monaco, range among -3.15% and 7.01%, the greatest differences were found on C-Shape plan for high gradient regions measurements. For Eclipse TPS, the average dose difference was 1.28%, range between -1.04% and 10.26%, the highest difference was found in high gradient region C-Shape plan. For Halcyon plans, IC measurements showed an average dose difference of 0.74%, ranging between -0.27% and 3.38%. <h3>Conclusion</h3> CLs agreed with TG119 reference values even when stricter requirements were used. New criteria are more appropriate for modern treatments, but TG119 methodology remains a valid procedure for VMAT. The methodology described for commissioning VMAT technique on a volumetric QA phantom shows to be robust and consistent for various TPS and lilacs.

On-line adaptive and real-time intrafraction motion management of spine-SBRT on an MR-linac

Purpose: The superior soft-tissue contrast of MRI-guided radiotherapy offers enhanced localization accuracy of the spinal cord in spine Stereotactic Body Radiotherapy (SBRT). This work includes a planning study for spine-SBRT on an MR-Linac. Additionally, a patient with spine metastasis was treated using an adaptive radiation therapy workflow. We report our initial experience of targeting accuracy, image-guided localization, on-line adaptive planning, and treatment with real-time intrafraction imaging with automatic beam gating.Methods: Six spine-SBRT patients were retrospectively re-planned to 18 Gy in 1-fraction on a commercial, Monte Carlo-based MR-Linac treatment planning system. Plans were generated using 9–13 step-and-shoot intensity-modulated radiation therapy 6 MV-flattening filter free beams and optimized to achieve plan quality criteria recommended by RTOG-0631. One thoracic vertebral body clinical case was treated to 27 Gy in 3-fractions utilizing ART, where daily anatomical changes were accounted for via re-planning and treatment in an on-line manner to account for limited ability to correct rotational setup uncertainties.Results: Plans met all critical-tissue constraints outlined in RTOG-0631 and AAPM Task Group-101, while covering 90% of the target with the prescription dose. Clinically, visibility of the spinal cord allowed for patient setup focusing on spinal cord-alignment. Utilization of the online ART workflow, while re-contouring the target and spinal cord, enabled an increase in prescription dose coverage from 89 to 95% in two of three fractions while maintaining acceptable doses to organs-at-risk. Real-time MR-cine imaging demonstrated sufficient quality for the automatic beam gating algorithm to provide intrafraction motion management of the spinal canal utilizing a 3.0 mm gating boundary and 1–2% region of excursion allowance, in the sagittal plane. A decrease in coverage, below the 95% threshold was noted in post-treatment volumetric imaging due to lateral movement not observed during real-time gating.Conclusion: Achieved plan quality and deliverability was within accepted standards. MR-guidance with an on-line ART workflow offered increased accuracy in the localization of the spinal cord at the time of treatment to enhance both tissue sparing and target volume coverage. Increased spatial resolution of cine-images, and tracking in three-dimensions would be beneficial for future spine-SBRT treatments on the MR-Linac.

3D star shot analysis using MAGAT gel dosimeter for integrated imaging and radiation isocenter verification of MR-Linac system.

PurposeThis study aims to investigate a star shot analysis using a three‐dimensional (3D) gel dosimeter for the imaging and radiation isocenter verification of a magnetic resonance linear accelerator (MR‐Linac).MethodsA mixture of methacrylic acid, gelatin, and tetrakis (hydroxymethyl) phosphonium chloride, called MAGAT gel, was fabricated. One MAGAT gel for each Linac and MR‐Linac was irradiated under six gantry angles. A 6 MV photon beam of Linac and a 6 MV flattening filter free beam of MR‐Linac were delivered to two MAGAT gels and EBT3 films. MR images were acquired by MR‐Linac with a clinical sequence (i.e., TrueFISP). The 3D star shot analysis for seven consecutive slices of the MR images with TrueFISP was performed. The 2D star shot analysis for the central plane of the gel was compared to the results from the EBT3 films. The radius of isocircle (ICr) and the distance between the center of the circle and the center marked on the image (ICd) were evaluated.ResultsFor MR‐Linac with MAGAT gel measurements, ICd at the central plane was 0.46 mm for TrueFISP. Compared to EBT3 film measurements, the differences in ICd and ICr for both Linac and MR‐Linac were within 0.11 and 0.13 mm, respectively. For the 3D analysis, seven consecutive slices of TrueFISP images were analyzed and the maximum radii of isocircles (ICr_max) were 0.18 mm for Linac and 0.73 mm for MR‐Linac. The tilting angles of radiation axis were 0.31° for Linac and 0.10° for MR‐Linac.ConclusionThe accuracy of 3D star shot analysis using MAGAT gel was comparable to that of EBT3 film, having a capability for integrated analysis for imaging isocenter and radiation isocenter. 3D star shot analysis using MAGAT gel can provide 3D information of radiation isocenter, suggesting a quantitative extent of gantry‐tilting.

Small static radiosurgery field dosimetry with small volume ionization chambers

PurposeTo evaluate the response of the four smallest active volume thimble type ionization chambers commercially available (IBA-dosimetry RAZOR Nano Chamber, Standard Imaging Exradin A16, IBA-dosimetry CC01 and PTW T31022) when measuring SRS cone collimated Flattening Filter Free (FFF) fields. MethodsWe employed Monte Carlo simulation for calculating correction factors as defined in IAEA TRS-483. Monte Carlo simulation beam model and ion chamber geometry definitions were supported by an extensive set of measurements. Type A and B uncertainty components were evaluated. ResultsCommissioning of Monte Carlo 6 MV and 10 MV FFF beam models yielded relative differences between measured and simulated dose distributions lower than 1.5%. Monte Carlo simulated output factors for 5 mm SRS field agree with experimental values within 1% local relative difference for all chambers. Smallest active volume ion chamber (IBA-dosimetry RAZOR Nano Chamber) exhibits smallest correction, being compatible with unity. Correction factor combined uncertainties range between 0.7% and 0.9%. Smallest uncertainties were recorded for smallest and largest active volume ion chambers, although the latter exhibited largest correction factor. Highest contribution to combined uncertainty was type B component associated with beam model initial electron spatial Full Width Half Maximum (FWHM) uncertainty. ConclusionsAmong the investigated chambers, the IBA RAZOR Nano Chamber was found to be an excellent choice for narrow beam output factor measurement since it requires minimum correction (in line with IAEA TRS-483 recommendations). This is caused by its tiny size and tissue equivalence materials which produce minimum volume averaging and fluence perturbation.

Comparison of calculation algorithms to predict the IQM detector response for various modulation degrees of VMAT treatment plans on linear accelerator equipped with the HD120 MLC.

The calculation model for the integral quality monitor (IQM) system does not take into account the characteristics of the HD120 multileaf collimator (MLC), which some Varian accelerators are equipped with. Some treatment plans prepared with this collimator are characterized by a high level of modulation. The aim of the work was to prepare a model for that collimator and to determine the influence of modulation on results of the verification carried out with the use of IQM system. The short and long stabilities of the IQM detector response were verified by measuring the signal for a 6MV flattening filter-free (FFF) beam with the static field of 10×10cm2 size. The obtained results were compared with the measurements performed with the PTW Farmer chamber. Next, the signals for 35 static square fields 4×4cm2 , covering the whole field 38×20cm2 , were measured with the IQM. Based on the results of these measurements, the original calculation model has been changed in order to achieve the smallest differences between calculations and measurements. While tuning the model, the characteristics of the HD120 MLC were included. Measurements were performed for 30 clinical plans (86 arcs) prepared with 6MV FFF beams. Among those 30 plans, there were were multitarget plans with single isocenter. For each plan, the modulation complexity score (MCS) was calculated. The measurement results were compared with the calculation results performed with the original and authors' calculation model. Very good stability of the short and long stabilities of the IQM detector response was obtained. Measurements performed for 35 static fields revealed that for the manufacturer's and for the authors' models, the deviation exceeded 3% for 12 and five of the 35 static fields, respectively. The differences for the manufacturer's and authors' algorithms were in the range of ±2% for the 15 and 26 of the fields, respectively. For original and the authors' models, the differences between measured and calculated signals (starting with the segment number 40) were within the range of ±3.5% for 87.6% and 96.7% of all arcs for the respective models. For both models, the dependence of the compliance of measurements and calculations on the MCS was observed. For most of the very modulated arcs, the measured signal was at least 3% lower than the calculated one. The largest differences between measurements and calculations were obtained for single-isocenter multitarget plans. The signal predicted by an algorithm taking into account the real geometry of the collimating system of the Edge accelerator (equipped with the HD120 MLC) made it possible to obtain greater consistency between the measurements and calculations. We characterized the dependence between the MCS of each arc and the compliance of the measurements and calculations. Much worse results were obtained for single-isocenter multitarget plans.

A regression-based approach to compute the pixels sensitivity map of linear accelerator portal imaging devices.

To determine the pixel sensitivity map (PSM) for amorphous silicon electronic portal imaging devices (EPIDs) using a single flood field signal. A raw EPID signal results from the incident particle energy fluence, the inherent pixels response, and the background signal. In large open fields, particle energy fluence is a slow-varying signal that is locally considered spatially constant. Pixels response is a fast and abrupt varying behavior. The background signal is due to the EPID panel electronics, which is determined during radiation absence. To determine the PSM, after correcting for the background signal, we apply a model that captures the underlying smooth particle energy fluence-induced signal. This fluence signal-fitted model is then used to determine the PSM. Here, we use a polynomial-based regression surface model in both x and y dimensions. To validate the generated PSM, we measure beams and compute PSMs for multiple beam energies with and without flattening filters and for multiple source-to-imager distances. Since the PSM is a detector characteristic, it should be independent of those variables. We also intercompare measurements of fixed slit fields with the EPID being shifted between measurements. The fluence signal of the flattening filter-free (FFF) beams was optimally modeled as a 12th degree polynomial surfaces, which had 0.1% residuals near the central axis. The 6 and 10MV FFF PSMs were within ˜0.1%, and independent of the EPID SID, suggesting that the PSM is energy independent. The 6, 10, and 15MV flattened-beam PSMs were well modeled as 12th degree polynomial surfaces, which were equivalent within ˜0.24% but differed from the FFF PSM by up to 0.5% near the beam central axis. Applying the FFF PSMs to the flattened-beam measurements reduced the central-axis deviation between the raw and corrected signal to 0.1%, confirming the PSM energy independence hypothesis. When the FFF PSM is utilized, output verification with shifted slit deliveries agreed within ˜0.5% for all beam energies, which is within the radiation delivery uncertainty of ˜0.57%. PSM for MV EPIDs can be determined by separating out the slowly varying, well-behaved fluence signal from the pixel-to-pixel sensitivity variations. The quality of the PSM is found to be dependent on the quality of the surface fit, which is best for the 6MV FFF beam measured at SID equal to 180cm. Within fitting errors, the PSM is independent of beam energy for 6, 10, and 15MV beams with and without flattening filters. The PSM generation does not require shifting the EPID panel nor multiple EPID panel irradiations and should be usable for linacs with fixed geometry EPIDs.

Out-of-field dose in stereotactic radiotherapy for paediatric patients.

Stereotactic radiotherapy combines image guidance and high precision delivery with small fields to deliver high doses per fraction in short treatment courses. In preparation for extension of these treatment techniques to paediatric patients we characterised and compared doses out-of-field in a paediatric anthropomorphic phantom for small flattened and flattening filter free (FFF) photon beams. Dose measurements were taken in several organs and structures outside the primary field in an anthropomorphic phantom of a 5year old child (CIRS) using thermoluminescence dosimetry (LiF:Mg,Cu,P). Out-of-field doses from a medical linear accelerator were assessed for 6MV flattened and FFF beams of field sizes between 2×2 and 10×10cm2. FFF beams resulted in reduced out-of-field doses for all field sizes when compared to flattened beams. Doses for FFF and flattened beams converged for all field sizes at larger distances (>40cm) from the central axis as leakage becomes the primary source of out-of-field dose. Rotating the collimator to place the MLC bank in the longitudinal axis of the patient was shown to reduce the peripheral doses measured by up to 50% in Varian linear accelerators. Minimising out-of-field doses by using FFF beams and aligning the couch and collimator to provide tertiary shielding demonstrated advantages of small field, FFF treatments in a paediatric setting.

Assessment of a conventional volumetric-modulated arc therapy knowledge-based planning model applied to the new Halcyon© O-ring linac in locoregional breast cancer radiotherapy.

The aim of this study was to evaluate the performance of a knowledge-based planning (KBP) model for breast cancer trained on plans performed on a conventional linac with 6 MV FF (flattening filter) beams and volumetric-modulated arc therapy (VMAT) for plans performed on the new jawless Halcyon© system with 6 MV FFF (flattening filter-free) beams. Based on the RapidPlan© (RP) KBP optimization engine, a DVH Estimation Model was first trained using 56 VMAT left-sided breast cancer treatment plans performed on a conventional linac, and validated on another 20 similar cases (without manual intervention). To determine the capacity of the model for Halcyon©, an additional cohort of 20 left-sided breast cancer plans was generated with RP and analyzed for both TrueBeam© and Halcyon© machines. Plan qualities between manual vs RP (followed by manual intervention) Halcyon© plans set were compared qualitatively by blinded review by radiation oncologists for 10 new independent plans. Halcyon© plans generated with the VMAT model trained with conventional linac plans showed comparable target dose distribution compared to TrueBeam© plans. Organ sparingwas comparable between the 2 devices with a slight decrease in heart dose for Halcyon© plans. Nine out of ten automatically generated Halcyon© plans were preferentially chosen by the radiation oncologists over the manually generated Halcyon© plans. A VMAT KBP model driven by plans performed on a conventional linac with 6 MV FF beams provides high quality plans performed with 6 MV FFF beams on the new Halcyon© linac.

Impact of 6 MV & 10 MV Flattened and Flattening Filter Free Beams in Whole Brain Radiotherapy: A Treatment Planning Study

Introduction:To study the impact of 6 MV and 10 MV flattened beam (FB) and flattening filter free (FFF) beam in whole brain radiotherapy (WBRT) by using volumetric modulated arc therapy (VMAT). Material and Methods: Twenty WBRTpatients were selected randomly. The dose prescription was 30 Gy, which was delivered in ten fractions. The planning target volume (PTV) and organs at risk (OARs) were contoured. Four VMAT plans, including 6 MV FB, 6 MV FFF, 10 MV FB, and 10 MV FFF beam plans, were generated. Results: The 6MV FB and FFF beam plans were statistically significant (p <0.05) in terms of the dose received by 98% of the PTV (D98%) (26.86 Gy vs. 27.31 Gy, P=0.006), the dose received by 95% of the PTV (D95%) (28.28 Gy vs. 28.52 Gy, P=0.038), 107% isodose (V107%) of the PTV (2.43% vs. 3.74%, P=0.001), D100% of the hippocampus (9.31 Gy vs. 9.16 Gy, P=0.009), and the Dmean scalp (16.7 Gy vs. 16.8 Gy, p=0.035). The 10 MV FB and FFF beam plans showed significant differences in the conformity index (0.9 vs. 0.85, P=0.01), V107% of the PTV (1.68% vs. 4.54%, P=0.001), D100% (10.08 Gy vs. 9.81 Gy, P=0.036), and Dmean of the hippocampus (12.78 Gy vs. 12.57 Gy, P=0.018). The 6 MV and 10 MV FFF beams showed homogeneous conformal plans, which required 18-19% more MUs, compared to the FB plans. Conclusion: The 6 MV and 10 MV FB and FFFB spared the hippocampus and the scalp with acceptable target coverage in WBRT cases.

Commissioning of the TrueBeam STx 6 MV FFF Beam in the RayStation Treatment Planning System for SRS and SBRT Treatments

Purpose: The purpose of this study is to provide technical information on commissioning the TrueBeam STx 6 MV flattening-filter free (FFF) beam in the RayStation treatment planning system (TPS) for stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) treatments. Methods: For beam modeling, percent depth dose curves, profiles and output factors for jaw-collimated fields and stereotactic cones as well as X-jaws transmission were measured. For multi-leaf collimator (MLC) modeling, MLC model parameters such as offset, gain, curvature, leaf tip width, tongue and groove and transmission were determined and output factors for MLC-collimated fields were measured. Absolute dose calibration was also performed. For beam model and MLC model validation, the American Association of Physicists in Medicine Task Group-119 plans, clinical SRS and SBRT plans and end-to-end testing were performed. Results: Beam characteristics of the 6 MV FFF beam agreed well with those in the literature. Validation results showed that our beam model and MLC model were acceptable for SRS and SBRT treatments. Conclusions: The technical information and dosimetric data provided in this study will be a useful reference for other clinics/institutions which will commission the same machine energy in the RayStation TPS.

First-stage validation of a portable imageable MR-compatible water calorimeter.

The purpose of this study is to design a water calorimeter with three goals in mind: (a) To be fully magnetic resonance (MR)-compatible; (b) To be imaged using kV cone beam computed tomography (CBCT), MV portal imaging or MRI for accurate positioning; (c) To accommodate both vertical and horizontal beam incidence, as well as volumetric deliveries or Gamma Knife®. Following this, the calorimeter performance will be measured using an accelerator-based high-energy photon beam. A portable 4°C cooled stagnant water calorimeter was built using MR-compatible materials. The walls consist of layers of acrylic plastic, aerogel-based material acting as thermal insulation, as well as tubing for coolant to flow to keep the calorimeter temperature stable at 4°C. The lid contains additional pathways for coolant to flow through as well as two hydraulically driven stirrers. The water calorimeter was positioned in an Elekta Versa using kV CBCT imaging as well as orthogonal MV image pairs. Absolute absorbed dose to water was then determined under a 6MV flattening filter-free (FFF) beam. This was compared against reference dosimetry results that were measured under identical conditions with an Exradin A1SL ionization chamber with a calibration coefficient directly traceable to the National Research Council Canada. The dose to water determined with the calorimeter (n=30) agreed with the A1SL ionization chamber reference dose measurements (n=15) to within 0.25%. The uncertainty associated with the water calorimeter absorbed dose measurement was estimated to be 0.54% (k=1). An MR-compatible water calorimeter was successfully built and absolute absorbed dose to water under a conventional 6MV FFF beam was determined successfully as a first-stage validation of the system.

Influence of 0.35T magnetic field on the response of EBT3 and EBT-XD radiochromic films.

To investigate the inconsistency of recent literature on the effect of magnetic field on the response of radiochromic films, we studied the influence of 0.35T magnetic field on dosimetric response of EBT3 and EBT-XD GafchromicTM films. Two different models of radiochromic films, EBT3 and EBT-XD, were investigated. Pieces of films samples from two different batches for each model were irradiated at different dose levels ranging from 1 to 20Gy using 6MV flattening filter free (FFF) x-rays generated by a clinical MR-guided radiotherapy system (B=0.35T). Film samples from the same batch were irradiated at corresponding dose levels using 6MV FFF beam from a conventional linac (B=0) for comparison. The net optical density was measured 48h postirradiation using a flatbed scanner. The absorbance spectra were also measured over 500-700nm wavelength range using a fiber-coupled spectrometer with 2.5nm resolution. To study the effect of fractionated dose delivery to EBT3 (/EBT-XD) films, 8 (/16)Gy dose was delivered in four 2 (/4)Gy fractions with 24h interval between fractions. No significant difference was found in the net optical density and net absorbance of the films irradiated with or without the presence of magnetic field. No dependency on the orientation of the film during irradiation with respect to the magnetic field was observed. The fractionated dose delivery resulted in the same optical density as delivering the whole dose in a single fraction. The 0.35T magnetic field employed in the ViewRay® MR-guided radiotherapy system did not show any significant influence on the response of EBT3 and EBT-XD GafchromicTM films.

Dosimetric Validation of Digital Megavolt Imager for Flattening Filter Free Beams in the Pre-Treatment Quality Assurance of Stereotactic Body Radiation Therapy for Liver Metastases.

Aim:The aim is to evaluate the use of digital megavolt imager (DMI) aS1200 in portal dosimetry with flattening filter free (FFF) beams. Materials and Methods:Dosimetric properties of DMI is characterized at 6MV FFF beams for signal saturation, dose linearity, dependency on dose-rate and source-detector distance (SDD), signal lag (ghosting), and back scatter. Portal dosimetry is done for twenty volumetric modulated arc therapy (VMAT) based stereotactic body radiotherapy (SBRT) plans for the treatment of liver metastases and the results are compared with repeated measurements of Octavius 4D. Results:The detector signal to monitor unit (MU) ratio drops drastically below 25MU. The detector linearity with dose is within 1% and no evidence of signal saturation as such. The aS1200 response variation across various dose rates and SDD is <0.4% and <0.2% respectively. The effect of ghosting increased distinctly at higher dose rate but however it is negligible (0.1%). The impact of back scatter is <0.3% because of additional shielding provided at the back of the detector. The portal dosimetry results of SBRT QA plans evaluated at the gamma criteria of 2mm/2% (DTA/DD) both under global and local mode analysis has shown an average gamma passing rate of area gamma (<1) 97.9±0.8% and 96.4±0.9%. The SBRT QA results observed in aS1200 are inline and consistent with Octavius 4D measured results. Conclusion:The characteristics of aS1200 evaluated at FFF beams have shown its potential ability as QA tool and can be used in SBRT QA for liver metastases with greater confidence.

Technical Note: Flattening filter free beam from Halcyon linac: Evaluation of the profile parameters for quality assurance.

The use of flattening filter free (FFF) beams generated by standard linear accelerators is increasing in the clinical practice. The radiation intensity peaked toward the beam central axis is properly managed in the optimization process of treatment planning through intensity modulation. Specific FFF parameters for profile analysis, as unflatness and slope for FFF beams, based on the renormalization factor concept has been introduced for quality assurance purposes. Recently, Halcyon, an O-ring based linear accelerator equipped with a 6MV FFF beam only has been introduced by Varian. Renormalization factors and related fit parameters according to Fogliata et al. ["Definition of parameters for quality assurance of FFF photon beams in radiation therapy," Med. Phys. 39, 6455-6464 (2012)] have been evaluated for the 6MV FFF beam generated by Halcyon units. The Halcyon representative beam data provided by Varian were used. Dose fall-off at the field edges was matched with an unflattened beam generated by a 6MV from a TrueBeam linac. Consistency of the results was evaluated against measurements on a clinical Halcyon unit, as well as a TrueBeam 6MV FFF for comparison. The five parameters in the analytical equation for estimating the renormalization factor were determined with an R2 of 0.997. The comparison of the unflatness parameters between the Halcyon representative and hospital beam data was consistent within a range of 0.6%. Consistently with the computed parameters, the Halcyon profiles resulted in a less pronounced peak than TrueBeam. Renormalization factors and related fit parameters from the 6MV FFF beam generated by the Varian Halcyon unit are provided.