K Kielar

An end-to-end examination of geometric accuracy of IGRT using a new digital accelerator equipped with onboard imaging system

The Varians new digital linear accelerator (LINAC), TrueBeam STx, is equipped with a high dose rate flattening filter free (FFF) mode (6 MV and 10 MV), a high definition multileaf collimator (2.5 mm leaf width), as well as onboard imaging capabilities. A series of end-to-end phantom tests were performed, TrueBeam-based image guided radiation therapy (IGRT), to determine the geometric accuracy of the image-guided setup and dose delivery process for all beam modalities delivered using intensity modulated radiation therapy (IMRT) and RapidArc. In these tests, an anthropomorphic phantom with a Ball Cube II insert and the analysis software (FilmQA (3cognition)) were used to evaluate the accuracy of TrueBeam image-guided setup and dose delivery. Laser cut EBT2 films with 0.15 mm accuracy were embedded into the phantom. The phantom with the film inserted was first scanned with a GE Discovery-ST CT scanner, and the images were then imported to the planning system. Plans with steep dose fall off surrounding hypothetical targets of different sizes were created using RapidArc and IMRT with FFF and WFF (with flattening filter) beams. Four RapidArc plans (6 MV and 10 MV FFF) and five IMRT plans (6 MV and 10 MV FFF; 6 MV, 10 MV and 15 MV WFF) were studied. The RapidArc plans with 6 MV FFF were planned with target diameters of 1 cm (0.52 cc), 2 cm (4.2 cc) and 3 cm (14.1 cc), and all other plans with a target diameter of 3 cm. Both onboard planar and volumetric imaging procedures were used for phantom setup and target localization. The IMRT and RapidArc plans were then delivered, and the film measurements were compared with the original treatment plans using a gamma criteria of 3%/1 mm and 3%/2 mm. The shifts required in order to align the film measured dose with the calculated dose distributions was attributed to be the targeting error. Targeting accuracy of image-guided treatment using TrueBeam was found to be within 1 mm. For irradiation of the 3 cm target, the gammas (3%, 1 mm) were found to be above 90% in all plan deliveries. For irradiations of smaller targets (2 cm and 1 cm), similar accuracy was achieved for 6 MV and 10 MV beams. Slightly degraded accuracy was observed for irradiations with higher energy beam (15 MV). In general, gammas (3%, 2 mm) were found to be above 97% for all the plans. Our end-to-end tests showed an excellent relative dosimetric agreement and sub-millimeter targeting accuracy for 6 MV and 10 MV beams, using both FFF and WFF delivery methods. However, increased deviations in spatial and dosimetric accuracy were found when treating lesions smaller than 2 cm or with 15 MV beam.

Verification of dosimetric accuracy on the TrueBeam STx: rounded leaf effect of the high definition MLC.

PURPOSE The dosimetric leaf gap (DLG) in the Varian Eclipse treatment planning system is determined during commissioning and is used to model the effect of the rounded leaf-end of the multileaf collimator (MLC). This parameter attempts to model the physical difference between the radiation and light field and account for inherent leakage between leaf tips. With the increased use of single fraction high dose treatments requiring larger monitor units comes an enhanced concern in the accuracy of leakage calculations, as it accounts for much of the patient dose. This study serves to verify the dosimetric accuracy of the algorithm used to model the rounded leaf effect for the TrueBeam STx, and describes a methodology for determining best-practice parameter values, given the novel capabilities of the linear accelerator such as flattening filter free (FFF) treatments and a high definition MLC (HDMLC). METHODS During commissioning, the nominal MLC position was verified and the DLG parameter was determined using MLC-defined field sizes and moving gap tests, as is common in clinical testing. Treatment plans were created, and the DLG was optimized to achieve less than 1% difference between measured and calculated dose. The DLG value found was tested on treatment plans for all energies (6 MV, 10 MV, 15 MV, 6 MV FFF, 10 MV FFF) and modalities (3D conventional, IMRT, conformal arc, VMAT) available on the TrueBeam STx. RESULTS The DLG parameter found during the initial MLC testing did not match the leaf gap modeling parameter that provided the most accurate dose delivery in clinical treatment plans. Using the physical leaf gap size as the DLG for the HDMLC can lead to 5% differences in measured and calculated doses. CONCLUSIONS Separate optimization of the DLG parameter using end-to-end tests must be performed to ensure dosimetric accuracy in the modeling of the rounded leaf ends for the Eclipse treatment planning system. The difference in leaf gap modeling versus physical leaf gap dimensions is more pronounced in the more recent versions of Eclipse for both the HDMLC and the Millennium MLC. Once properly commissioned and tested using a methodology based on treatment plan verification, Eclipse is able to accurately model radiation dose delivered for SBRT treatments using the TrueBeam STx.

MO‐D‐BRB‐11: Evaluation of the Accuracy of Eclipse AAA Modeling for Flatten Filter Free Beam Used for SBRT

Purpose: To evaluate the Eclipse analytical anisotropic algorithm (AAA) modeling of flattening filter free (FFF) megavoltage photon beams for a new Varian linear accelerator, TruebeamSTx ,TM, used for Gated SBRT. Materials and Methods: With the new Varian TruebeamSTx, Eclipse AAA was configured by using the measured dosimetric data. The TruebeamSTx can operate 6 and 10 MV photon beams free of the flattening filter (FFF). FFF photon beams are especially suitable for use of SBRT since they can operate at high dose rate (1400MU/min for 6MVFFF, 2400MU/min for 10MVFFF). Gated IMRT, RapidArc and gated RapidArc have routinely been used for SBRT, using the FFF photon beams. The accuracy of treatment plans with field sizes as small as 1×1 sq. cm are evaluated by measurement performed using films, ion chamber, and also with a Delta4 measurement system (ScandiDos). For gated treatments, measurements were done on a Respiratory Gating Platform (Standard Imaging) with motion simulating the actual patient breathing pattern. Results: The dosimetric leaf gap of the HD120MLC was adjusted to 1.7 mm with leaf transmission set to 0.015 so the calculated isodose distribution has the best fit with the measurement. The comparison between Eclipse and measured isodose distribution show an agreement of better than 95% using a gamma index of (3%/3mm). For gated RapidArc, the difference of FWHM of the measured and calculated profiles is found to be less than 0.2 mm with target size larger than 3 cm in diameter, and less than 1 mm for 1 cm diameter target size. Conclusions: The Eclipse AAA (V8.9) is found to be within 1mm accuracy for target volume as small as 1 cm in diameter. No major difference of the isodose distribution produced between the WFF and the FFF beams.

TH‐C‐BRA‐12: Tomographic Measurement of X‐Ray Beam Spot Profiles Using a Rotating Edge

Purpose: X‐ray beam spot size and shape are critical performance determinants of an imaging or treatment system. However, quantitative assessment of such spot profiles can prove difficult, particularly at MV energies. We have developed a novel and convenient tomographic spot measurement technique that uses a rotating edge phantom. The method can be applied to x‐ray systems equipped with flat panel imagers.Methods: Data were acquired at 10MV and 6MV on a Varian TrueBeam system. A 0.5 mm thick tantalum sheet (attenuation ∼5%) was placed on the systems rotatable treatment head with an edge abutting the axis of rotation. A total of 144 projections, each 10MU, were acquired at 2.5 degree steps. For each projection, a line‐spread function (LSF) is generated by differentiating the measured edge spread function. For sufficiently high source magnifications, the LSF, taken at a given rotation angle, is a tomographic projection of the x‐ray beam spot at that angle. The LSFs were then assembled into a sinogram and the corresponding spot profile was reconstructed using a parallel‐beam CT algorithm. The reconstructed profiles were compared to those measured using a film‐based ‘spot camera’ made from a 15 cm thick tungsten cylinder pierced with an array of small holes. Monte Carlo simulations of the edge and the spot camera experiments were also performed. Results: The edge technique produced profiles similar to those from the spot camera yet with higher resolution (0.17mm vs. 0.25mm). These results were confirmed by Monte Carlo simulations. The measured FWHM of the 10 MV spot was 1.6 mm. The 6 MV spot was slightly asymmetric with an average FWHM of 1.5 mm. Conclusions: A thin rotating edge with low attenuation can be used to accurately and conveniently measure x‐ray beam spot profiles. NIH NIH 1R01CA138426 Employees of Varian Medical Systems

SU‐GG‐T‐271: Dosimetric Properties of Flattening Filter Free Photon Beams from a New Clinical Accelerator

Purpose: To determine the dosimetricproperties of flattening filter free (FFF) megavoltage photon beams for a new Varian linear accelerator, Trilogy MX. Materials and methods: A new Varian linac consists of 6, 10 and 15 MV photon beams equipped with flattening filters (WFF), and 6 and 10 MV photon beam free of the flattening filter (FFF). Dosimetric data including depth dose, profiles, output factors, phantom scatter factors, and surface doses operated under the FFF mode were examined. Measurements of leaf transmission, leakage radiation, and off axis beam quality were also performed. Treatment plans using the FFF beams were evaluated to determine differences between WFF and FFF beams. Results: Depth of dose maxima for photon beams with WFF and FFF were compared. It was found that for FFF beams, dmax was shallower. The PDDs for the FFF beams were found to be lower than WFF beams. Surface doses were found to be larger for FFF. The flatness for a 10×10 cm2 field is 6.1 % (FFF) versus 2.5 % (WFF) for a 6MV at a depth of 10 cm. Sc,p were found to be higher for FFF beams for field sizes < 10×10 cm2, but lower otherwise. No significant dose distribution differences were observed between IMRTtreatment plans using the FFF or WFF beams. Conclusions: The results indicate that the physical characteristic of the FFF photon beams are not dramatically different from the WFF photon beams, except for uniformity across the field. However, FFF mode does result in a softer beam due to reduced beam hardening. The increased dose rate for FFF beams will reduce treatment time. Conflicts of interest: Research funding from Varian Medical Systems.

SU‐GG‐T‐145: Inverse Planning for IMRT with Flattening Filter Free (FFF) Beams

Purpose: High dose rate photon beam with FFF has recently become available for clinical use. For IMRT with FFF beams, the strategy of inverse planning should be modified to minimize the segments and MUs by accommodating the cone‐shaped beam fluence. Method and Materials: FFF beam data measured from a Varian Trilogy MX unit was employed for dose calculation and optimization. An incident beam is divided into a collection of 0.5×0.5cm beamlets. To accommodate the inherent non‐flatness of FFF fluence, a total‐variation regularization (TVR) is introduced in the objective function to encourage piece‐wise constant fluence in the FFF fluence domain, which is mathematically equivalent to working in the conventional flat fluence domain when the beamlet intensities are normalized according to the measured FFF fluence profile. The system was optimized by using MOSEK software package. The performance of the method is evaluated by using phantom and clinical cases. Results: TVR‐based inverse planning utilizes the known profiles of the incident beams and provides clinically sensible IMRTsolutions with much reduced number of segments as compared to conventional approaches. To obtain an inverted cone‐shaped isodose distribution (phantom case), the solution requires only one segment when TVR is used. Multiple segments and much higher MUs would be required when conventional beamlet‐based or direct aperture optimization is used. For clinical cases, the total numbers of segments are reduced significantly as compared to the beamlet‐based optimization without regularization. The final dose distributions are found to be more conformai than that obtained of DAO using 6 segments for each field. Conclusion: A TVR‐based inverse planning with explicit inclusion of FFF profiles provides an effective way to find IMRT plans with minimized number of segments or MUs and may find natural application in IMRT with FFF beams.

SU‐E‐T‐509: Photon Spectrum Modeling of Flattening Filter Free (FFF) Beam and the Optimization of Model Parameters

PURPOSE To determine the distribution of photon spectrum on flattening filter free (FFF) beams, novel and fast optimization methods that were applicable on a convolution/superposition dose calculation algorithm were implemented. METHODS Two-step optimization method was designed to model the virtual photon spectrums for FFF beams. At first, simple functional form of photon spectrums proposed by E. S. M. Ali was modified and used to make rough shapes of photon spectrum. The distributions of photon spectrums were defined at various field sizes (FSs) to consider the changes of the contribution for scattered photons. Percent depth doses (PDDs) at various FSs were used, and collapsed cone convolution (CCC) algorithm was used to calculate PDDs by considering cone-shaped photon fluence in fields. At next, an arbitrary functional form of photon spectrums where the values of photon intensity itself were free parameters was designed. Line search method was used for optimization and gradient terms at each free parameter were derived from CCC algorithm to enhance the speed of iterations. RESULTS The mean energies of the optimized spectrums were decreased from 1.40 to 1.21 MeV for 6 MV FFF beams and from 2.45 to 1.27 MeV for 10MV FFF beams as FSs were increased from 3×3 to 40×40 cm2 because of the contributions of scattered photons. The shape of the spectrums were not greatly changed with field sizes, but root mean squared differences (RMSDs) between the measured PDDs and the calculated PDDs using optimized spectrums were increased upto 0.87% as the FSs were decreased to 3×3 cm2 . CONCLUSIONS Developed method for spectrum modeling showed good agreements when the PDDs were calculated with the optimized results. Suggested method is proper to the radiation treatment planning systems because it only requires measured PDDs, and based on the analytic dose calculation algorithm. This work was supported by the National Research Foundation of Korea(NRFK)⌉ grant funded by the Korean government (MEST)⌉ (Grant No. K20901000001-09E0100-00110⌉), Varian Medical Systems, NCI Grant No.1R01 CA98523⌉, and NSF Grant No. 0854492⌉.

Development of a Beam Source Modeling Technique for a Flattening Filter Free (FFF) Beam

This study was focused on a new beam source modeling technique for a flattening filter free (FFF) beam. The model was based on a previous three source model, and improved by introducing off axis ratio (OAR) of photon fluence to the primary and scattered photon sources to generate cone shaped dose profiles. The model parameters and the OAR were optimized from measured head scatter factors and a dose profile with 40 x 40 cm2 field size by using line search optimization algorithm. The model was validated by comparing various dose profiles on 6 and 10 MV FFF beam from a True Beam STx linear accelerator. Planar dose distributions for clinically used radiation fields were also calculated and compared with measured data. All calculated dose profiles were agreed with the measured data within 1.5% for 6 MV FFF beam, and within 1% for 10 MV FFF beam. The calculated planar doses showed good passing rates (> 94%) at 3%/3 mm of gamma indexing criteria. This model expected to be easily applicable to any FFF beams for treatment planning systems because it only required measured PDD, dose profiles and output factors which were easily acquired during conventional beam commissioning process.

MO‐F‐BRB‐04: Multi‐Source Modeling of Flattening Filter Free (FFF) Beam and the Optimization of Model Parameters

Purpose: To suggest a novel analytic beam source model for a flattening filter free(FFF) beam, a multi‐source model and its optimization method applicable to a treatment planning system were developed. Methods: Previous three source model was improved by introducing off axis ratio (OAR) of primary photon fluence to generate cone shape profiles. The parameters of the model and the OAR were determined from measured head scatter factors and a measured dose profile of a 40 × 40 cm2 field size using a line search optimization technique. A new method to acquire gradient terms for OARs was developed to enhance the speed of the optimization process. The improved model was validated with measured dose profiles from 3 × 3 cm2 to 40 × 40 cm2 field sizes at 6 and 10 MV from a TrueBeamTM STx linear accelerator. Furthermore, planar dose distributions for clinically used radiation fields were also calculated and compared with measurements using a 2D array detector using the gamma index method. Results: All dose values for the calculated profiles agreed with the measured dose profiles within 0.5% at 6 and 10 MV beams, except for some low dose regions for larger field sizes. A slight overestimation was seen in the lower penumbra region near the field edge for the large field sizes by 1 % ∼ 4%. The planar dose calculations showed comparable passing rates (> 98%) when the criterion of the gamma index method was selected to be 3%/3 mm. Conclusions: Developed source model showed good agreements between measured and calculated dose distributions. The model is easily applicable to any other linear accelerator using FFF beams as the data required include only the measured PDD, dose profiles and output factors for various field sizes, which are easily acquired during conventional beam commissioning process.

SU‐E‐T‐564: End‐To‐End Test for LINAC‐Based SBRT with Onboard Planar and Volumetric Imaging System

Purpose: The Varian TrueBeam, TM, STx linear accelerator is equipped with flattening filter free (FFF) mode (6 MV and 10 MV), high definition multileaf collimators (HDMLC), as well as onboard imaging (OBI) devices, making it ideal for SBRT delivery. The purpose of this work is to design an end‐to‐end test to determine the dosimetric and targeting accuracy achievable with the TrueBeam system. Methods: An anthropomorphic head phantom with a Ball Cube II insert and FilmQA software were used to evaluate the accuracy of TrueBeam image guidance and dose delivery. Laser cut Gafchromic EBT2 films with 0.15 mm accuracy were used. The phantom with previously irradiated films inserted was first scanned with a CT scanner, and imported into the planning system. Four RapidArc plans and five IMRT plans were created with different energies, target sizes and delivery modalities. Plans were then delivered on the TrueBeam using image guidance to locate the isocenter. Films were analyzed using FilmQA, TM, (3cognition) software. The film dose was compared with Eclipse v8.9 calculated dose, and was analyzed using a Gamma index criterion of 3%/1 mm and 3%/2 mm. The shifts required to align the film with the calculated dose after the auto registration was estimated to be the targeting accuracy. Results: Targeting accuracy was found to be within 1 mm in all three orthogonal directions. Gamma index (3%, 1 mm) for all the plans was found to be above 90% except the plans with the smaller targets (2 cm and 1 cm) and higher energy (15 MV). Gammas index (3%, 2 mm) for all the plans was found to be above 97%. Conclusions: An end‐to‐end test has been designed and found to be excellent in accessing the targeting and dosimetric accuracy of the overall delivery of the TrueBeam system.