Versa HD Research Articles

191. Commissioning of a high definition multileaf collimator with flattening filter free photon beams

PurposeApex is a micro multileaf collimator mounted on the Elekta Versa HD. It has 56 leaf pairs with leaf thickness of 2.46 mm at isocenter and maximum field size of 12 × 14 cm2. Apex has been commissioned on treatment planning system (TPS) Pinnacle 9.10 for high dose rate stereotactic treatment using volumetric modulated arc therapy (VMAT) technique in conventional (6/10 MV) and flattening-filter free energies (6FFF/10FFF). MethodsAll measurements were done according the request of the TPS [1]: Percentage Dose Depth, Profiles, Output factor, Absolute dose with PTW MP3-M water tank phantom using a synthetic diamond detector (PTW microdiamond 60019) and ionization chamber (PTW Farmer 30013, PTW Semiflex 31010) [2] for field dimension showed in Fig. 1; the validation of TPS model, was carried out by comparing calculated and measured doses acquired in water phantom for different source surface distance (SSD), detector depth and field configuration. Leaf leakage and transmission test was evaluated with PTW Octavius SRS 100. A comparison with multileaf Agility has been done. ResultsComparison between Agility and Apex showed an average reduction of 25%. Discrepancies between calculated and measured dose were in tolerance, 2% or 3%, according the complexity of the geometry. Mean intra leaf transmission were 0.31% and 0.40% (6/10 MV FFF). Mean inter leaf leakage were 0.32% for 6 MV FFF and 0.39% for 10 MV FFF. ConclusionsThe high resolution of multileaf gives dosimetric advantages in penumbra of profiles and less dose outside the filed, thanks to smaller leaves double focused (Table 1). Unfortunately, there is few literatures on this specific topic so our experience could be useful to other center starting a similar commissioning. The time needed to mount the micromultileaf and manual fine tuning takes several minutes. Must be taking account several electronic troubles of this technologies with high dose FFF delivered during a commissioning.

Feasibility of photon beam profile deconvolution using a neural network.

Ionization chambers are the detectors of choice for photon beam profile scanning. However, they introduce significant volume averaging effect (VAE) that can artificially broaden the penumbra width by 2-3mm. The purpose of this study was to examine the feasibility of photon beam profile deconvolution (the elimination of VAE from ionization chamber-measured beam profiles) using a three-layer feedforward neural network. Transverse beam profiles of photon fields between 2×2 and 10×10cm2 were collected with both a CC13 ionization chamber and an EDGE diode detector on an Elekta Versa HD accelerator. These profiles were divided into three datasets (training, validation and test) to train and test a three-layer feedforward neural network. A sliding window was used to extract input data from the CC13-measured profiles. The neural network produced the deconvolved value at the center of the sliding window. The full deconvolved profile was obtained after the sliding window was moved over the measured profile from end to end. The EDGE-measured beam profiles were used as reference for the training, validation, and test. The number of input neurons, which equals the sliding window width, and the number of hidden neurons were optimized with a parametric sweeping method. A total of 135 neural networks were fully trained with the Levenberg-Marquardt backpropagation algorithm. The one with the best overall performance on the training and validation dataset was selected to test its generalization ability on the test dataset. The agreement between the neural network-deconvolved profiles and the EDGE-measured profiles was evaluated with two metrics: mean squared error (MSE) and penumbra width difference (PWD). Based on the two-dimensional MSE plots, the optimal combination of sliding window width of 15 and 5 hidden neurons was selected for the final neural network. Excellent agreement was achieved between the neural network-deconvolved profiles and the reference profiles in all three datasets. After deconvolution, the mean PWD reduced from 2.43±0.26, 2.44±0.36, and 2.46±0.29mm to 0.15±0.15, 0.04±0.03, and 0.14±0.09mm for the training, validation, and test dataset, respectively. We demonstrated the feasibility of photon beam profile deconvolution with a feedforward neural network in this work. The beam profiles deconvolved with a three-layer neural network had excellent agreement with diode-measured profiles.

Profile- and output-determined Prp output corrections for a Varian TrueBeam and Elekta Versa HD

For calibration of flattening filter-free (FFF) linac beams, one needs to either use a chamber small enough to avoid volume averaging that occurs with the conventional Farmer-type chamber, or apply a gradient correction (Prp) to the Farmer chamber. Continued use of the Farmer chamber is attractive due to the desirable characteristics of the chamber. We measured Prp using two methods, the recommended profile gradient method, and an output method with a small chamber having negligible volume averaging, for both a Varian TrueBeam and Elekta Versa 6MV and 10MV FFF beams for 90 cm source-to-surface distance (SSD), 100 cm source-to-axis (SAD) calibrations. For profile-determined Prp, profiles were measured along the calibration plane (100 cm SAD, 90 cm SSD, 10 cm depth) using a diode having a sensitive diameter of 0.8mm. The profiles were integrated numerically along both the length and width of both chamber types, with the deviation from unity giving the magnitude of Prp. The Farmer chamber was 23 mm in length, 3.05 mm in radius, and had a volume of 0.6cc. The smaller chamber was 6.5 mm in length, 2.75 mm in radius, and a volume of 0.125cc. For the output-determined Prp, beam outputs were measured for 6MV conventional flattening filter (cFF), 6FFF, 10cFF, and 10FFF beams using the TG51 protocol for six Farmer and six 0.125cc chambers. Use of six chambers was thought to improve statistics. The 0.125cc chamber was not reference grade, and required cross calibration with the Farmer chamber using cFF beams. To validate the lack of a gradient for the cFF beams, profiles of the cFF beams were measured as well, with a corresponding profile-determined Prp. A ratio of the FFF beam output between Farmer and 0.125cc chambers gives the output-determined Prp. Values of profile-determined Prp for the TrueBeam were 1.003 and 1.007 for 6FFF and 10FFF beams, respectively. Values of profile-determined Prp for the Versa were 1.005 and 1.007 for 6FFF and 10FFF beams, respectively. Surprisingly, the Versa 10cFF beam used was not “flat”, having a central axis divot, giving a Prp 0.995. This beam was not included in the cross-calibration of the 0.125cc chambers. Values of the output-determined Prp were within 0.1% of the profile-determined Prp for the TrueBeam, and within 0.4% for the Versa. Repeated Versa measurements gave similar results. One reference for the 0.125cc chamber used here indicates an anomalous dose-per-pulse issue for cFF beams, which may be exacerbated for FFF beams. Values of Prp from other linacs available to us and from the literature indicate good agreement (within 0.1%, one 0.2% outlier) for the TrueBeam and profile-determined Prp for the Versa. Profile- and output-determined Prp values for the TrueBeam are consistent. A Prp correction for the Versa 10cFF may be warranted, but appears linac or energy dependent. The profile-determined Prp appears to be more reproducible.

Effect of the Number of Arcs and Target Dose Homogeneity on the Normal Tissue Sparing for SBRT Plan in Liver Cancer Patients

To evaluate the effect of the number of arcs and target dose homogeneity on the dose sparing of normal tissue for stereotactic radiotherapy (SBRT) plans in liver cancer patients. Ten patients (4 males and 6 females) with primary or metastatic liver cancer were analyzed. Two groups of volumetric modulated arc therapy (VMAT) plans (Group A and B) with 1 to 4 arcs (239-359°) were generated by RayStation treatment planning system and customized to the Versa HD accelerator. In Group A, the PTV dose was controlled in 90% - 120% of the prescription dose (based on RTOG 0438), and the dose homogeneity of target was not restricted in Group B. A total dose of 48 Gy in 4 fractions was prescribed to the PTV. Ten rings (5 mm width for each ring) outside the PTV were created to limit the dose of normal tissue and evaluate the dose fall-off. All the plans were optimized by using an automatic optimizer developed by our team to avoid the effects of man-made factors. The dose-volume parameters, delivery time among ≥3 plans were analyzed by Friedman test and those between two values were compared by Wilcoxon signed ranks test; p < 0.05 (2-tailed, Friedman) and p < 0.05/N (2-tailed, Wilcoxon, N = n(n-1)/2, where n is the number of groups) was considered statistically significant. All the VMAT plans achieved acceptable target coverage with 43.2 Gy delivered to more than 99% of the PTV and 95% of the target receiving 48 Gy. As the number of arc increased, the doses of organs at risk (OARs, such as the mean liver dose, the maximum dose of stomach, etc.) were reduced by up to 21.5% (2 arcs vs. 1 arc), 17.8% (3 arcs vs. 2 arcs) in Group A (p < 0.008) and 23.4% (2 arcs vs. 1 arc), 32.2% (3 arcs vs. 2 arcs) in Group B (p < 0.008), while the value was only 1.38% (4 arcs vs. 3 arcs) in Group A and there was no statistic difference between 4 arcs and 3 arcs in Group B. The dose of ring 1-10 was improved when using more arcs, in which the mean dose was reduced by 9.1-13.3% (2 arcs vs. 1 arc), 1.5-5.1% (3 arcs vs. 2 arcs) for Group A and 12.3-17.2% (2 arcs vs. 1 arc), 0.9-3.2% (3 arcs vs. 2 arcs) for Group B. The gradient index (GI) was improved by approximately 14% (2 arcs vs. 1 arc) for both Group A and B (p < 0.008). The doses of OARs in Group B were improved by up to 7.5% compared to those in Group A (p < 0.05). Similarly, the value was 5.2% and 5.0% for the mean rings dose and GI (p < 0.05). The monitor units (MUs) and delivery time increased with the number of arc (p < 0.008). For 1-4 arcs VMAT plans, the median delivery time increased from approximately 3 min to 5min. The VMAT SBRT plans with inhomogeneous target dose yielded better dose fall-off and OARs sparing for liver cancer patients. Increasing the number of arcs reduced the dose of normal tissue but at the expense of more delivery time. The three arc VMAT plans without constraint of target dose homogeneity seemed to provide a good trade-off between delivery efficiency and plan quality for liver cancer SBRT.

Retrospective Study of the Correlation between Radiation Isocenter Shifts and the Delivered Dose in Stereotactic Body Radiation Therapy

To determine the magnitude of dosimetric variations in stereotactic body radiation therapy (SBRT) with respect to radiation isocenter positional errors measured with the Winston-Lutz method. We hypothesize that a correlation between maximum dose errors and radiation isocenter instabilities can be determined to predict the maximum error in the dose delivered to a malignancy in SBRT treatments. We have performed a retrospective analysis of dosimetric variations due to mechanical instabilities such as shifts of the radiation isocenter caused by gantry sag and couch isocenter walkout during patient set up for non-coplanar beams. We analyzed the dose distributions of 20 patients using a commercial treatment planning system (TPS). We simulated the expected dose differences by shifting the treatment isocenter in the TPS to emulate the daily 3D vector shift of the radiation isocenter detected by the W-L test. The dose errors were classified according to the W-L detected shift magnitude and planning target volume (PTV). Simulated shifts were limited to a maximum of 2.0 mm from the ideal isocenter according to the maximum deviations measured during two consecutive years of W-L data on a SBRT calibrated VERSA HD. A family of hyperbolic curves based on an in-house dose error model of a spherical or ellipsoidal tumor was used to investigate the parametrization of dosimetric variations in the treated malignancy. No stratification between treatment technique and tumor site was performed for our analysis. In the considered sample, the dosimetric variations do not follow the expected trend from the spherical or elliptical tumor model enclosing the targeted malignancy. However, the maximum dose errors are bounded by a family of hyperbolic curves that diverge at sample tumor sizes less than 3 cm in diameter and are asymptotic to a minimum error limit for large tumors. Furthermore, for small tumors (< 20 cm3) there are multiple cases where the expected dose error is minimal (<3%) with irradiation isocenter shifts of up to 2.0 mm. No significant difference is shown between shifts oriented at each orthogonal axis. Dose variations to the target malignancy due to radiation isocenter perturbations do not follow a parametric curve. However, the maximum dose variation can be modeled using a family of parametric curves where the curve selection is a function of the shift magnitude. This is in part due to high stability cases with respect to radiation isocenter perturbations. The study presents the first steps to determine the factors that improve dose delivery stability to increase dose accuracy in SBRT treatments.

Organisation et gestion des risques en radiothérapie stéréotaxique au poste de traitement

The development of stereotaxic body radiotherapy in the last decade has forced the radiotherapy departments to redouble their efforts in the fields of quality and risk management. For this purpose, increasingly complex and rigorous controls of high performance machines as well as a solid team training must be put in place. Extreme hypofractionation requires both increased vigilance at the treatment desk and well-defined and known procedures. The in place organizations contribute to the control of the risks related to the stereotaxic body radiotherapy machines. The medical presence at the beginning of the treatment fractions has been specified as mandatory in a regulatory way since January 2017. This not only ensures security, but also transmits information to the radiation therapy technicians. At the Eugène-Marquis center, the skills of the technicians for stereotaxic body radiotherapy on two dedicated machines (Cyberknife® and Versa HD® Novalis® type) have been upgraded. An accreditation is formalized after a training period and re-evaluated annually. The communication inside and outside the radiotherapy field plays also an important role in maintaining a high level of exchange and sharing of essential information. The means implemented at the Eugène-Marquis center increase the risk control of SBRT, by paying attention to the management of skills at the treatment station.

P151] Implementation of advanced techniques in radiotherapy and comparison to standard 3DCRT plans- initial institutional experience

Purpose The transition from three-dimensional conformal therapy (3DCRT) to intensity modulated (IMRT) or volumetric modulated arc therapy (VMAT) requires implementation of complex advanced dosimetry, fully commissioned treatment planning system (TPS), machine and other equipment and skills of employees. IMRT/VMAT is implemented in our clinic one year ago. Methods Twenty-one gynecological patient treated in our clinic was selected for the study. Patients were planned for 3D CRT, but due to unacceptable doses to organs at risk (QUANTEC), treatment plans for IMRT or VMAT were generated, and dosimetrically verified before treatment delivery. The TPS used was Monaco 5.11 (Elekta) with Monte Carlo (VMAT) and Collapsed Cone (3DCRT) algorithm.The treatments were delivered by Versa HD (Elekta, Crawley, UK) using 10 MV (VMAT) and 15MV (3DCRT). The patients were prescribed 50.4 Gy/28 fractions (4) and 45 Gy/25 fractions (17 patients). The coverage of PTV and doses to organs at risk were recorded, both for VMAT/IMRT treatment and 3DCRT. Results Coverage PTV: The ICRU 83 criteria for PTV coverage were fulfilled in all 3DCRT/VMAT/IMRT plans. Doses to OARS: in average, the V45 in small bowel in IMRT/VMAT plans was approximately 6 times smaller than the same of 3DCRT plans. The V45 of small bowels was in average 45cm3 in IMRT/VMAT plans, while in 3DCRT plans it was 233 cm3. In case of femoral head, significant reduction in V30 (9.8 % vs. 33.1%) and mean dose in case of IMRT/VMAT plans. Rectum was planned with significantly less dose in terms of V30 (79.5% vs 95.2%) in IMRT/VMAT plans. Bladder was better spared in VMAT plans in terms of V40 (51% vs. 91%), but maximum dose was higher in VMAT plans than in 3DCRT (50.1 Gy to 48.1 Gy in average). Homogeneity index was in average 0.11 for VMAT plans and 0.09 for 3DCRT plans. Conclusion Both 3DCRT and IMRT/VMAT provided good coverage of PTV, but analysis of dosimetric data revealed significant differences in normal tissue doses. The advanced treatment planning, evaluation, dosimetric evaluation and delivery is far more complex than the same for 3DCRT,but patient benefit (OAR sparing) confirms necessity for implementation of advanced techniques.

Adaptation of NCS report 24 for flattening filter free VMAT quality assurance

Volumetric Arc radiotherapy (VMAT) was introduced nearly 10 years ago and is an advancement on the technology of Intensity-Modulated Radiotherapy (IMRT). The main difference between VMAT & IMRT are the increased number of degrees of freedom during patient treatment including gantry rotation, dose rate variation, multi-leaf collimator (MLC) movement and in some cases collimator movement1. Because of the aforementioned differences between IMRT and VMAT, the established IMRT Quality Assurance (QA) procedures alone are not sufficient for VMAT QA. To address this shortfall, in 2015 the Nederlandse Commissie voor Stralingsdosimetrie (NCS) published report 24 which is the first fully comprehensive guide of its kind aimed explicitly at VMAT QA and therefore has the potential to be utilised to improve the quality assurance of VMAT. The aim of this work was to integrate machine QA aspects of NCS report 24 as part of Galway Clinic’s VMAT QA process & furthermore investigate if this can be adapted to flattening filter free (FFF) beams. QA tests outlined by NCS 24 were performed on an Elekta Versa HD linac. The methods, equipment required to conduct the test set out by NCS 24 was investigated. Analysis of the test results was performed to the standards set by NCS 24 and compared to the standards of TG 142, which is the standard currently employed by the Galway Clinic. The tests set out by NCS 24 were also translated to FFF beam energies using additional sources of information.

OA151] Error sensitivity of forward- and back-projection EPID dosimetry for VMAT treatment of the prostate

Introduction Volumetric modulated arc therapy (VMAT) treatments may be verified dosimetrically using an electronic portal imaging device (EPID), either using the forward-projection method to calculate integrated predicted images, which are compared with acquired images, or using the back-projection method to reconstruct delivered dose, for comparison with the planned dose. This study compares the error sensitivity of both methods for prostate treatments. Materials and Methods VMAT treatment plans for prostate and seminal vesicles were created using AutoBeam in-house software, with final dose calculation carried out in Pinnacle3. Patients were treated using the 6 MV beam of a Versa HD accelerator. Portal images were acquired for 46 fractions in 13 patients, and EPID dosimetry was carried out using AutoBeam (forward-projection) and iViewDose (back-projection). Differences between measurements (M) and predictions (P) were evaluated on the central axis of the beam (forward-projection) and at the isocentre (back-projection). The plans were also retrospectively delivered to a Solid Water phantom and errors were deliberately introduced. Errors consisted of a 2–10% increase in monitor units, a 2–10 mm increase in aperture size, a 2–10 mm shift in aperture and introduction of a 10–50 mm air gap into the phantom. Results For the patient images, M-P = 3.1% ± 2.1% (1 SD) for forward-projection and 2.2% ± 2.2% for back-projection, with good correlation between forward-projection and back-projection. For the phantom images, M-P = 0.7% ± 2.0% for forward-projection and 3.0% ± 1.0% for back-projection. Monitor unit and aperture size errors are detected equally by both methods, aperture shift errors are mainly detected in forward-projection due to removal of shifts by the back-projection image processing software, while air gaps have a slightly larger impact on back-projection than forward-projection. Conclusions Each method of EPID dosimetry has its own distinct sensitivity to errors, but overall, both methods can be used reliably for verification of delivered dose. The back-projection method, in clinical use at this centre, enables convenient visualisation of delivered dose to the patient. Other methods, such as cone-beam imaging, are required for verification of patient position, while further work is needed to investigate the potential of time-resolved EPID dosimetry.

4D Imaging Experience for Treatment of Lung SBRT

So far we have treated 17 patients at our centre since February 2017 using VMAT on Elekta Versa HD linacs. Patients: T stages, a mixture of T1a, T1b and T2a with an even mix of tumour location. Our standard protocol involves a free-breathing (no abdominal compression) 4DCT for planning from which we have generated phase based CT datasets including the average intensity projection (AVIP) which is used for export to the image verification CBCT system. 10 phase images, a MIP (maximum intensity projection), an AVIP and MIP (minimum intensity projection) are exported to the TPS for contouring and planning. The treatment verification process includes a day 0 check for patient position and clearance on the linac. This includes a 4DCBCT image of the patient to ensure tumour motion is the same as at the planning CT. On treatment days, patients receive a 4DCBCT followed by a 3DCBCT acquisition immediately prior to treatment. Following treatment another 3DCBCT is acquired to assess intra-fraction motion.

Comparison of kQ factors measured with a water calorimeter in flattening filter free (FFF) and conventional flattening filter (cFF) photon beams

Recently flattening filter free (FFF) beams became available for application in modern radiotherapy. There are several advantages of FFF beams over conventional flattening filtered (cFF) beams, however differences in beam spectra at the point of interest in a phantom potentially affect the ion chamber response. Beams are also non-uniform over the length of a typical reference ion chamber and recombination is usually larger. Despite several studies describing FFF beam characteristics, only a limited number of studies investigated their effect on kQ factors. Some of those studies predicted significant discrepancies in kQ factors (0.4% up to 1.0%) if TPR20,10 based codes of practice (CoPs) were to be used. This study addresses the question to which extent kQ factors, based on a TPR20,10 CoP, can be applied in clinical reference dosimetry. It is the first study that compares kQ factors measured directly with an absorbed dose to water primary standard in FFF-cFF pairs of clinical photon beams. This was done with a transportable water calorimeter described elsewhere. The measurements corrected for recombination and beam radial non-uniformity were performed in FFF-cFF beam pairs at 6 MV and 10 MV of an Elekta Versa HD for a selection of three different Farmer-type ion chambers (eight serial numbers). The ratio of measured kQ factors of the FFF-cFF beam pairs were compared with the TPR20,10 CoPs of the NCS and IAEA and the %dd(10)x CoP of the AAPM. For the TPR20,10 based CoPs differences less than 0.23% were found in kQ factors between the corresponding FFF-cFF beams with standard uncertainties smaller than 0.35%, while for the %dd(10)x these differences were smaller than 0.46% and within the expanded uncertainty of the measurements. Based on the measurements made with the equipment described in this study the authors conclude that the kQ factors provided by the NCS-18 and IAEA TRS-398 codes of practice can be applied for flattening filter free beams without additional correction. However, existing codes of practice cannot be applied ignoring the significant volume averaging effect of the FFF beams over the ion chamber cavity. For this a corresponding volume averaging correction must be applied.

Energy response corrections for profile measurements using a combination of different detector types.

Different detector properties will heavily affect the results of off-axis measurements outside of radiation fields, where a different energy spectrum is encountered. While a diode detector would show a high spatial resolution, it contains high atomic number elements, which lead to perturbations and energy-dependent response. An ionization chamber, on the other hand, has a much smaller energy dependence, but shows dose averaging over its larger active volume. We suggest a way to obtain spatial energy response corrections of a detector independent of its volume effect for profiles of arbitrary fields by using a combination of two detectors. Measurements were performed at an Elekta Versa HD accelerator equipped with an Agility MLC. Dose profiles of fields between 10×4cm² and 0.6×0.6cm² were recorded several times, first with different small-field detectors (unshielded diode 60012 and stereotactic field detector SFD, microDiamond, EDGE, and PinPoint 31006) and then with a larger volume ionization chamber Semiflex 31010 for different photon beam qualities of 6, 10, and 18MV. Correction factors for the small-field detectors were obtained from the readings of the respective detector and the ionization chamber using a convolution method. Selected profiles were also recorded on film to enable a comparison. After applying the correction factors to the profiles measured with different detectors, agreement between the detectors and with profiles measured on EBT3 film was improved considerably. Differences in the full width half maximum obtained with the detectors and the film typically decreased by a factor of two. Off-axis correction factors outside of a 10×1cm² field ranged from about 1.3 for the EDGE diode about 10mm from the field edge to 0.7 for the PinPoint 31006 25mm from the field edge. The microDiamond required corrections comparable in size to the Si-diodes and even exceeded the values in the tail region of the field. The SFD was found to require the smallest correction. The corrections typically became larger for higher energies and for smaller field sizes. With a combination of two detectors, experimentally derived correction factors can be obtained. Application of those factors leads to improved agreement between the measured profiles and those recorded on EBT3 film. The results also complement so far only Monte Carlo-simulated values for the off-axis response of different detectors.

52. Transmission detector with 3D dose reconstruction: compass system validation for patient specific dose verification

Introduction Methods used for external radiotherapy plan verification do not generally provide clinically relevant results. Compass 4.0 software combined with Dolphin (IBA) transmission detector is a plan verification system which reconstructs the delivered dose distribution inside the patient’s CT-dataset. This study aimed to evaluate this system for VMAT plans verification. Methods The transmission detector is a 2D array of 1513 ionization chambers. With an independent beam modeling and a collapsed cone dose engine, the system provides two ways for treatment plans verification [ Fig. 1 ]: a dose calculation verification with an independent dose calculation; a verification of the actual dose delivered by the linear accelerator (measurement-based dose reconstruction). VMAT plans, using 6 MV photons beams, were generated in Pinnacle v9.10 (Philips) TPS. Elekta linear accelerators Synergy and Versa HD, equipped with an Agility multi leaf collimator (MLC) were used for beams delivery. System validation was realized in two steps: (i) Verification of beam modelling and dose reconstruction method: both Compass calculated dose distribution and reconstructed dose distribution were compared to radiochromic films measurements (gamma index 2D, criteria 3% local/3 mm, threshold 30%) for 11 Head and Neck (H&N) plans. (ii) Evaluation of error detection abilities for two treatment plan complexities (prostate and H&N). Two kinds of error were introduced: MLC positional uncertainties from 1 to 5 mm (one leaf bank shift, two banks shifts in same and in opposite direction) and output errors (MU increase from 1% to 5%). The system was used for 17 clinical cases (nine H&N, eight lungs including SBRT). Measurement-based dose distributions ere compared with planned dose distributions (volume of PTV receiving at least 95% of prescribed dose, V95%, mean dose to organs at risks, Dmean). Results On average, Compass calculated dose distributions compared with film measurements showed a mean gamma value ( γ mean) of 0.422 (standard-deviation σ = 0.078) and a percentage of accepted points (%accepted pts) of 95.7% ( σ = 3.7%). Reconstructed dose distributions compared with film measurements showed a γ mean of 0.380 ( σ = 0.072) and a %accepted pts of 96.8 % ( σ = 2.1%). The system could detect 1 mm leaf bank shift for all the simulated leaf banks shifts, 1% and 3% MU increase for H&N and prostate plan respectively. For clinical cases, differences between planned dose and Compass reconstructed dose were on averaged 1.5% ( σ = 3.8%) for V95% and 53 cGy ( σ = 137 cGy) for the organs at risk. Conclusions The performances of Compass software combined with Dolphin transmission detector were validated for clinical implementation: Compass calculated and reconstructed doses were in agreement with film measurements, 1 mm leaf bank shift error was detected. The dose reconstruction on patient CT-dataset gives a relevant clinical meaning to patient quality assurance results. An interesting evolution of the product would be the delivered dose reconstruction on daily CBCT, for adaptative radiotherapy purpose.

A universal parameterized gradient-based method for photon beam field size determination.

To propose a universal, parameterized gradient-based method (PGM) for radiation field size determination. The PGM locates the beam profile's edge by parameterizing its penumbra region with a modified sigmoid function where the inflection point can be determined in a closed form. The parametrization was validated with filter-flattened (FF), flattening-filter-free (FFF) and wedged profiles measured on two Elekta linac models (Synergy and Versa HD). Gamma analysis with the delta dose function set to zero was used to quantitatively assess the parameterization accuracy. Field sizes of FF beams were calculated with the PGM and the full width at half maximum (FWHM) methods for comparison. To assess the consistency of the PGM and the FWHM method with geometric scaling across different depths, the calculated field size at a reference depth was scaled to other depths and compared with the field sizes calculated from the measured profiles. The method was also validated against a maximum-slope method (MSM) with wedge and FFF profiles. We also evaluated the robustness of the three methods with respect to measurement noise, varying scanning step sizes, detector characteristics, and beam energy/modality. Small distance-to-agreement (0.02±0.02mm) between the measured and parameterized penumbra region was observed for all profiles. The differences between the field sizes calculated with the FWHM method and the PGM were consistent (0.9±0.3mm), with the FWHM method yielding larger values. With geometrical scaling, the PGM and the FWHM method produced maximum differences of 0.26 and 1.16mm, respectively. For wedge and FFF beams, the mean differences relative to FF fields were 0.15±0.09mm and 0.57±0.91mm for the PGM and the MSM, respectively. The PGM was also found to produce more consistent results than the FWHM method and the MSM when measurement noise, scanning step size, detector characteristics, and beam energy/modality changed. The proposed PGM is universally applicable to all beam modalities (FF, wedge and FFF) for accurate field size determination. Compared to the FWHM and the MSM, it is more robust to variations in measurement condition and detection system.

Implementation of respiratory-gated VMAT on a Versa HD linear accelerator.

The accurate delivery of respiratory‐gated volumetric modulated arc therapy (VMAT) treatment plans presents a challenge since the gantry rotation and collimator leaves must be repeatedly stopped and set into motion during each breathing cycle. In this study, we present the commissioning process for an Anzai gating system (AZ‐733VI) on an Elekta Versa HD linear accelerator and make recommendations for successful clinical implementation. The commissioning tests include central axis dose consistency, profile consistency, gating beam‐on/off delay, and comparison of gated versus nongated gamma pass rates for patient‐specific quality assurance using four clinically commissioned photon energies: 6 MV, 6 FFF, 10 MV, and 10 FFF. The central axis dose constancy between gated and nongated deliveries was within 0.6% for all energies and the analysis of open field profiles for gated and nongated deliveries showed an agreement of 97.8% or greater when evaluated with a percent difference criteria of 1%. The measurement of the beam‐on/off delay was done by evaluating images of a moving ball‐bearing phantom triggered by the gating system and average beam‐on delays of 0.22–0.29 s were observed. No measurable beam‐off delay was present. Measurements of gated VMAT dose distributions resulted in decrements as high as 9% in the gamma passing rate as compared to nongated deliveries when evaluated against the planned dose distribution at 3%/3 mm. By decreasing the dose rate, which decreases the gantry speed during gated delivery, the gamma passing rates of gated and nongated treatments can be made equivalent. We present an empirically derived formula to limit the maximum dose rate during VMAT deliveries and show that by implementing a reduced dose rate, a gamma passing rate of greater than 95% (3%/3 mm) was obtained for all plan measurements.

EP-1455: IGRT kV-CBCT dose calculations using Virtual Source Models and validated in phantoms using OSL

Purpose or Objective: With the growing use of X-ray imaging equipment in Image-Guided RadioTherapy (IGRT), the need to evaluate the dose-to-organs delivered by kV-CBCT imaging acquisition increases. This study aims to propose accurate Monte Carlo (MC) calculations of the patient dose-to-organs delivered by two commercially available kV-CBCT systems: the XVI from Elekta’s VERSA HD accelerator and the OBI from Varian’s TrueBeam system. Simulations are to be validated using in phantom OSL measurements. Material and Methods: For both kV-CBCT systems, the kV irradiation head geometry was implemented in the MC simulation code Penelope. As a first step, the resulting photon distributions were expressed as Virtual Source Models (VSM) for every standard irradiation condition (kVp, filtration, collimation); it was then validated and adjusted using in water-phantom measurements performed with a calibrated Farmer-type ionization chamber. In a second step, the validated VSMs were used to simulate the dose delivered by both the XVI and OBI systems in anthropomorphic phantoms, using standard clinical imaging protocols. Simulated dose-to-organs were then confronted to dose measurements performed using OSL inserted into the same phantoms, following a dosimetric protocol for OSLs previously established [1]. In addition, VSM results were confronted to their direct MC counterparts in order to evaluate the benefit of using such technique. Results: The current study highlights the possibility to reproduce OSL dose-to-organ measurements using VSM-driven Monte Carlo simulation with an overall agreement better than 20 %. In addition, the use of VSM in the MC simulation enables to speed-up the calculation time by a factor better than two (for the same statistical uncertainty) compared to direct MC simulation. Nevertheless, if direct and VSM calculations are in agreement inside the irradiation field, outside, VSM results tend to be significantly lower (10-30%). Conclusion: The use of a VSM was demonstrated to simplify and fasten MC simulations for personalized kV-CBCT MC dose estimation. In addition, OSLs enable to perform the low dose measurement in the kV range needed for in phantom X-ray imaging equipment dose QA. This study is to be completed in the near future by the addition of other standard X-ray imaging equipment dedicated to IGRT.

Accounting for the ion recombination factor in relative dosimetry of flattening filter free photon radiation

Flattening filter free (FFF) linear accelerators (linacs) are increasingly being used to deliver external beam radiotherapy. Advantages of FFF include lower out-of-field dose to the patient due to reduced head scatter, and shorter treatment times due to increased dose rate. The ion recombination factor (kion) is known to increase linearly with dose per pulse in pulsed photon beams. This variation is neglected when performing relative dosimetry in flattened beams, however could be significant in the higher dose rates encountered with FFF. kion has been derived from theory and measured using Jaffe plots and two voltage analysis for three ionisation chambers at a range of depths and points off-axis in 6 and 10 MV FFF beams produced by an Elekta Versa HD linac. The chambers under investigation were the PTW 30013 ‘Farmer’ (0.60 cc), the Exradin A1SL (0.053 cc) and the IBA CC04 (0.04 cc). The effect of neglecting ion recombination corrections in commissioning data was quantified using a 1D gamma analysis (1%/2 mm) on depth dose curves and beam profiles. The largest variation in kion was for a Farmer chamber in a 10 MV FFF beam, where kion varied by up to 1.2% ± 0.15% (95% CL) between different points in a water tank. The Farmer variation with depth was comparable to that measured by Kry et al (2012 J. Appl. Clin. Med. Phys. 13 318–325). Other chambers exhibited negligible changes in kion. It was concluded that neglecting ion recombination in relative measurements at 6 MV FFF would have a negligible effect on dosimetric accuracy. Furthermore kion could be safely neglected when acquiring relative data with small volume chambers for commissioning 10 MV FFF, although may need to be accounted for if relative data are acquired with a Farmer chamber and when routine energy checks are performed.

Dose-to-medium vs. dose-to-water: Dosimetric evaluation of head and neck VMAT cases using Monaco treatment planning system

Purpose: In this paper, we evaluate the dosimetric differences between absorbed dose to water and absorbed dose to medium in Monte Carlo (MC)-based calculations used for radiation therapy treatment plans. Methods: Thirty-four treated Head and Neck simultaneously integrated boost cases were analyzed retrospectively. All of them were planned by Monaco treatment planning system (TPS), calculated and reviewed on absorbed dose to medium (D m ) calculations and treated in Elekta Versa HD LINAC. Absorbed dose to medium D m was converted to absorbed dose to water D w in Monaco treatment planning system using the procedure based on stopping power ratios and the Bragg-Gray cavity theory. Dosimetric parameters were then compared and analyzed with respect to absorbed dose to medium (D m ) calculations for multiple planning target volumes (PTVs) and critical organs such as brainstem, spinal cord, left and right lens, left and right parotids, larynx, left and right middle ear and lips. Results: It was found that mean and minimum D w (i.e. D w mean and D w min ) of organs at risk did not differ much (hardly differing by 0.8-2%) with respect to those of the absorbed dose to medium. However maximum D w (i.e. D w max ) in case of lips, left and right middle ear were found to differ more than 4% with respect to D m max . For serial organs brainstem and spinal cord, maximum dose D w max were found to vary around 1% and 2%, respectively, with respect to absorbed dose to medium dose calculation. In case of PTVs, the mean percentages variation of D w min and D w mean were found to be less than 1 %, although the variation of maximum D w was found to be high around 5-7% with respect to that of D m . Conclusion: The comparative analysis of dosimetric parameters in the present study shows that the selection of either D m or D w in Monaco planning system is less likely to produce any significant clinical effect in tumor control and to the damage of organs at risk.

33. Dosimetry evaluation for VMAT breast and nodes treatment

Introduction Feedback of VMAT breast and nodes dosimetry based of an international competition organized by the King Faisal Specialist Hospital & Research Centre (Saudi Arabia) in 2016, in order to develop the technic at the Clinical Center of Porte de Saint Cloud. Methods Treatment plans for breast and nodes have been realized with Pinnacle 9.10 TPS and for a Versa HD linac with Agility collimator (160 leafs). Dosimetry is optimized for different PTV and OAR constraints. Dosimetry has been evaluated with the competition parameters and with gamma index method (using PTW’s Octavius4D and Verisoft 6.2) to analyze the plan treatment feasibility on linac. A DVH comparison between VMAT and conformational dosimetry was also realized. Results Competition criteria have been successfully realized with 92% points of measure accept 4%/4 mm gamma index. Gamma index analyze show better results for less optimize treatment plans but worst competition score. VMAT dosimetry have better OAR protection and PTV uniformity and conformity than conformational dosimetry. Conclusions Competition allowed us better knowledge and mastery for breast VMAT. Dosimetry analyze show us that too much optimization are not able to be realized on the linac. Compromise must be define to perform the best VMAT dosimetry and make a feasible treatment on the linac. VMAT dosimetry improve DHV constraints and allow a shorter time treatment. Feedback of VMAT breast and nodes dosimetry based of an international competition organized by the King Faisal Specialist Hospital & Research Centre (Saudi Arabia) in 2016, in order to develop the technic at the Clinical Center of Porte de Saint Cloud. Treatment plans for breast and nodes have been realized with Pinnacle 9.10 TPS and for a Versa HD linac with Agility collimator (160 leafs). Dosimetry is optimized for different PTV and OAR constraints. Dosimetry has been evaluated with the competition parameters and with gamma index method (using PTW’s Octavius4D and Verisoft 6.2) to analyze the plan treatment feasibility on linac. A DVH comparison between VMAT and conformational dosimetry was also realized. Competition criteria have been successfully realized with 92% points of measure accept 4%/4 mm gamma index. Gamma index analyze show better results for less optimize treatment plans but worst competition score. VMAT dosimetry have better OAR protection and PTV uniformity and conformity than conformational dosimetry. Competition allowed us better knowledge and mastery for breast VMAT.

Total body irradiation’s dosimetric features by volumetric modulated ARC therapy method

Purpose To compare three different types of volumetric modulated arc therapy(VMAT) planning tecniques using Elekta Versa HD lineer accelerator to deliver total body irradition(TBI) treatment. Methods Five TBI patient’s treatment planning performed using Monaco5.1® treatment planning system with three different VMAT tecniques for each patient.First one was single arc VMAT tecnique, second one was double arc VMAT tecnique and third one was 2 fields arc VMAT tecnique for one isocentre.The VMAT-TBI tecnique consisted of three isocentres and three overlapping arcs.The prescribed dose was 95% of target volume receiving dose of 12 Gy.Mean dose to lung and kidney were restricted less than 9.0 Gy. Results An average total delivery time was determined 835 ± 86 seconds and an average monitor unit (MU)s was determined 2202 ± 254 MUs for double arc VMAT technique. This study demonstrates that double arc VMAT technique has got less MUs than other technique but an average delivery time is %17 more than single arc VMAT technique. When we compared organ at risk (OAR)s, we had less dose and better dose coverage to target with double arc VMAT technique. Conclusion The tecnique for TBI using VMAT was found feasible.The results show that dose coverage of target and OAR’s doses also depend significantly VMAT techniques. Based on the results we have decided to plan TBI in our clinic with double arc VMAT technique.