Flattening Filter Free Photon Research Articles

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

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

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.

Evaluation of dosimetric parameters of small fields of 6 MV flattening filter free photon beam measured using various detectors against Monte Carlo simulation – CORRIGENDUM

Evaluation of dosimetric parameters of small fields of 6 MV flattening filter free photon beam measured using various detectors against Monte Carlo simulation – CORRIGENDUM

Dosimetric Comparison between 6MV Flattened Filter and Flattening Filter Free Photon Beams in the Treatment of Glioblastoma with IMRT Technique: A Treatment Planning Study

Introduction: The present study evaluated the dosimetric comparison between 6MV flattened filter (FF) and flattening filter-free (FFF) photon beams in intensity-modulated radiation therapy (IMRT) technique for the treatment of glioblastoma (GBM) patients. Material and Methods: The present study was conducted on 10 patients with GBM previously planned and treated with 6MV FF photon beam by IMRT technique. Additional IMRT plans were retrospectively created using 6MV FFF photon beam for each patient plan. The dose prescription, beam parameters, and planning objective were kept same in both plans. The plans were evaluated using cumulative dose-volume histogram (c-DVH). Both types of plans were compared on the basis of homogeneity index (HI), conformity index (CI), beam-on time (BOT), monitor unit (MU), and doses to organs at risk (OARs). Results: Dose received by 95% (D95%) of planning target volume (PTV) coverage was observed significantly higher in 6MV_FF_IMRT plan than 6MV_FFF_IMRT plan (P<0.05). No significant dose differences were noticed for HI, CI, D98%, and D2% between both plans. Significantly lower Dmax for the brainstem, eyes, and eye lens was observed in 6MV_FFF_IMRT plan. For the brain, less than 2% mean dose was observed in 6MV_FFF_IMRT plan than 6MV_FF_IMRT plan (P=0.017). In 6MV_FFF_IMRT plan, mean BOT decreased by 39% in comparison to that in 6MV_FF_IMRT plans. Conclusion: The 6MV FFF beam provides a desirable and clinically acceptable IMRT plan for the treatment of GBM than 6MV FF beam. In addition, 6MV FFF beam provides higher MUs, better OARs sparing, lower scattered dose, and lower beam delivery time.

Dosimetric characteristics of a 2D silicon diode array for stereotactic radiotherapy end-to-end patient-specific QA

The measurement accuracy of stereotactic radiotherapy (SRT) patient-specific quality assurance (QA) is challenging as small-field dosimetry contains lateral charge disequilibrium, partial blocking of the beam source, and limitations based on the detector size. The aim of this study was to investigate the dosimetric characteristics of a two-dimensional high-resolution silicon diode array for SRT end-to-end patient-specific QA.The dosimetric characteristics of the diode array, delivered to an end-to-end QA phantom, were evaluated for 6 MV, with a flattening filter (FF) and flattening filter-free (FFF), and 10 MV FFF photon beams. Dose linearity, dose rate dependence, dose-per-pulse dependence, field size dependence, angular dependence, and beam directivity were investigated for all photon beams. A total of 27 clinical plans were selected to evaluate the SRT end-to-end patient-specific QA for treatment delivery using volumetric modulated arc therapy (VMAT).For all beam qualities, the differences of dose and dose rate linearity of the diode array were below 0.7% (10–1400 MU range) and 0.6% (100–2400 MU/min range), respectively. The diodes had a decrease in sensitivity of about 4% for the 21-fold decrease in dose-per-pulse. Output factors were in good agreement for field size down to 2 × 2 cm2 (<0.6% for both 6 MV FF and FFF beams, <1.6% for the 10 MV FFF beam), while the field size down to 0.6 × 0.6 cm2 had an under-response (<4% for all beam qualities). Angular dependence was within ±2% and ±4%, for coplanar and non-coplanar beams, respectively. However, this was not the case for the beam direction nearly parallel to the diode array plane. Beam directivity response on both sides of the array was in excellent agreement. For the end-to-end patient-specific QA, the average gamma passing rate (GPR) with a 2%/1 mm criterion was 96.8% and the minimum value was 93.0%. There was no significant difference in the GPRs between coplanar and non-coplanar VMAT plans.The diode array, with several corrections, demonstrated good dosimetric characteristics for SRT QA. The GPRs of end-to-end patient-specific QA for non-coplanar and multiple metastases were as high as that for coplanar and single-target, and a gamma criterion of 2%/1 mm would be suitable for SRT QA when using the array.

Evaluation of dosimetric parameters of small fields of 6 MV flattening filter free photon beam measured using various detectors against Monte Carlo simulation

AbstractPurpose:This study aims to evaluate dosimetric parameters like percentage depth dose, dosimetric field size, depth of maximum dose surface dose, penumbra and output factors measured using IBA CC01 pinpoint chamber, IBA stereotactic field diode (SFD), PTW microDiamond against Monte Carlo (MC) simulation for 6 MV flattening filter-free small fields.Materials and Methods:The linear accelerator used in the study was a Varian TrueBeam® STx. All field sizes were defined by jaws. The required shift to effective point of measurement was given for CC01, SFD and microdiamond for depth dose measurements. The output factor of a given field size was taken as the ratio of meter readings normalised to 10 × 10 cm2 reference field size without applying any correction to account for changes in detector response. MC simulation was performed using PRIMO (PENELOPE-based program). The phase space files for MC simulation were adopted from the MyVarian Website.Results and Discussion:Variations were seen between the detectors and MC, especially for fields smaller than 2 × 2 cm2 where the lateral charge particle equilibrium was not satisfied. Diamond detector was seen as most suitable for all measurements above 1 × 1 cm2. SFD was seen very close to MC results except for under-response in output factor measurements. CC01 was observed to be suitable for field sizes above 2 × 2 cm2. Volume averaging effect for penumbra measurements in CC01 was observed. No detector was found suitable for surface dose measurement as surface ionisation was different from surface dose due to the effect of perturbation of fluence. Some discrepancies in measurements and MC values were observed which may suggest effects of source occlusion, shift in focal point or mismatch between real accelerator geometry and simulation geometry.Conclusion:For output factor measurement, TRS483 suggested correction factor needs to be applied to account for the difference in detector response. CC01 can be used for field sizes above 2 × 2 cm2 and microdiamond detector is suitable for above 1 × 1 cm2. Below these field sizes, perturbation corrections and volume averaging corrections need to be applied.

Beam flatness modulation for a flattening filter free photon beam utilizing a novel direct leaf trajectory optimization model.

Flattening filter free (FFF) linear accelerators produce a fluence distribution that is forward peaked. Various dosimetric benefits, such as increased dose rate, reduced leakage and out of field dose has led to the growth of FFF technology in the clinic. The literature has suggested the idea of vendors offering dedicated FFF units where the flattening filter (FF) is removed completely and manipulating the beam to deliver conventional flat radiotherapy treatments. This work aims to develop an effective way to deliver modulated flat beam treatments, rather than utilizing a physical FF. This novel optimization model is an extension of the direct leaf trajectory optimization (DLTO) previously developed for volumetric modulated radiation therapy (VMAT) and is capable of accounting for all machine and multileaf collimator (MLC) dynamic delivery constraints, using a combination of linear constraints and a convex objective function. Furthermore, the tongue and groove (T&G) effect was also incorporated directly into our model without introducing nonlinearity to the constraints, nor nonconvexity to the objective function. The overall beam flatness, machine deliverability, and treatment time efficiency were assessed. Regular square fields, including field sizes of 10 × 10 cm2 to 40 × 40 cm2 were analyzed, as well as three clinical fields, and three arbitrary contours with "concave" features. Quantitative flatness was measured for all modulated FFF fields, and the results were comparable or better than their open FF counterparts, with the majority having a quantitative flatness of less than 3.0%. The modulated FFF beams, due to the included efficiency constraint, were able to achieve acceptable delivery time compared to their open FF counterpart. The results indicated that the dose uniformity and flatness for the modulated FFF beams optimized with the DLTO model can successfully match the uniformity and flatness of their conventional FF counterparts, and may even provide further benefit by taking advantage of the unique FFF beam characteristics.

Impact of Flattening Filter Free Photon Beam on Rapidarc Radiotherapy for Gynaecological Malignancies: A Comparative Study

Introduction: To compare the dosimetric outcomes of 6 and 10 MV flattening filter free beam (FFFB) energies in gynaecological malignancies RapidArc (RA) planning. Material and Methods: The RA plans were generated for a cohort of 20 patients using 6 and 10 MV FFFBs. The plans aimed to deliver a dose of 50.4Gy in 28 fractions to planning target volume (PTV); moreover, planning objectives were kept as low as reasonably achievable for organs at risk (OARs). Dosimetric analysis was performed in terms of PTV coverage, conformity index (CI), homogeneity index (HI), dose to OAR’s, integral dose to normal tissue (NTID), and total number of monitor units (MU’s). Results: According to the results, volumes of PTV receiving prescription dose and CI values were 95.03±0.10% and 95.02±0.18%, as well as 1.018±0.028 and 1.024±0.027, respectively. Moreover, HI values were estimated at 1.063±0.008 and 1.068±0.010. Additionally, the corresponding values of mean NTID and MUs were 280.3±42.5 and 267.9±39.1 (liter-Gy), as well as 610.3±30.3 and 630.6±39.7 for FFFB using 6 and 10 MV, respectively. The 6 and 10 MV FFFBs were statistically similar in terms of mean dose to bladder, rectum and both femoral heads, while comparison yielded significant difference (p <0.05) in terms of HI, CI, MUs and NTID. Conclusion: The FFFB of 6MV was found superior, compared to 10MV, for RA planning in case of gynaecological malignancies. Moreover, it offers better HI and CI values, as well as fewer numbers of MUs (3.33%). In addition, it delivers more NTID (4.42%) for similar target coverage and OAR’s sparing.

Clinical experience with lung-specific electromagnetic transponders for real-time tumor tracking in lung stereotactic body radiotherapy.

Motion management is crucial for optimal stereotactic body radiotherapy (SBRT) of moving targets. We aimed to describe our clinical experience with real-time tracking of lung-specific electromagnetic transponders (EMTs) for SBRT of early stage non-small cell lung cancer in free-breathing (FB) or deep inspiration breath-hold (DIBH). Seven patients were implanted with EMTs. Simulation for SBRT was performed in FB and in DIBH. We prescribed 60Gy in 3, 5 or 8 fractions to the tumor and delivered SBRT with volumetric modulated arcs and a 6MV flattening filter free photon beam. Patients' setup at the linac was performed using EMT positions and cone-beam CT (CBCT) verification. Four patients were treated in DIBH because of a dosimetric benefit. We analysed patient alignment and treatment delivery parameters using DIBH or FB and EMT real-time tracking. There were no complications from the EMT implantation. Visual inspection of CBCT before and/or after SBRT revealed good alignment of structures and EMTs. The median setup time was 9.8min (range: 4.6-34.1min)and the median session time was 14.7min (range: 7.3-36.5min). EMT positions in lungs remained stable during overall treatment and allowed real-time tracking both in FB and in DIBH SBRT. The treatment beam was gated when EMT centroid position exceeded tolerance thresholds ensuring correct delivery of radiation to the tumor. Using EMTs for real-time tracking of tumor motion during lung SBRT proved to be safe, accurate and easy to integrate clinically for treatments in FB or DIBH.

Effects of metal implants and a metal artifact reduction tool on calculation accuracy of AAA and Acuros XB algorithms in small fields.

In this study, the dosimetric accuracy of analytical anisotropic algorithm (AAA) and Acuros XB (AXB) dose calculation algorithms (Varian Medical Systems, Palo Alto, CA) was investigated for small radiation fields incident on phantoms of various metals that include stainless steel grade 316L (SS316L) and titanium alloy grade 5 (Ti5) implants. In addition, the effects of using metal artifact reduction for orthopedic implants (O-MAR, Philips Healthcare, Cleveland, OH) were evaluated. The evaluations of AAA and AXB were performed by comparing the crossline profiles calculated by AAA and AXB with GafChromicTM EBT3 film measurements at the phantom-implant interfaces and in close vicinity of implant materials for small field sizes (1×1cm2 , 2×2cm2 , 3×3cm2 , and 4×4cm2 ) of a 6MV flattening filter free photon beam. O-MAR corrected and uncorrected (UC) computed tomography (CT) images were used for dose calculations. The values of average and standard deviations (SD) of Hounsfield unit (HU) for selected regions of each case were evaluated. The differences in average dose percentages in defined regions were calculated to quantify the relative dosimetric changes between doses calculated on UC and O-MAR corrected CT images. Compared to UC images, the values of SD were reduced, and the average HU became closer to its reference value in the O-MAR images. There was some discrepancy in average dose percentage differences between calculations using UC and O-MAR images at 1cm above the SS316L implant (average dose percentage differences were AXB/UC=5.9% and AXB/O-MAR=-1.2%; AAA/UC=2.2%, and AAA/O-MAR=-0.8%). Neither AAA nor AXB algorithms predict increase in dose at upper phantom-implant interface (4.9%, 9.9%. 13.5%, and 13.8% for the fields from 1×1cm2 to 4×4cm2 , respectively). At the side of the SS316L implant (where dark streak artifacts exist), dose difference averages were estimated as-1.1% and 22.3% when AXB/O-MAR and AXB/UC calculations are compared with EBT3 measurements, respectively. Dose predictions at 1cm below the SS316L implant were underestimated by AXB/O-MAR (average -0.5%) and AXB/UC (average 2.0%). The O-MAR tool was shown to have a favorable dosimetric effect or no effect on the calculations in the upper proximity of the implant materials. The dose differences between EBT3 film measurements and calculations at upper phantom-implant interfaces were smaller when they were calculated using O-MAR images. However, the dose differences increased when O-MAR corrected images were used for AAA calculations at lower phantom-implant interfaces. Use of O-MAR enabled closer agreement for the AXB algorithm, especially in the dark streak artifact regions. The O-MAR algorithm should be used when the dose is calculated with the AXB algorithm in cases of patients with the metal implants. The estimations using AAA and AXB algorithms, in phantom setups, with Ti5 implant material were found to be closer to the EBT3 film measurements, when compared with the same estimations using SS316L implant material.

Response characterization of EBT-XD radiochromic films in megavoltage photon and electron beams.

The purpose of this study was to investigate the beam quality dependence of the EBT-XD radiochromic films for megavoltage photon and electron beam irradiations by studying their spectral response. Dose, dose rate, and interbatch dependencies were investigated as well. EBT-XD and EBT3 radiochromic films were cut into 5cm2 ×5cm2 pieces, placed between solid water phantoms, and irradiated with 6 and 15MV photon beams, 6 and 10MV flattening filter free photon beams, and 6 and 20MeV electron beams at dose levels between 0.5 and 50Gy. In order to measure the spectral response, the films were illuminated by a deuterium and tungsten-halogen lamp and net absorption was measured in 400-800nm spectral range using a fiber-coupled optical spectrometer. Film samples were then analyzed using a flatbed scanner in the red, green, and blue channel to obtain the optical density of the films. The net absorption spectra of the EBT-XD and EBT3 films showed two absorption bands centered at 635 and 585nm, characteristic of the EBT model. However, for a given dose, the EBT-XD films showed lower net absorbance compared to the EBT3 films. At a given dose level, the net absorption spectra and dose-response curves for EBT-XD films showed only slight variations across various beam qualities. When a calibration curve obtained from a given beam quality is used to measure dose from films irradiated with other beam qualities, the maximum uncertainty in the calculated dose, in ranges 5-40 and 8-50Gy for red and green channels, respectively, were ~3.1% and ~2.4%. The response of the EBT-XD films is dose rate independent but batch dependent. No significant beam quality dependence was observed in EBT-XD films in the dose range up to 50Gy.

Study of detectors response on the scatter radiation of 6 MV flattening filter free beams

Small field dosimetry which related to the use of Flattening Filter Free (FFF) beams is a great challenge because of the perturbation effects caused by the size of the detector’s active volume or detector’s materials. This phenomenon encourages researchers to do a further study about detector response. In this study, scatter and primary radiation from 6 MV FFF photon beams were studied to evaluate the detector response in small fields for a better understanding of the contribution of every phenomenon. Ion chamber and Gafchromic films were used in three measurement configurations representing open field measurement and approximations of both primary and scatter part of the beam, described as follows: (i) detectors positioned under steel block, exposing the detectors only to scatter part of radiation field, (ii) detectors positioned inside mini phantom, approximating the detector response to primary radiation with minimum scatter, (iii) detectors positioned in the standard open field, which was the superposition of the primary and scatter radiation. The results show that detector responses were heavily depended on its design (i.e. active volume) especially in small beams. The detector response in the open field could be reproduced from the blocked and primary beam set-up with a discrepancy ranging from 1.0% to 36%. Moreover, the volume averaging and other phenomenon affecting the detector response could be observed in the blocked beam.

Evaluating out-of-field doses during radiotherapy of paediatric brain tumours using lead shielding and flattening-filter free beams

Paediatric radiotherapy comes at the expense of increased risk of late effects due to out-of-field dose caused not only by the treatment itself but also by image guidance. This study examined how the out-of-field dose to selected radiosensitive organs was affected by applying a 1 mm lead shielding during delivery of volumetric-modulated arc therapy (VMAT) for paediatric brain cancer. The study also investigated how the out-of-field dose to the same organs was affected by the use of flattening-filter free (FFF) beams.Out-of-field doses to the thyroid, breast and testes were measured using thermoluminescence dosimeters inserted in two anthropomorphic phantoms equivalent to a 1-year and 5-year old child. Coplanar VMAT plans were prepared for 6 MV and 6 MV FFF photon beams and delivered using a Varian TrueBeam linear accelerator, with and without lead shielding applied to the phantoms.The measured out-of-field doses were as large as 200,9 cGy for the whole treatment, with associated secondary cancer risk being as large as 1,1%. Shielding of the phantoms was found to decrease the out-of-field dose by up to 24%. The use of 6 MV FFF beams yielded a decrease in the dose to the testes by 21–42% compared to 6 MV, while in one case increasing the dose to the thyroid by 18%. The observation that only doses to organs distant to the primary irradiated volume were significantly decreased for FFF can be explained by an increase in internal scatter caused by the softer energy spectrum of the Varian FFF beam.

A method of describing the characteristics of flattening filter-free photon beam

Objective To establish a novel method of describing the off-axis ratio (OAR) characteristics of the flattening filter-free (FFF) beam. Methods The OAR curves at a depth of 1.5, 5, 10 and 20 cm were measured for Varian Edge, Elekta VersaHD and Tomotherapy using the water tank. The second derivatives of the OAR in the positive and negative directions were calculated. The center of the line connecting the maximum and minimum second derivatives was defined as the field edge. The distance between the left and right field edges was defined as the dosimetric field size. The OAR curve within the 80% of dosimetric field size was fitted using the gaussian function and the fitting parameters were adopted to describe the shape of OAR. Results The calculated field size error was less than 0.11 cm and the central axis position error was less than 0.05 cm. The fitting correlation coefficient was greater than 0.998.The fitting maximum error of OAR curve did not significantly alter with the depth, whereas slightly increased over the increased field size. The maximum error for a field size of 10, 20, 30 and 40 cm was 0.49%, 0.67%, 1.25% and 2.52%, respectively. Conclusions A method which can independently and accurately describe the OAR characteristics of FFF photon beam is established for the first time, which can calculate the field size of FFF beam and fit the OAR curve of FFF beam using the gaussian function. Key words: Flattening filter free photon beam; Field size; Off-axis ratio characteristics

Modelling the x-ray source for the Australian MRI-Linac

MRI-guided radiotherapy allows real-time imaging during treatment however the magnetic field influences the dose distribution in the patient. An accurate model of the radiation beam and the encompassing magnetic field is important to predict dosimetry changes. The purpose of this work is to develop a Monte Carlo model of the Australian MRI-Linac to be used as input into a dose calculation tool for treatment planning. The Australian MRI-Linac is a 1 T inline system with a 6MV flattening filter free photon beam. Commissioning measurements were undertaken both with and without the magnetic field present, PDDs and profiles were used to develop a model with the Geant4 toolkit. To date the model at 0 T matches within ±2% of measured data. Ongoing work involves measurements at 1 T at various linac to MRI isocentre distances, the magnetic field model at each configuration is also under development.

Study of Dosimetric Properties of Flattening Filter Free Photon Beam Passing through Cadmium Free Compensator Alloy.

Background: This study aims to investigate radiation beam geometry of Cyberknife beam and change in dosimetric characteristics of six megavoltage (6MV) flattening filter free (FFF) beam after passing through high density cadmium free compensator alloyMaterial and Methods: In this experimental study, changes in FFF beam dosimetric characteristics after passing through compensator alloy was measured. Transmitted intensity of FFF beam was measured in air by an ion chamber at a source to detector distance (SDD) of 800mm. Extended SDD measurement also has been performed at a distance of 1270mm to analyze scattering due to compensator. Linear attenuation coefficient (µeff) was measured for cadmium free compensator alloy using simple exponential attenuation model. Percentage depth doses (PDDs) have been measured by a radiation field analyzer with compensator material to observe the beam hardening and change in surface doses and depth doses.Results: Linear attenuation coefficient of compensator alloy was measured 0.042 (Standard Deviation ±0.00099) mm-1 and it was found that there is no change with increase in collimator size. Even after increasing distance source from detector, µeff has no change. PDDs were found to increase with thickness of compensator. PDD from a 60mm collimator size increased by 5% and 6% at a depth of 100mm and 200mm, respectively in water. PDD also increased with collimator size less significantly. Surface dose was found to decrease with increase in compensator thicknessConclusion: Cyberknife beam has been found to be narrow beam geometry. FFF beam contains lesser scattered photons. Presence of high density compensator filters out the soft x-ray photon causes significant dosimetric changes

16 Dosimetry improvement of an EPID-based pre-treatment verification software by the implementation of a dual photon beam model from linear accelerators

Introduction Point-kernel convolution-based dose calculation algorithms require an accurate modelling of the incident energy. The contribution of diffused photons by the primary collimator and the flattening filter (extra-focal source) is added to the primary source. Deviations up to 5% can be observed (Alaei, 2010) compared to the measurement for certain settings of collimation elements without an explicit modelling of the extra-focal source according to the position of the various collimation elements. EPID-based pre-treatment verification solutions generally uses fluence-based convolution models with the impulsional response of the detector. To be perfectly reliable, these methods must accurately predict the portal image to be compared with the acquired image. The aim of this work concerns the implementation and validation of an extra-focal source model for the computation of the predicted EPID image for standard (FF) and FFF (Flattening Filter Free) beams. Methods The linear accelerator used in this study is a TrueBeam STx® equipped with MLC HD120 and EPID as1200 with 6MV FF and FFF photons. Square fields with swapped openings of jaws and MLC leafs of 5 to 22 cm are designed with Eclipse® (Varian). Reference dose measurements in water are carried out on the central axis, with a microDiamond detector at SSD = 95 cm and 5 cm deep. The predicted and acquired portal images are processed with the EPIbeam software (Dosisoft). The prediction model calculates the expected dose map at SSD = 95 cm and 5 cm depth in water from the DICOM RT plan file, which is then compared to the acquired portal image converted to dose. The accuracy of the TPS calculations, dose conversion and prediction models in EPIbeam, without and with implementation of an extra-focal source model including only the effects of the flattening filter, are confronted with the reference measures in water. Results The maximum discrepancy between the TPS calculations and the measurements is less than 1%. The dose values on the converted EPID image accurately match to microDiamond measurements (less than 1% of error). With the primary source model alone, the EPIbeam prediction model for FF photons shows dose differences up to 4% with the microDiamond detector with small MLC openings and retracted jaw positions. The observed deviations after the implementation of the extra-focal source model remain less than 1% for all collimations. For the FFF photons, deviations are in all cases less than 1%. Conclusions For FFF photons, the implementation of an extra-focal source model is optional since the contribution of the primary collimator appears negligible. However, this study confirms the importance of modelling the extra-focal source for FF photons to enhance the accuracy of the dosimetry calculations for pre-treatment checking.

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.

15 Characterization of an aSi-1000 EPID response in integrated and continuous acquisition modes for SBRT dosimetry applications

Introduction The amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) mounted on linacs have been extensively studied and have shown dose-response characteristics suitable for dose verification. Their use for dosimetric purposes is already in clinical routine. However, the response of EPID in continuous acquisition mode was not so studied in literature, and in less extent when considering it for dosimetric applications in stereotactic body radiotherapy (SBRT) conditions (i.e. small fields, and high dose rates using flattening filter free (FFF) beams). The purpose of this work was therefore to characterize the EPID response in terms of relevant dosimetric properties, mainly for the potential use of continuous mode for further SBRT dosimetry with dynamic techniques. Methods The EPID used in this study was the aSi-1000 model installed on a Varian TrueBeamSTx. Its response was characterized in FFF photon beams with nominal energies of 6 and 10 MV and maximum available dose rates (1400 and 2400 MU/min, for 6 and 10 MV respectively). Different sets of images were acquired in both integrated and continuous modes, with EPID at 150 cm from the source, to study dose linearity, repeatability and reproducibility of response, field-size dependence, ghosting effect and response to energy spectrum variations. Before data acquisition, dark field and flood filed images were acquired for offset and gain corrections. As fluctuations in dose rate can potentially influence the response of EPID when using dynamic techniques with continuous acquisition mode, the linearity of EPID response was further evaluated by irradiating a 10 × 10 cm2 field in 20° and 90° arcs focusing on low MU range (3–100 MU), and was also compared to static irradiations. Results Conclusions The characterization of the aSi-1000 EPID response to relevant dosimetric properties was investigated, showing the feasibility of its use for dosimetry with FFF beams and high dose rates. Results for continuous mode, which differed from dosimetric non-linearity reported in previous published studies [ [1] McCurdy B.M.C. Greer P.B. Dosimetric properties of an amorphous-silicon EPID used in continuous acquisition mode for application to dynamic and arc IMRT. Med Phys. 2009; 36: 3028-3039 Crossref PubMed Scopus (73) Google Scholar ], also encourage the use of EPID for SBRT with dynamic techniques in future works for 3D dosimetry applications (e.g. quality assurance including in vivo dosimetry). The amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) mounted on linacs have been extensively studied and have shown dose-response characteristics suitable for dose verification. Their use for dosimetric purposes is already in clinical routine. However, the response of EPID in continuous acquisition mode was not so studied in literature, and in less extent when considering it for dosimetric applications in stereotactic body radiotherapy (SBRT) conditions (i.e. small fields, and high dose rates using flattening filter free (FFF) beams). The purpose of this work was therefore to characterize the EPID response in terms of relevant dosimetric properties, mainly for the potential use of continuous mode for further SBRT dosimetry with dynamic techniques. The EPID used in this study was the aSi-1000 model installed on a Varian TrueBeamSTx. Its response was characterized in FFF photon beams with nominal energies of 6 and 10 MV and maximum available dose rates (1400 and 2400 MU/min, for 6 and 10 MV respectively). Different sets of images were acquired in both integrated and continuous modes, with EPID at 150 cm from the source, to study dose linearity, repeatability and reproducibility of response, field-size dependence, ghosting effect and response to energy spectrum variations. Before data acquisition, dark field and flood filed images were acquired for offset and gain corrections. As fluctuations in dose rate can potentially influence the response of EPID when using dynamic techniques with continuous acquisition mode, the linearity of EPID response was further evaluated by irradiating a 10 × 10 cm2 field in 20° and 90° arcs focusing on low MU range (3–100 MU), and was also compared to static irradiations. The characterization of the aSi-1000 EPID response to relevant dosimetric properties was investigated, showing the feasibility of its use for dosimetry with FFF beams and high dose rates. Results for continuous mode, which differed from dosimetric non-linearity reported in previous published studies [ [1] McCurdy B.M.C. Greer P.B. Dosimetric properties of an amorphous-silicon EPID used in continuous acquisition mode for application to dynamic and arc IMRT. Med Phys. 2009; 36: 3028-3039 Crossref PubMed Scopus (73) Google Scholar ], also encourage the use of EPID for SBRT with dynamic techniques in future works for 3D dosimetry applications (e.g. quality assurance including in vivo dosimetry).

Effect of dose-delivery time for flattened and flattening filter-free photon beams based on microdosimetric kinetic model.

The effect of dose-delivery time with flattening filter (FF) and flattening filter-free (FFF) photon beams based on microdosimetric kinetic model (MKM) was investigated in this study. Monte Carlo simulation with the particle and heavy ion transport code system (PHITS) was performed to calculate the dose-mean lineal energy yD (keV/μm) of FF and FFF 6 MV photon beams using the IAEA phase-space files of Varian TrueBeam linear accelerator. Human non-small cell lung cancer NCI-H460 cells were used to determine the MKM parameters under the condition that dose-delivery times with continuous irradiation were 1, 5, 10, 30, and 60 min, and the adsorbed dose was 2, 4, and 8 Gy in this study. In addition, the relative biological effectiveness (RBE) of FF and FFF photon beams were calculated for evaluating the effect of dose delivery time. The RBE of FF decreased to 99.8% and 97.5% with 5 and 60 min for 2 Gy in comparison to 99.6% and 95.1% for 8 Gy, respectively. Meanwhile, that of FFF decreased to 99.5% and 94.9% with 5 and 60 min for 2 Gy in comparison to 99.5% and 94.9% for 8 Gy, respectively. Dose-delivery time has an effect on the RBE with photon beams. In other words, the dose-delivery time should be considered during radiation therapy. Furthermore, FFF photon beams were an effective irradiation method compared to FF in dose-delivery time on account of improving clinic throughput.