Flattening Filter Free Photon Beams Research Articles

P288] Use of organic plastic scintillators for determination of the beam quality correction factor for flattening filter free photon beams

Purpose The purpose was to use organic plastic scintillators for determination of the beam quality correction factor ( k Q , Q 0 ) for flattening filters free (FFF) photon beams increasingly used in modern radiotherapy. The values were compared with those recommended in the recent TRS-483 code of practice from the International Agency of Atomic Energy. Methods The formalism in TRS-483 covers not only small conventional photon beams with flattening filter (WFF), but also flattening filter free (FFF) beams. TRS-483 splits the k Q , Q 0 into two correction factors for FFF beams: k Q WFF , Q 0 f ref and k Q FFF , Q WFF f ref . An IBA FC-65G ionization chamber, calibrated for conventional MV photon beams, was used to experimentally determine the k Q FFF , Q 0 . The fiber-coupled scintillator (1 mm diameter; 2 mm length) was used as a water-point surrogate detector for determination of the k Q , Q 0 from quality Q FFF with respect Q WEF . Given the high degree of water-equivalence of this detector, and in line with the studies presented in TRS-483, it was assumed that the detector was perturbation free. Corrections were, however, made for ionization quenching based on published parameters for the used scintillator and Birks formalism implemented in the Monte Carlo toolkit EGSnrc. Results The spectral difference between WFF and FFF resulted in changes in the ionization quenching which in turn affected the FFF/WFF-ratio in the TRS-483 formalism by about 0.6% for 6 MV and by 0.35% for 10 MV, respectively. After correcting for this, the beam quality correction factor for the FFF 6 MV and 10 MV beam was found to differ insignificantly, 0.3%, compared with those recommended in TRS-483 when given that uncertainty of the ionization quenching correction factor alone was about 0.15% and that additional uncertainty components (>0.5%) are associated with the measurements. Conclusions An experimental method for determining the beam quality correction factor for FFF to WFF was carried out. The study supports that organic plastic scintillators can be used as a perturbation-free detector in MV photon beams. However, considerations should be given to the change in quenching when spectrally different beams are involved such as when computing the dose ratio for WFF and FFF beams in the TRS-483 formula for beam quality correction factor.

P190] Investigation of beam quality correction factors for flattening filter free photon beams

Purpose It has previously been shown that the use of a dual beam quality specifier consisting of two tissue-phantom ratios, TPR20/10 and TPR10/5, provides an accurate prediction of stopping power ratios (sw,air) for flattening filter free (FFF) photon beams with different filtration, using a bi-linear relationship between the two TPRs and sw,air. For the current project, the intention is to evaluate this dual beam quality specifier on kQ factors used in radiotherapy reference dosimetry, calculated using Monte Carlo modelling. Methods Two different ionisation chambers were used in the simulations, NE2571 and FC65-G. Calculations of kQ factors, TPR20/10 and TPR10/5 were performed using EGSnrc, and the chamber geometry was acquired from technical specifications from the manufacturer and from previously published articles. A total of 18 photon beams (10 with flattening filter (FF) and 8 flattening filter free) were used. Results For the FFF beams, the calculated beam quality correction factors were found to be 0.1–0.3% lower than for the beams with a flattening filter at the same TPR20/10. Using data from TRS398 overestimated kQ by up to 1% for FFF beams but also up to 0.9% for FF beam for the FC65-G chamber. The corresponding numbers for the NE2571 were 0.6% for both FF and FFF beams. Using the dual beam quality metric fitted to the calculations, kQ factors could be predicted within 0.12% for all beams. Conclusions Beam quality conversion factors for NE2571 and FC65-G farmer type ionisation chambers have been calculated, for beams with and without a flattening filter. Differences between beams with and without a flattening filter were found to be 0.1–0.3%. An overestimation of kQ factors for conventional photon beams of up to 0.9% was found using TRS398. Using a dual beam quality measure, consisting of both TPR20/10 and TPR10/5, KQ factors could be predicted to within 0.12%. Although the difference between FF and FFF beams is small, it is a systematic difference which can be accounted for.

P140] Flattening filter-free radiotherapy for paediatric brain cancer: Impact on the dose to selected out-of-field organs

Purpose Removal of the flattening filter has been shown to alter the out-of-field dose during radiotherapy. In this study, the dose to selected out-of-field organs is compared for conventional flattened and flattening filter free (FFF) volumetric-modulated arc therapy (VMAT) of paediatric brain cancer. Methods Out-of-field doses to the thyroid, breast and testes were measured using thermoluminescence dosimeters inserted in two anthropomorphic phantoms resembling a 1-year and 5-year old child. Coplanar VMAT plans were prepared for 6 MV and 6 MV FFF photon beams, for two different PTV sizes (2 cm and 5 cm diameter), simulating post-operative radiotherapy of an ependymoma in the posterior fossa. The phantoms were subject to three VMAT fractions of 1,8 Gy each, delivered using a Varian TrueBeam linear accelerator. Some additional irradiations were performed for the 1-year phantom with the head of the phantom removed, to investigate the contribution from internal scatter to the total out-of-field dose. Results For the testes, measured doses showed a statistically significant decrease (Student’s t-test, significance level p 0,05) of 21–42% for FFF compared with conventional plans, for both phantoms as well as both PTV sizes. Doses to the breast showed a significant decrease of approximately 12% for the 5-year phantom for the small PTV, but other breast doses did not show a significant difference with and without the flattening filter. Doses to the thyroid showed a significant increase of approximately 18% for the 1-year phantom for the small PTV, with other dose differences being insignificant. However, doses to all three organs were significantly reduced (19–37%) when removing the head of the 1-year phantom, minimizing the contribution from internal scatter. Conclusions The results show that FFF VMAT of paediatric brain cancer leads to reduced doses to organs far from the field compared to VMAT with conventional flattened beams, while doses to organs close to the field seem to increase. The latter finding can be explained by an increase in internal scatter, which is possibly caused by the softer energy spectrum of the 6 MV FFF beam for the investigated linear accelerator model compared to conventional 6 MV.

Radiobiological effects of flattening filter-free photon beams on A549 non-small-cell lung cancer cells.

Flattening filter–free (FFF) photon beams minimize the intrafraction motion of tumors, and this feature is useful in pulmonary malignancies, such as non-small-cell lung cancer (NSCLC). However, the radiobiological effects of such beams on NSCLC cells, which are often treated with stereotactic body radiotherapy (SBRT), have not been investigated sufficiently. Although cell motility may be promoted by photon beams with a low dose, the relationship between cell motility and the dose rate of photon beams has not been evaluated. The purpose of this study was to evaluate the radiobiological effects of FFF photon beams on cell survival and motility in NSCLC. A human lung cancer cell line (A549) was irradiated with conventional flattening filter (FF) and FFF photon beams at dose rates of 300 (FF), 500 and 2000 MU/min (FFF). While cell survival was estimated using the colony formation assay, cell motility was evaluated using the Boyden chamber and Matrigel invasion assays. FFF photon beams with a high dose rate neither affected the survival of A549 cells nor caused any significant difference in their motility. On the other hand, high-dose irradiation reduced cell survival and motility regardless of the dose rate. Photon beams with a high dose rate used for radiation therapy are suitable for SBRT from the standpoint of both cell survival and motility, in addition to their physical characteristics.

Evaluation of dosimetric impact of Flattening Filter-Free (FFF) Photon Beams in 3D conformal radiotherapy

Flattening Filter Free (FFF) photon beams have been frequently used in radiotherapy clinical practice. This study aimed to analyze the dosimetric quality of the FFF beams, as compared to the standard flattened beams (FB) for plans of treatment of three patients involving three cancer sites using the 6 and 10 MV photon energies and the 3D conformal radiotherapy technique (3D-CRT). The doses to the planning target volumes (PTVs) and organs at risk (OARs) were compared. The dose-volume histogram curves analysis show that no significant differences between FFF and FB plans were found in the coverage and mean dose for the PTV, the OAR revealed slight differences. So the FFF photon beam achieved comparable dosimetric quality as the standard flat beam.

22. Feasibility study for the implementation of aS1000 EPID controls of RapidArc® treatments using FFF photon beams

Introduction The development of VMAT (Volumetric Modulated Arc Therapy) treatments by flattening filter free photon beams (FFF) involves the adaptation of dosimetric verification methods. A number of physical limits (detector saturation, backscattering…) or algorithmic limits, constrain us to use 3D phantoms of which implementation is time-consuming. In order to optimize the patient workflow, the purpose of this study concerns the technical feasibility of using the EPID (Electronic Portal Imaging Device) and the performances of the algorithms PDIP (Varian) and GlAaS (EPIQA) for the verification of plans of treatment Varian RapidArc® FFF. Methods The chain of treatment includes a Varian Clinac 2100C accelerator with an EPID aS1000 imager and the PDIP algorithm V13.6 and EPIQA V2.2.1. A calibration of the portal imager is performed for the energy X6 FFF. To test the criticality of the two algorithms for the local and global gamma index validation methods at 3% 3 mm, the results obtained with the portal imager are compared with those obtained with the ArcCheck. Results The first results show the feasibility of implementing FFF treatment plan validation checks with EPID for CLINAC 2100C accelerators equipped with the aS1000 portal imaging system. Conclusions This study makes it possible to validate the control of the RapidArc® treatment plans by FFF beams using the portal imager. This method is an alternative to 3D phantom validations such as ArcCheck, Delta 4.

Monte Carlo Simulation of 6 MV Flattening Filter Free Photon Beam of True Beam STx LINAC at Songklanagarind Hospital

In this study, 6 MV photon beam of TrueBeam STx Varian LINAC with Flattening Filter Free (FFF) was simulated using PRIMO code. The depth dose profiles for various jaws open fields and cross beam profiles for various depths inside water phantom were determined using Monte Carlo (MC) simulation technique and validated with experimental result. The experiments were performed using the Source to Surface Distance (SSD) technique with a 100 cm distance from target to the surface of water. Simulation used 109 histories with the same configurations with experiments. The depth dose profiles and cross beam profiles of 6 MV FFF photon beam was determined using MC simulations and compared with experimental results. The results showed that depth dose profiles and cross beam profiles by MC simulation accurately matched with experimental results. The best result of depth dose profile was obtained at 10×10 cm2 jaws open field with 98.53% passing criterion whereas cross beam profile was obtained at 10 cm depth inside water phantom with 88.96% passing criterion.The discrepancies were caused by scatter of particle and incompatibility of primary beam in PRIMO with experiment.

Equivalent (uniform) square field sizes of flattening filter free photon beams

Various types of treatment units, such as CyberKnife, TomoTherapy and C-arm linear accelerators (LINACs) are operated using flattening filter free (FFF) photon beams. Their reference dosimetry, however, is currently based on codes of practice that provide data which were primarily developed and tested for high-energy photon beams with flattening filter (WFF). The aim of this work was to introduce equivalent uniform square field sizes of FFF beams to serve as a basis of a unified reference dosimetry procedure applicable to all aforementioned FFF machines. For this purpose, in-house determined experimental data together with published data of the ratio of doses at depths of 20 cm and 10 cm in water (D20,10) were used to characterize the depth dose distribution of 6 and 10 MV WFF and FFF beams. These data were analyzed for field sizes ranging from 2 × 2 cm2 to 40 × 40 cm2. A scatter function that takes the lateral profiles of the individual beams into account was fitted to the experimental data. The lateral profiles of the WFF beams were assumed to be uniform, while those of the FFF beams were approximated using fourth or sixth order polynomials. The scatter functions of the FFF beams were recalculated using a uniform lateral profile (the same as the physical profile of the WFF beams), and are henceforth denoted as virtual uniform FFF beams (VUFFF). The field sizes of the VUFFF beams having the same scatter contribution as the corresponding FFF beams at a given field size were defined as the equivalent uniform square field (EQUSF) size. Data from four different LINACs with 18 different beams in total, as well as a CyberKnife beam, were analyzed. The average values of EQUSFs over all investigated LINACs of the conventional 10 × 10 cm2 reference fields of 6 MV and 10 MV FFF beams for C-arm LINACs and machine-specific reference fields for CyberKnife and TomoTherapy were 9.5 cm, 9 cm, 5.0 cm and 6.5 cm respectively. The standard deviation of the mean of these EQUSFs was below 0.1 cm. It has been shown that with the introduction of a VUFFF beam, EQUSFs can be consistently defined for a variety of energies and collimations. These EQUSFs can form the basis for a unified reference dosimetry protocol for all different types of FFF machines.

Small field characterization of a Nanochamber prototype under flattening filter free photon beams

IntroductionNanochambers present some advantages in terms of energy independence and absolute dose measurement for small field dosimetry in the SBRT scenario. Characterization of a micro-chamber prototype was carried out both under flattened and flattening-filter-free (FFF) beams with particular focus on stem effect. MethodsThe study included characterization of leakage and stem effects, dose rate and dose per pulse dependence, measurement of profiles, and percentage depth doses (PDDs). Ion collection efficiency and polarity effects were measured and evaluated against field size and dose per pulse. The 6_MV, 6_MV_FFF and 10_MV FFF beams of a Varian EDGE were used. Output factors were measured for field sizes ranging from 0.8×0.8cm2 to 20×20cm2 and were compared with other detectors. ResultsThe 2mm diameter of this chamber guarantees a high spatial resolution with low penumbra values. In orthogonal configuration a strong stem (and cable) effect was observed for small fields. Dose rate and dose per pulse dependence were <0.3% and 0.6% respectively for the whole range of considered values. The Nanochamber exhibits a field size (FS) dependence of the polarity correction >2%. The OF values were compared with other small field detectors showing a good agreement for field sizes >2×2cm2. The large field over-response was corrected applying kpol(FS). ConclusionsNanochamber is an interesting option for small field measurements. The spherical shape of the active volume is an advantage in terms of reduced angular dependence. An interesting feature of the Nanochamber is its beam quality independence and, as a future development, the possibility to use it for small field absolute dosimetry.

Dose evaluation of Grid Therapy using a 6 MV flattening filter-free (FFF) photon beam: A Monte Carlo study.

Spatially fractionated radiotherapy is a strategy to overcome the main limitation of radiotherapy, i.e., the restrained normal tissue tolerances. A well-known example is Grid Therapy, which is currently performed at some hospitals using megavoltage photon beams delivered by Linacs. Grid Therapy has been successfully used in the management of bulky abdominal tumors with low toxicity. The aim of this work was to evaluate whether an improvement in therapeutic index in Grid Therapy can be obtained by implementing it in a flattening filter-free (FFF) Linac. The rationale behind is that the removal of the flattening filter shifts the beam energy spectrum towards lower energies and increase the photon fluence. Lower energies result in a reduction of lateral scattering and thus, to higher peak-to-valley dose ratios (PVDR) in normal tissues. In addition, the gain in fluence might allow using smaller beams leading a more efficient exploitation of dose-volume effects, and consequently, a better normal tissue sparing. Monte Carlo simulations were used to evaluate realistic dose distributions considering a 6MV FFF photon beam from a standard medical Linac and a cerrobend mechanical collimator in different configurations: grid sizes of 0.3×0.3cm2 , 0.5×0.5cm2 , and 1×1cm2 and a corresponding center-to-center (ctc) distance of 0.6, 1, and 2cm, respectively (total field size of 10×10cm2 ). As figure of merit, peak doses in depth, PVDR, output factors (OF), and penumbra values were assessed. Dose at the entrance is slightly higher than in conventional Grid Therapy. However, it is compensated by the large PVDR obtained at the entrance, reaching a maximum of 35 for a grid size of 1×1cm2 . Indeed, this grid size leads to very high PVDR values at all depths (≥10), which are much higher than in standard Grid Therapy. This may be beneficial for normal tissues but detrimental for tumor control, where a lower PVDR might be requested. In that case, higher valley doses in the tumor could be achieved by using an interlaced approach and/or adapting the ctc distance. The smallest grid size (0.3×0.3cm2 ) leads to low PVDR at all depths, comparable to standard Grid Therapy. However, the use of very thin beams might increase the normal tissue tolerances with respect to the grid size commonly used (1×1cm2 ). The gain in fluence provided by FFF implies that the important OF reduction (0.6) will not increase treatment time. Finally, the intermediate configuration (0.5×0.5cm2 ) provides high PVDR in the first 5cm, and comparable PVDR to previous Grid Therapy works at depth. Therefore, this configuration might allow increasing the normal tissue tolerances with respect to Grid Therapy thanks to the higher PVDR and thinner beams, while a similar tumor control could be expected. The implementation of Grid Therapy in an FFF photon beam from medical Linac might lead to an improvement of the therapeutic index. Among the cases evaluated, a grid size of 0.5×0.5cm2 (1-cm-ctc) is the most advantageous configuration from the physics point of view. Radiobiological experiments are needed to fully explore this new avenue and to confirm our results.

Surface dose measurements in and out of field: Implications for breast radiotherapy with megavoltage photon beams

This study examines the difference in surface dose between flat and flattening filter free (FFF) photon beams in the context of breast radiotherapy. The surface dose was measured for 6MV, 6MV FFF, 10MV, 10MV FFF and 18MV photon beams using a thin window ionisation chamber for various field sizes. Profiles were acquired to ascertain the change in surface dose off-axis. Out-of-field measurements were included in a clinically representative half beam block tangential breast field. In the field centres of FFF beams the surface dose was found to be increased for small fields and decreased for large fields compared to flat beams. For FFF beams, surface dose was found to decrease off-axis and resulted in lower surface dose out-of-field compared to flat beams.

Clinically relevant investigation of flattening filter‐free skin dose

As flattening filter‐free (FFF) photon beams become readily available for treatment delivery in techniques such as SBRT, thorough investigation of skin dose from FFF photon beams is necessary under clinically relevant conditions. Using a parallel‐plate PTW Markus chamber placed in a custom water‐equivalent phantom, surface‐dose measurements were taken at 2×2,3×3,4×4,6×6,8×8,10×10,20×20, and 30×30 cm2 field sizes, at 80, 90, and 100 cm source‐to‐surface distances (SSDs), and with fields defined by jaws and multileaf collimator (MLC) using multiple beam energies (6X, 6XFFF, 10X, and 10XFFF). The same set of measurements was repeated with the chamber at a reference depth of 10 cm. Each surface measurement was normalized by its corresponding reference depth measurement for analysis. The FFF surface doses at 100 cm SSD were higher than flattened surface doses by 45% at 2×2 cm2 to 13% at 20×20 cm2 for 6 MV energy. These surface dose differences varied to a greater degree as energy increased, ranging from +63% at 2×2 cm2 to −2% at 20×20 cm2 for 10 MV. At small field sizes, higher energy increased FFF surface dose relative to flattened surface dose; while at larger field sizes, relative FFF surface dose was higher for lower energies. At both energies investigated, decreasing SSD caused a decrease in the ratios of FFF‐to‐flattened surface dose. Variability with SSD of FFF‐to‐flattened surface dose differences increased with field size and ranged from 0% to 6%. The field size at which FFF and flattened beams gave the same skin dose increased with decreasing beam energy. Surface dose was higher with MLC fields compared to jaw fields under most conditions, with the difference reaching its maximum at a field size between 4×4 cm2 and 6×6 cm2 for a given energy and SSD. This study conveyed the magnitude of surface dose in a clinically meaningful manner by reporting results normalized to 10 cm depth dose instead of depth of dose maximum.PACS number(s): 87.53.Bn, 87.53.Ly, 87.55.‐x, 87.55.N‐, 87.56.N‐

Dosimetric influence of filtered and flattening filter free photon beam on rapid arc (RA) radiotherapy planning in case of cervix carcinoma

AimTo investigate the dosimetric influence of filtered and flattening filter free (FFF) photon beam of 6 and 10MV energies on cervix RA radiotherapy planning and to find possibilities to develop the clinically acceptable RA plans with FFFB photon beam and explore their potential benefits to cervix cancer patients. BackgroundFFF photon beams enhances the treatment delivery by increased dose rate which results in shorter treatment time, this shorter treatment time reduces intrafraction motion and enhance comfort to the patients. Materials and methodsRA plans were generated for filtered and flattening filter free photon beams of 6 and 10MV energies using same dose–volumes constraints. RA plans were generated to deliver a dose of 50.4Gy in 28 fractions, for a cohort of eleven patients reported with cervix carcinoma. RA plans were evaluated in terms of PTV coverage, dose to OAR's, CI, HI, total no. of monitor units (MUs) and NTID and low dose volume of normal tissues. ResultsClinically acceptable and similar plans were generated for filtered and flattening filter free photon beams. FFFB delivered slightly higher mean target dose (52.28Gy vs. 52.0Gy, p=0.000 for 6MV and 52.42Gy vs. 52.0Gy, p=0.000 for 10MV) less homogeneous (1.062 vs. 1.052, p=0.000 for 6MV and 1.066 vs. 1.051, p=0.000 for 10MV) and less conformal (1.007 vs. 1.004, p=0.104 for 6MV and 1.012 vs. 1.003, p=0.010 for 10MV) RA plans compared to FB. FFFB delivered more doses to the bladder and rectum, also required more numbers of MUs in comparison to FB. ConclusionsThis study concludes that FB is more beneficial for cervix RA planning in comparison to FFFB, as FB generates more conformal and homogenous rapid arc plans and offers better OAR's sparing.

Sci‐Thur AM: YIS – 02: Imaging dose distributions through the detection of radiation‐induced acoustic waves

Purpose:X‐ray acoustic computed tomography (XACT) is an emerging technique that images the dose deposited within an object following linac irradiation by detecting acoustic waves induced via the photoacoustic effect. This work shows that XACT images can be formed in soft‐tissue equivalent material and that dosimetric information can be extracted from such images.Methods:Acoustic waves induced in a water tank following irradiation by a 10 MV flattening filter free photon beam were detected with an immersion ultrasound transducer at 60 angles surrounding the radiation field. A back‐projection algorithm was used to reconstruct an XACT image from the detected transducer signals. Profiles extracted from XACT images were compared to profiles measured with ion chambers as per the current clinical protocol.Results:XACT images were successfully formed of simple 4 cm × 4 cm and 6 cm × 3 cm fields, as well as of more complicated multi‐leaf collimator defined fields. For the 6 cm × 3 cm field, 74% and 87% of the XACT profile points in the 6 cm and 3 cm dimensions, respectively, passed a 7% / 4 mm gamma test when compared to ion chamber measurements. In a complicated puzzle piece shaped field, 86% of the pixels in an extracted profile passed a 7% / 4 mm gamma test.Conclusions:XACT is capable of imaging the dose distribution delivered by a variety of field sizes and shapes in water, and is a viable technique for both water tank and in vivo dosimetry.

Technical Report: Evaluation of peripheral dose for flattening filter free photon beams.

To develop a comprehensive peripheral dose (PD) dataset for the two unflattened beams of nominal energy 6 and 10 MV for use in clinical care. Measurements were made in a 40 × 120 × 20 cm(3) (width × length × depth) stack of solid water using an ionization chamber at varying depths (dmax, 5, and 10 cm), field sizes (3 × 3 to 30 × 30 cm(2)), and distances from the field edge (5-40 cm). The effects of the multileaf collimator (MLC) and collimator rotation were also evaluated for a 10 × 10 cm(2) field. Using the same phantom geometry, the accuracy of the analytic anisotropic algorithm (AAA) and Acuros dose calculation algorithm was assessed and compared to the measured values. The PDs for both the 6 flattening filter free (FFF) and 10 FFF photon beams were found to decrease with increasing distance from the radiation field edge and the decreasing field size. The measured PD was observed to be higher for the 6 FFF than for the 10 FFF for all field sizes and depths. The impact of collimator rotation was not found to be clinically significant when used in conjunction with MLCs. AAA and Acuros algorithms both underestimated the PD with average errors of -13.6% and -7.8%, respectively, for all field sizes and depths at distances of 5 and 10 cm from the field edge, but the average error was found to increase to nearly -69% at greater distances. Given the known inaccuracies of peripheral dose calculations, this comprehensive dataset can be used to estimate the out-of-field dose to regions of interest such as organs at risk, electronic implantable devices, and a fetus. While the impact of collimator rotation was not found to significantly decrease PD when used in conjunction with MLCs, results are expected to be machine model and beam energy dependent. It is not recommended to use a treatment planning system to estimate PD due to the underestimation of the out-of-field dose and the inability to calculate dose at extended distances due to the limits of the dose calculation matrix.

Technical Note: On the impact of the incident electron beam energy on the primary dose component of flattening filter free photon beams.

For commercially available linear accelerators (Linacs), the electron energies of flattening filter free (FFF) and flattened (FF) beams are either identical or the electron energy of the FFF beam is increased to match the percentage depth dose curve (PDD) of the FF beam (in reference geometry). This study focuses on the primary dose components of FFF beams for both kinds of settings, studied on the same Linac. The measurements were conducted on a VersaHD Linac (Elekta, Crawley, UK) for both FF and FFF beams with nominal energies of 6 and 10 MV. In the clinical setting of the VersaHD, the energy of FFFM (Matched) beams is set to match the PDDs of the FF beams. In contrast the incident electron beam of the FFFU beam was set to the same energy as for the FF beam. Half value layers (HVLs) and a dual parameter beam quality specifier (DPBQS) were determined. For the 6 MV FFFM beam, HVL and DPBQS values were very similar compared to those of the 6 MV FF beam, while for the 10 MV FFFM and FF beams, only %dd(10)x and HVL values were comparable (differences below 1.5%). This shows that matching the PDD at one depth does not guarantee other beam quality dependent parameters to be matched. For FFFU beams, all investigated beam quality specifiers were significantly different compared to those for FF beams of the same nominal accelerator potential. The DPBQS of the 6 MV FF and FFFM beams was equal within the measurement uncertainty and was comparable to published data of a machine with similar TPR20,10 and %dd(10)x. In contrast to that, the DPBQS's two parameters of the 10 MV FFFM beam were substantially higher compared to those for the 10 MV FF beam. PDD-matched FF and FFF beams of both nominal accelerator potentials were observed to have similar HVL values, indicating similarity of their primary dose components. Using the DPBQS revealed that the mean attenuation coefficient was found to be the same within the uncertainty of 0.8% for 6 MV FF and 6 MV FFFM beams, while for 10 MV beams, they differed by 6.4%. This shows that the DPBQS can provide a differentiation of photon beam characteristics that would remain hidden by the use of a single beam quality specifier, such as %dd(10)x or HVL.

SU‐F‐T‐498: A Comparative Evaluation of 6MV Flatten Beam and Flattening Filter Free Photon Beam in Carcinoma Breast

Purpose:Aim of the current study is to look plan quality, treatment beam ON time for IMRT using 6MV FB (Flatten Beam) and FFFB (Flattening Filter Free Beam) in left breast cancer cases.Methods:Ten left breast cancer patients treated with breast conserving surgical (BCS) procedure approach and adjuvant radiotherapy were selected from the department database. Simultaneous Integrated boost (SIB) technique was used to irradiate the total left breast (PTV) to a dose of 50.40Gy with concomitant enhance to the lumpectomy cavity known as gross tumour volume (GTV) to a dose of 59.40Gy in 28 fractions. Plans 6MV FB IMRT and 6MV FFFB IMRT had been generated to achieve dose to 95% target volume (TV) and spare Organ at risks (OAR's). Homogeneity index (HI), conformity index (CI), treatment monitor unit (MU),normal tissues integral dose (NTID) and low dose volume of normal tissue were compared.Results:There was no statistically huge difference among the plans with respect to target volume coverage, CI HI, Ipsilateral Lung and Breast. But statistically significant difference (p&lt; 0.05) as observed in Heart, V5Gy of Contralateral Lung, MU's NTID and low dose volume of normal tissue.Conclusion:6MV FB and FFF beam produce almost equivalent plans in IMRT modality with admire to target volume coverage, HI, CI. Beam on time and NTID was determined to be much less in 6MV FFFB IMRT. FFF beam leads to a time saving treatment delivery and fewer NTID in cancer of left breast cases

SU-F-T-338: Flattening Filter Free Photon Beams Can Achieve the Same Plan Quality as Conventional Flattened Beams for Prostate Radiotherapy

Purpose: Some modern linear accelerators are equipped with one low energy flat beam and two flattening filter free (FFF) beams at high and low energies. The purpose of this study is to investigate whether the high energy FFF beam can produce the same plan quality as the conventional low energy flat beam, using a volumetric modulated arc (VMAT) technique for prostate patients. Methods: Ten prostate cancer patients were selected with a prescription of 78Gy. For each patient, three plans were created: (a) double arc flat 6MV plan used clinically; (b) double arc 10MV FFF plan; (c) single arc 10MV FFF plan. Each plan was prescribed so that at least 95% of the PTV received the prescription dose. The following dosimetric endpoints were evaluated: volume receiving 78Gy (V78) of the CTV and PTV, PTV conformality index (CI, ratio of prescription isodose volume to the PTV volume), bladder volume receiving 70Gy (V70) and 60Gy (V60), rectum volume receiving 70Gy (V70) and 50Gy (V50), dose to 10cc of the rectum, and volume of both femoral heads receiving 50Gy (V50). Total monitor units for each plan were recorded. Results: No significant difference was found for all dosimetric endpoints between all plans (p>0.05). Compared to the 6MV plans, monitor units were higher with the double arc 10MV FFF plans and lower with the single arc 10MV FFF plans, 29% and 4% respectively. Conclusion: Both single arc and double arc 10MV FFF VMAT can achieve equivalent plan quality as 6MV flat beam double arc treatment plans. With the gantry speed restriction, a high dose rate of 2400MU/min may allow the optimizer to use more MUs than actually needed. Single arc 10MV FFF VMAT plans are a reasonable alternative to double arc 6MV flat beam VMAT plans.

SU‐F‐T‐461: Dosimetric Evaluation of Indigenous Farmer Type Chamber FAR65‐ GB for Reference Dosimetry of FFF MV Photon Beam

Purpose:Indigenous Farmer type chamber FAR 65 GB is a reference class 0.6 cc ion chamber. It can be used for dosimetric evaluation of photon and high energy electron beams. We studied dosimetric characteristics of the chamber for 6MV and 10MV Flattening filter free FFF photon beams available on trueBEAM STx Linac.Methods:The study was carried out on trueBEAM STx Linac having 6 and 10 MV FFF photon beam with maximum dose rate 1400 and 2400 MU per min respectively. The dosimetric device to be evaluated is Rosalina Instruments FAR 65‐GB Ion Chamber with active volume 0.65 cc, total active length 23.1cm, inner diameter of cylinder 6.2mm, wall thickness 0.4mm, inner electrode diameter 1mm. Inner and outer electrodes are made from Aluminium 2.7 gm per cc and graphite 1.82 gm per cc respectively. The ion chamber was placed along central axis of beam at 10cm depth and irradiated for 10cm × 10cm field size at SAD of 100 cm in plastic phantom. We studied Precision, Dose Linearity, Dose Rate dependence, directional dependence, Recombination effect. Recombination effect was determined using standard two‐voltage method.Results:1. Measurements were reproducible std deviation of 0.0105 and type A uncertainty 0.003265 under same set of reference conditions 2. Chamber exhibit dose linearity over a wider dose range. 3. Chamber shows dose rate independence for all available dose rate range. 4. Response of chamber with the angle of incidence of radiation is constant. 5. Recombination correction factors were 1.01848 and 1.02537 for dose rate 1400 and 2400 MU per min resp.Conclusion:Our study reveals that the chamber is prone to saturation effect at dose rate of 2400 MU per min. FAR 65‐GB can be used for reference dosimetry of FFF MV photon beam with proper calculation of recombination effect.

Modeling of flattening filter free photon beams with analytical and Monte Carlo TPS

Recent conventional medical linear particle accelerators (linacs) come with the capacity to deliver flattening filter free (FFF) beams, allowing higher dose rates. Intensity modulated radiation therapy (IMRT) and small-fields stereotactic techniques no longer seem to require flattened beams. To implement such beams, it is important to correctly measure and model the dose distribution they deliver. The objective of this work was to model the recent VERSA-HD (Elekta®) FFF beams using RayStation® TPS, as done externally on Monaco TPS. There exists no publication on the modeling of this machine in FFF mode. Also, no published work relates to the use of RayStation® to model unflattened beams. Flattened X6 and unflattened 6FFF and 10FFF beams have been modeled. Lateral and depth dose profiles as well as output factors were measured using ionization chambers and diodes detectors. Models were created in RayStation® and Monaco® treatment planning systems (TPS), offering analytically- and Monte Carlo-algorithms, respectively. Surface dose, out-of-field dose and collimator transmission were also measured to improve models. All steps and parameters used during the modeling process are presented, along with measured and calculated dose profiles. Agreement between measurements and models was assessed by one-dimensional gamma analysis. Stereotactic and VMAT (volumetric modulated arc therapy) treatment plans were controlled by measuring isocenter absolute dose and dose distribution with the ArcCheck device. The impacts of the model parameters on each region of the depth and lateral dose profiles were explained. FFF beams models exhibited different energy spectra, smaller source sizes and lower electron contamination compared to flattened beams, in both TPS. Monaco® and RayStation® exhibit good agreement with depth and lateral dose profile measurements, 1% and 1 mm criteria being respected in the high dose regions. Stereotactic and VMAT treatment plans measured led to good agreement with calculation.