Varian Clinac Research Articles

21. Assessing image quality of kilo-voltage cone beam CT (kV-CBCT) for adaptive radiotherapy

Introduction The scope of this study was to review and compare quality of kV-CBCT imaging highlighted from different generation systems and their respective main issues in regards with fan-beam CT (FBCT). Indeed kV-CBCT beam geometry is different from FBCT, introducing much more artefacts and leading to inaccuracies to convert Hounsfield Units (HU) into the relative electron density. This should be explored in depth because kV-CBCT is potentially the reference data for new approaches such computation of the daily actual delivered dose. Methods Different kV-CBCT acquisition modes with various tube voltage (kV) and exposure (mAs) were studied: “pelvis” and “pelvis obese” with a Varian Truebeam™ (unit 1), “pelvis” and “low dose thorax” with a Varian Clinac 2100CD (unit 2). Using the TomoTherapy® Cheese phantom (Gammex RMI, Middleton, WI) for HU to electron density (de) calibration curve, and the NEMA body water phantom for HU uniformity, noise and mean water number analysis, were investigated. From the FBCT and kV-CBCT HU differential histograms, frequency peaks representing water, air, lung, soft tissue and bone were compared. Results For unit 2, streaking and ring artifacts are much greater than for unit 1 as well as HU uniformity is deteriorated. Noise values are comparable between units but standard deviation is twice at unit 2. Peaks observed in the differential histograms are broader in kV-CBCT than those of FBCT especially for high tissue density. kV-CBCT uniformity is more degraded than in FBCT because CBCT is more prone to artefacts in images such as: streaking, cupping, or ring artifacts. Variations in calibration protocols were observed between FBCT and kV-CBCT that could explain differences for mean water number. Conclusions The two commercial kV-CBCT systems demonstrate different performances. We found that HU differential histograms are an efficient way for comparing and controlling image quality in addition to existing quality metrics.

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.

29. Dosimetric response of ferrous gels in a low energy beam produced by a mini-accelerator

Introduction The INTRABEAM® system is a miniature accelerator producing low energy photons (50 keV maximum). The published dosimetric characterization of the INTRABEAM system for flat and surface applicators [1] , [2] was based on detectors (radiochromic films or ionization chambers) not allowing measuring the absorbed dose in the first millimeters of the irradiated medium, where the dose is actually prescribed. This study aims at measuring the dose deposit produced with INTRABEAM surface applicators in the first millimeters by determining gel sensitivity with Magnetic Resonance Imaging (MRI). Methods The irradiations at different dose levels were performed with the INTRABEAM® Carl Zeiss Surgical system (Oberkochen, Germany)]. The gel used in this study is a new “sensitis” material which is described by Stien et al. [3] and V. Dedieu et al. [4] . Gel irradiation in tin containers was carried out for twelve dose levels between 2 Gy and 50 Gy at the gel surface with a 4 cm surface applicator. In order to compare gel sensitivity at low energy with high energy, gels in tins containers were irradiated by a 18 MV photon beam produced by a Varian Clinac 2100 CD. T2 weighted multi echo MRI sequences were performed with 16 echo times and TR = 2000 ms. The response in T2 was determined on Matlab (Version 7.11, MathWorks, Natrick, MA, US). Results The T2 signal versus echo times can be fitted with a mono-exponential function with 95% of confidence. The calibration curve determined from experiments with tins at low energy is a linear function (R2 = 0.964) with a sensitivity of 0.0422 s−1 Gy−1. It is of the same order of magnitude as the gel sensitivity found from experiments at high energy (0.0391 s−1 Gy−1 with R2 = 0.924). The calibration curve at low energy was used to obtain Isodose maps from images (Fig. 1). The PDD (Percent Depth Dose) determined in gel is similar to the one measured with an ionization chamber. Conclusions The dosimetric response of ferrous gels in a low energy beam produced by a mini -accelerator was determined with MRI measurements. It allowed measuring the dose distribution in particular in the first millimeters for an irradiation with the INTRABEAM® miniature accelerator equipped with a surface applicator. These results must be confirmed for the other diameters of surface applicators and for flat applicators as well.

2. Entropic closure for MRI-guided radiotherapy

Introduction The majority of patients affected by cancer are nowadays treated by radiotherapy, which consists in delivering a homogeneous dose with energetic particles. Magnetic Resonance Imaging (MRI)-guided radiotherapy shows precisely the tumor position in real time and allows to direct the photon beams to the tumor while sparing organs at risks. Our work consists in the validation of a new model developped to simulate the energy deposition of the particles used in radiotherapy (electrons, photons and protons) in the presence of strong magnetic field, within human tissues, and its adaptation to this new technology. Material and methods This model is based on a kinetic entropic closure of the linearized Boltzmann equation [1] , which describes the transport of energetic particles in the matter. This equation takes a lot of computation time to be solved due to the high number of variables. To simplify this, we replace fluences by angular moments, which allows getting rid of the angular variables and improve the calculation time. We obtain a set of angular moments equations, and we close this set using the Boltzmann’s principle of entropy maximization on the two first equations of the set. This model has an accuracy comparable to references Monte-Carlo (MC) codes [2] (Geant4, MCNPx, Penelope), and is less time-consuming than these ones. We added the Lorentz force into our code to study the influence of a magnetic field on the dose deposition, and compared it to the reference full MC code FLUKA. To further validate our model, we have carried out an experimental campaign in which we irradiated a composition of materials of different mass densities, with 6 and 18 MV photon beams of a Varian Clinac 21Ex accelerator, without and with the influence of a 0.87 T magnetic field of amplitude induced by a permanent magnet. Results We show a good agreement between our model and the FLUKA code ( Fig. 1 ). We highlight the effects that occur on the propagation of particles in the matter in presence of a magnetic field of a few Tesla. Indeed, it modifies the dose deposition by shifting the deposition of energy, leading to an increased diffusion depending on the orientation of the magnetic field. Moreover, it also leads to an increase or decrease of the dose deposited at the interfaces between materials depending on their difference between mass densities. The results of our experiments lead to the same conclusions. These effects have to be taken into account in order to prevent creation of hot spots or a spread of energy distribution in a human body, within computation times compatible with the clinical environment. Conclusion Our model is a good candidate for future Treatment Planning Systems since it allows a faster and efficient way to plan a viable treatment for a patient in presence of strong magnetic fields. A magnetic field modifies the profile of the dose deposition such as it has to be taken into account in calculations to prevent the dramatic changes which occur at interfaces between media of different density. This work takes place in the framework of POPRA (Programme Optique Physique et Radiotherapie en Aquitaine), which involves several laboratories around problematics on the topic of cancer treatment.

Determination of small field synthetic single-crystal diamond detector correction factors for CyberKnife, Leksell Gamma Knife Perfexion and linear accelerator

PurposeThe aim of this study was to determine small field correction factors for a synthetic single-crystal diamond detector (PTW microDiamond) for routine use in clinical dosimetric measurements. Materials and methodsCorrection factors following small field Alfonso formalism were calculated by comparison of PTW microDiamond measured ratio MQclinfclin/MQmsrfmsr with Monte Carlo (MC) based field output factors ΩQclin,Qmsrfclin,fmsr determined using Dosimetry Diode E or with MC simulation itself. Diode measurements were used for the CyberKnife and Varian Clinac 2100C/D linear accelerator. PTW microDiamond correction factors for Leksell Gamma Knife (LGK) were derived using MC simulated reference values from the manufacturer. ResultsPTW microDiamond correction factors for CyberKnife field sizes 25–5 mm were mostly smaller than 1% (except for 2.9% for 5 mm Iris field and 1.4% for 7.5 mm fixed cone field). The correction of 0.1% and 2.0% for 8 mm and 4 mm collimators, respectively, needed to be applied to PTW microDiamond measurements for LGK Perfexion. Finally, PTW microDiamond MQclinfclin/MQmsrfmsr for the linear accelerator varied from MC corrected Dosimetry Diode data by less than 0.5% (except for 1 × 1 cm2 field size with 1.3% deviation). ConclusionsRegarding low resulting correction factor values, the PTW microDiamond detector may be considered an almost ideal tool for relative small field dosimetry in a large variety of stereotactic and radiosurgery treatment devices.

Energetic properties’ investigation of removing flattening filter at phantom surface: Monte Carlo study using BEAMnrc code, DOSXYZnrc code and BEAMDP code

The Monte Carlo calculation method is considered to be the most accurate method for dose calculation in radiotherapy and beam characterization investigation, in this study, the Varian Clinac 2100 medical linear accelerator with and without flattening filter (FF) was modelled. The objective of this study was to determine flattening filter impact on particles’ energy properties at phantom surface in terms of energy fluence, mean energy, and energy fluence distribution. The Monte Carlo codes used in this study were BEAMnrc code for simulating linac head, DOSXYZnrc code for simulating the absorbed dose in a water phantom, and BEAMDP for extracting energy properties. Field size was 10 × 10 cm2, simulated photon beam energy was 6 MV and SSD was 100 cm. The Monte Carlo geometry was validated by a gamma index acceptance rate of 99% in PDD and 98% in dose profiles, gamma criteria was 3% for dose difference and 3mm for distance to agreement. In without-FF, the energetic properties was as following: electron contribution was increased by more than 300% in energy fluence, almost 14% in mean energy and 1900% in energy fluence distribution, however, photon contribution was increased 50% in energy fluence, and almost 18% in mean energy and almost 35% in energy fluence distribution. The removing flattening filter promotes the increasing of electron contamination energy versus photon energy; our study can contribute in the evolution of removing flattening filter configuration in future linac.

Photon beam softening coefficient determination with slab thickness in small filed size: Monte Carlo study

Determination and understanding of photon beam softening using material soften photon beam for clinical usage is important for material study for attenuation and for beam modifier enhancements and linac improvements. Monte Carlo model was used to simulate 6 MeV photon beams produced by Varian Clinac 2100 accelerator with flattening filter thereafter the flattening filter was replaced by a slab of aluminum and copper with different of 0.5, 1, 1.5 and 2 mm. The Monte Carlo geometry was validated by a gamma index acceptance rate of 99% in PDD and 98% in dose profiles, the gamma criteria was 3% for dose difference and 3mm for distance to agreement. The purpose was to investigate the beam softening for small size and beam attenuation as a function of inserted slab thickness of copper and aluminum and also as a function of off-axis distance. For beam softening evaluation, variation amplitude of beam softening coefficient a1 was very high near the beam central axis and decreased with off-axis distance and also it was high for aluminum slab compared to copper slab. For aluminum slab, variation amplitude of beam softening coefficient a1 have a minimum at–0.5 cm–1 and a maximum at 0.5 cm–1 and for copper slab, variation amplitude of beam softening a1 have a minimum at–0.15 cm–1 and a maximum at 0.11 cm–1. Variation amplitude of beam softening coefficient a2 was very high near the beam central axis and decreased with off-axis distance and it was very high for aluminum slab compared to copper slab. For aluminum slab, variation amplitude of beam softening coefficient a2 have a minimum at–0.54 cm–2 and a maximum at 0.44 cm–2 and for copper slab, variation amplitude of beam softening coefficient a2 have a minimum at–0.111 cm–2 and a maximum at 0.0825 cm–2.

Dosimetry Investigation and Evaluation for Removing Flattening Filter Configuration of Linac: Monte Carlo Study

The objective of this study is to build a Monte Carlo geometry of Varian Clinac 2100 linear accelerator as realistically as possible and then to investigate the removing of flattening filter impact on dosimetry for a high radiotherapy efficiency. Monte Carlo codes used in this work were BEAMnrc code to simulate photons beam and DOSXYZnrc code to examinate absorbed dose in water phantom. PDDs and beam profiles were calculated for 6 × 6 cm2 and 10 × 10 cm2 field sizes. Good agreement was found between calculated PDD and beam profile compared to measurements. Gamma index acceptance rate was more than 98% of both distribution comparisons PDDs and dose profiles and our results were more developed and accurate. Varian Clinac 2100 linear accelerator was accurately modeled using Monte Carlo codes: BEAMnrc, DOSXYZnrc and BEAMDP codes package. Varian Clinac 2100 with removing flattening filer could increase the dose by a gain was approximately 80% for 6 × 6 cm2 field size and it was approximately 110% for 10 × 10 cm2 at the build-up dose region but for all depth in water phantom, the dose of without FF configuration of linac was increased by more than 40% of dose of with FF configuration of linac.

Evaluation of plan quality between two treatment planning systems for volumetric modulated arc therapy

Objective To investigate the plan quality between two treatment planning systems (TPSs) for volumetric modulated arc therapy (VMAT). Methods VMAT plans based on Varian Eclipse and Philips Pinnacle TPS were designed for 10 cervical cancer patients (2, 3 Arcs) and 10 prostate cancer patients (1, 2 Arcs). The delivery system of VMAT was Varian Clinac®iX machine, and RapidArc was used. The treatment plan quality between the two TPSs was evaluated based on dose distribution, delivery efficiency, and parameter settings. The differences between the two TPSs were compared using paired t-test. Results For cervical cancer patients, the 2-Arc VMAT plans based on the Pinnacle was slightly better than those based on the Eclipse in terms of the conformal index (CI) of planning target volume (PTV), rectum V30 and V40, and bladder V30 and V40, and the homogeneity index (HI) of PTV and PTV1 as well as CI of PTV1 in the Eclips were slightly better than those in the Pinnacle(P 0.05). The number of monitor units with 2-Arcs and 3-Arcs plans of the Eclipse was significantly smaller than those in the Pinnacle (P 0.05). Conclusions For patients with cervical cancer and prostate cancer, the VMAT plans based on Varian Eclipse and Philips Pinnacle TPS can achieve a clinically acceptable dose distribution and show a little difference in the treatment plan quality. However, we will still need more cases to further study and determine the performance characteristics of the commercial TPSs for optimizing VMAT. Key words: Cervical cancer/radiotherapy; Prostate cancer/radiotherapy; Volumetric modulated arc treatment; Plan quality

Abstract ID: 65 Dosimetry for treatment of retinoblastoma with external photon beams

Retinoblastoma is the most common intraocular malignancy in the early childhood. Patients treated with external beam radiotherapy respond very well to the treatment. The University Hospital of Essen has successfully treated these patients on a daily basis during nearly 30 years using a dedicated “D”-shaped collimator inserted in the head of a Varian Clinac 2100 C/D operating at 6 MV. The collimator conforms a “D”-shaped off-axis field whose irradiated area can be either 5.2 or 3.1 cm2. In this work, a dosimetric analysis of the technique has been carried out by using the Monte Carlo (MC) code PENELOPE. Experimental depth dose distributions and lateral profiles were compared with those obtained with Monte Carlo and with the analytical anisotropic algorithm (AAA). PENELOPE simulations agree well with the experimental data with differences in the dose profiles less than 3 mm/3%. Discrepancies between AAA and the experiment reach 3 mm/6%. [1] Absolute dosimetry has been estimated for the collimator with two MC approaches in which the backscattered dose in the monitor chamber is determined [2] . Results are compared to experiment and to those provided by AAA. The MC results for the absolute doses differ from the experimental ones by 2.6% and 1.7%, depending on the tracking parameter set used for the electron transport in the target of the linac head. For the studied radiation fields, the two MC approaches produce absolute doses that are statistically compatible. AAA underestimates the experimental absolute doses with discrepancies larger than those found for MC results. A new “D”-shaped optimized collimator has been proposed [3] . It permits to reduce the risk of radio-induced secondary malignancies and may be easily built up. The new collimator delivers a dose distribution that is 0.3 cm narrower than that obtained with the current collimator, maintaining the other relevant dosimetric characteristics (depth doses at clinically relevant positions, penumbrae width, and shape of the lateral profiles). This allows using the clinical experience gained in the past.

Validation of Monte Carlo simulation of 6 MV photon beam produced by Varian Clinac 2100 linear accelerator using BEAMnrc code and DOSXYZnrc code

The Monte Carlo model for the photon-beam output from the Varian Clinac 2100 linear accelerator was validated to compare the calculated to measured PDD and beam dose profiles The Monte Carlo calculation method is considered to be the most accurate method for dose calculation in radiotherapy. The objective of this study is to build a Monte Carlo geometry of Varian Clinac 2100 linear accelerator as realistically as possible. The Monte Carlo codes used in this work were the BEAMnrc code to simulate the photons beam and the DOSXYZnrc code to examinate the absorbed dose in the water phantom. We have calculated percentage depth dose (PDD) and beam profiles of the 6 MV photon beam for the 6 × 6 cm2, 10 × 10 cm2 and 15 × 15 cm2 field sizes. We have used the gamma index technique for the quantitative evaluation to compare the measured and calculated distributions. Good agreement was found between calculated PDD and beam profile compared to measured data. The comparison was evaluated using the gamma index method and the criterions were 3% for dose difference and 3 mm for distance to agreement. The gamma index acceptance rate was more than 97% of both distribution comparisons PDDs and dose profiles and our results were more developed and accurate. The Varian Clinac 2100 linear accelerator was accurately modeled using Monte Carlo codes: BEAMnrc and DOSXYZnrc codes package.

Clinical implementation and evaluation of the Acuros dose calculation algorithm

PurposeThe main aim of this study is to validate the Acuros XB dose calculation algorithm for a Varian Clinac iX linac in our clinics, and subsequently compare it with the wildely used AAA algorithm.Methods and materialsThe source models for both Acuros XB and AAA were configured by importing the same measured beam data into Eclipse treatment planning system. Both algorithms were validated by comparing calculated dose with measured dose on a homogeneous water phantom for field sizes ranging from 6 cm × 6 cm to 40 cm × 40 cm. Central axis and off‐axis points with different depths were chosen for the comparison. In addition, the accuracy of Acuros was evaluated for wedge fields with wedge angles from 15 to 60°. Similarly, variable field sizes for an inhomogeneous phantom were chosen to validate the Acuros algorithm. In addition, doses calculated by Acuros and AAA at the center of lung equivalent tissue from three different VMAT plans were compared to the ion chamber measured doses in QUASAR phantom, and the calculated dose distributions by the two algorithms and their differences on patients were compared. Computation time on VMAT plans was also evaluated for Acuros and AAA. Differences between dose‐to‐water (calculated by AAA and Acuros XB) and dose‐to‐medium (calculated by Acuros XB) on patient plans were compared and evaluated.ResultsFor open 6 MV photon beams on the homogeneous water phantom, both Acuros XB and AAA calculations were within 1% of measurements. For 23 MV photon beams, the calculated doses were within 1.5% of measured doses for Acuros XB and 2% for AAA. Testing on the inhomogeneous phantom demonstrated that AAA overestimated doses by up to 8.96% at a point close to lung/solid water interface, while Acuros XB reduced that to 1.64%. The test on QUASAR phantom showed that Acuros achieved better agreement in lung equivalent tissue while AAA underestimated dose for all VMAT plans by up to 2.7%. Acuros XB computation time was about three times faster than AAA for VMAT plans, and computation time for other plans will be discussed at the end. Maximum difference between dose calculated by AAA and dose‐to‐medium by Acuros XB (Acuros_Dm,m) was 4.3% on patient plans at the isocenter, and maximum difference between D100 calculated by AAA and by Acuros_Dm,m was 11.3%. When calculating the maximum dose to spinal cord on patient plans, differences between dose calculated by AAA and Acuros_Dm,m were more than 3%.ConclusionCompared with AAA, Acuros XB improves accuracy in the presence of inhomogeneity, and also significantly reduces computation time for VMAT plans. Dose differences between AAA and Acuros_Dw,m were generally less than the dose differences between AAA and Acuros_Dm,m. Clinical practitioners should consider making Acuros XB available in clinics, however, further investigation and clarification is needed about which dose reporting mode (dose‐to‐water or dose‐to‐medium) should be used in clinics.

Computation of the electron beam quality [formula omitted] factors for the NE2571, NE2571A and NE2581A thimble ionization chambers using PENELOPE

The quality correction factor kQ,Q0 for electron beams was calculated for three thimble ionization chambers, namely, NE2571, NE2571A and NE2581A. The Monte Carlo code PENELOPE was used to estimate the overall correction factor fc,Q of these chambers for electron beams with nominal energies ranging between 6 and 22MeV, corresponding to a Varian Clinac 2100 C/D. A 60Co beam was used as reference quality Q0. Also eight monoenergetic electron beams reproducing the quality index R50 of the Clinac beams were considered. The kQ,Q0 factors were calculated as the ratio between fc,Q and fc,Q0. Those obtained for the NE2571 ionization chamber show a nice agreement with those calculated by Muir and Rogers with EGSnrc. As it occurred to other ionization chambers analyzed in previous works, the kQ,Q0 factors found for the monoenergetic beams are larger (smaller) than those corresponding to the Clinac beams at low (high) R50 values, the differences being slightly above 0.5%. Finally, the kQ,Q0 factors obtained in the case of the NE2571A chamber are systematically ∼0.5% below those of its predecessor chamber, the NE2571.

Photon beam softening coefficients evaluation for a 6 MeV photon beam for an aluminum slab: Monte Carlo study using BEAMnrc Code, DOSXYZnrc Code, and BEAMDP code

Determination and understanding the photon beam attenuation by the photon beam modifier and the radiation beam softening for clinical use is more important part of material study for the beam modifier enhancements and the linac improvements. A Monte Carlo model was used to simulate 6 MeV photon beams from a Varian Clinac 2100 accelerator with the flattening filter and the later was replaced by the aluminum slab with variable thickness. The Monte Carlo geometry was validated by a gamma index acceptance rate of 99% in PDD and 98% in dose profiles, the gamma criteria was 3% for dose difference and 3 mm for distance to agreement. The purpose was to investigate aluminum material attenuation and beam softening coefficients as a function of the inserted aluminum slab thickness and of off-axis distance. The attenuation and beam softening coefficients were not identical for the same off-axis distance and they varied as a function of aluminum slab thickness. The results of our study were shown that the beam softening coefficients were varied with thickness beam modifier material used for beam softening and the off-axis distance inside the irradiation field. Thereafter, the softening coefficient a 1 have a maximum of 2.5 × 10–1 cm–1 for the aluminum slab thickness of 1 mm, 1.4 × 10–1 cm–1 for the aluminum slab thickness of 1.5 mm and 4.47 × 10–2 cm–1 for the aluminum slab thickness of 2 mm. The maximum of the second softening coefficient a 2 was 1.02 × 10–2 cm–2 for the aluminum slab thickness of 1 mm, was 1.92 × 10–2 cm–2 for the aluminum slab thickness of 1.5 mm and was 1.93 × 10–2 cm–2 for the aluminum slab thickness of 2 mm. Our study can be a basic investigation of photon beam softening material that will be used in the future linac configuration and also in the photon beam modifiers.

Study of 6 MV Photon Beam Dose Profile Peaks, Investigation and Evaluation of the Backscattering Effects on the Beam Dose Profiles

The dose profile peaks are mainly produced by the flattening filter of the head of the medical linear accelerator due to the backscattering effects on the beam dose profiles; in this study we have investigated the backscattering effects on the beam profile peaks; the purpose was to evaluate the backscattering effects at the beam edge due to the flattening filter shape by calculating the increasing relative dose on the beam profile peaks as functions of depth along the central axis and filed size. The accent is on the photon beam dose profiles at different depths in a water phantom along central beam axis for a set of irradiation field sizes. The measures of dose profiles of the 6 MV photon beam in a water phantom, source to surface distance is 100 cm, for the set of the irradiation field sizes of 3×3 cm², 6×6 cm², 10×10 cm², 15×15 cm², and 20×20 cm² at different depths on the beam central axis, the linear accelerator is VARIAN Clinac 2100. First, we have plotted the dose profiles as function of the normalized depth to show the beam profile peaks; then we have calculated the increasing relative dose on the beam profile peaks as a function of the field size and depth along central beam axis from the measurements data. Then we have normalized the increasing relative dose on the beam profile peaks to the increasing relative dose on the beam profile peak of the standard filed size of 10×10 cm². We have plotted the results for interpretation and evaluation the backscattering effects on the beam dose profiles; the pervious study have shown that the flattening filter produced a flat profile at a depth of 10 cm and beam profile peaks; and in our study we have quantified the increasing relative dose on the beam profile peaks; then, we have shown that the variation of the normalized increasing relative dose presented the peaks at depth of 10 cm along the central beam axis, e.g. We have found that the increasing relative dose at depth of 10 cm along central beam axis for the field size of 25×25 cm² is almost 20 times of the standard field size at the same depth, but the increasing relative dose is very close for both filed sizes at a depth of 20 cm along central beam axis. Throughout this work we have use a method to evaluate the backscattering effects on the dose profiles; we have found the backscattering effects are very high at a depth 10 cm for the large irradiation field size.

Comparison of Flattening Filter (FF) and Flattening-Filter-Free (FFF) 6 MV photon beam characteristics for small field dosimetry using EGSnrc Monte Carlo code

The present study is focused to compare the characteristics of Varian Clinac 600C/D flattened and unflattened 6MV photon beams for small field dosimetry using EGSnrc Monte Carlo Simulation since the small field dosimetry is considered to be the most crucial and provoking task in the field of radiation dosimetry. A 6MV photon beam of a Varian Clinac 600C/D medical linear accelerator operates with Flattening Filter (FF) and Flattening-Filter-Free (FFF) mode for small field dosimetry were performed using EGSnrc Monte Carlo user codes (BEAMnrc and DOSXYZnrc) in order to calculate the beam characteristics using Educated-trial and error method. These includes: Percentage depth dose, lateral beam profile, dose rate delivery, photon energy spectra, photon beam uniformity, out-of-field dose, surface dose, penumbral dose and output factor for small field dosimetry (0.5×0.5cm2 to 4×4cm2) and are compared with magna-field sizes (5×5cm2 to 40×40cm2) at various depths. The results obtained showed that the optimized beam energy and Full-width-half maximum value for small field dosimetry and magna-field dosimetry was found to be 5.7MeV and 0.13cm for both FF and FFF beams. The depth of dose maxima for small field size deviates minimally for both FF and FFF beams similar to magna-fields. The depths greater than dmax depicts a steeper dose fall off in the exponential region for FFF beams comparing FF beams where its deviations gets increased with the increase in field size. The shape of the lateral beam profiles of FF and FFF beams varies remains similar for the small field sizes less than 4×4cm2 whereas it varies in the case of magna-fields. Dose rate delivery for FFF beams shows an eminent increase with a two-fold factor for both small field dosimetry and magna-field sizes. The surface dose measurements of FFF beams for small field size were found to be higher whereas it gets lower for magna-fields than FF beam. The amount of out-of-field dose reduction gets increased with the increase in field size. It is also observed that the photon energy spectrum gets increased with the increase in field size for FFF beam mode. Finally, the output factors for FFF beams were relatively quite low for small field sizes than FF beams whereas it gets higher for magna-field sizes. From this study, it is concluded that the FFF beams depicted minimal deviations in the treatment field region irrespective to the normal tissue region for small field dosimetry compared to FF beams. The more prominent result observed from the study is that the shape of the beam profile remains similar for FF and FFF beams in the case of smaller field size that leads to more accurate treatment planning in the case of IMRT (Image-Guided Radiation Therapy), IGAT (Image-Guided Adaptive Radiation Therapy), SBRT (Stereotactic Body Radiation Therapy), SRS (Stereotactic Radio Surgery), and Tomotherapy techniques where homogeneous dose is not necessary. On the whole, the determination of dosimetric beam characteristics of Varian linac machine using Monte Carlo simulation provides accurate dose calculation as the clinical golden data.

Ion recombination and polarity corrections for small-volume ionization chambers in high-dose-rate, flattening-filter-free pulsed photon beams.

To investigate ion recombination and polarity effects in scanning and microionization chambers when used with digital electrometers and high-dose-rate linac beams such as flattening-filter-free (FFF) fields, and to compare results against conventional pulsed and continuous photon beams. Saturation curves were obtained for one Farmer-type ionization chamber and eight small-volume chamber models with volumes ranging from 0.01 to 0.13 cm3 using a Varian TrueBeam™ STx with FFF capability. Three beam modes (6 MV, 6 MV FFF, and 10 MV FFF) were investigated, with nominal dose-per-pulse values of 0.0278, 0.0648, and 0.111 cGy/pulse, respectively, at dmax . Saturation curves obtained using the Theratronics T1000 60 Co unit at the UWADCL and a conventional linear accelerator (Varian Clinac iX) were used to establish baseline behavior. Jaffé plots were fitted to obtain Pion , accounting for exponential effects such as charge multiplication. These values were compared with the two-voltage technique recommended in TG-51, and were plotted as a function of dose-per-pulse to assess the ability of small-volume chambers to meet reference-class criteria in FFF beams. Jaffé- and two-voltage-determined Pion values measured for high-dose-rate beams agreed within 0.1% for the Farmer-type chamber and 1% for scanning and microionization chambers, with the exception of the CC01 which agreed within 2%. With respect to ion recombination and polarity effects, the Farmer-type chamber, scanning chambers and the Exradin A26 microchamber exhibited reference-class behavior in all beams investigated, with the exception of the IBA CC04 scanning chamber, which had an initial recombination correction that varied by 0.2% with polarity. All microchambers investigated, with the exception of the A26, exhibited anomalous polarity and ion recombination behaviors that make them unsuitable for reference dosimetry in conventional and high-dose-rate photon beams. The results of this work demonstrate that recombination and polarity behaviors seen in conventional pulsed and continuous photon beams trend accordingly in high-dose-rate FFF linac beams. Several models of small-volume ionization chambers used with a digital electrometer have been shown to meet reference-class requirements with respect to ion recombination and polarity, even in the high-dose-rate environment. For such chambers, a two-voltage technique agreed well with more rigorous methods of determining Pion . However, the results emphasize the need for careful reference detector selection, and indicate that ionization chambers ought to be extensively tested in each beam of interest prior to their use for reference dosimetry.

A Monte Carlo model for photoneutron generation by a medical LINAC

For an optimal tuning of the radiation protection planning, a Monte Carlo model using the MCNPX code has been built, allowing an accurate estimate of the spectrometric and geometrical characteristics of photoneutrons generated by a Varian TrueBeam Stx© medical linear accelerator. We considered in our study a device working at the reference energy for clinical applications of 15MV, stemmed from a Varian Clinac©2100 modeled starting from data collected thanks to several papers available in the literature. The model results were compared with neutron and photon dose measurements inside and outside the bunker hosting the accelerator obtaining a complete dose map. Normalized neutron fluences were tallied in different positions at the patient plane and at different depths. A sensitivity analysis with respect to the flattening filter material were performed to enlighten aspects that could influence the photoneutron production.

A portal dosimetry dose prediction method based on collapsed cone algorithm using the clinical beam model.

Amorphous silicon electronical portal imaging devices (EPIDs) are widely used for dosimetric measurements in Radiation Therapy. The purpose of this work was to determine if a portal dose prediction method can be utilized for dose map calculations based on the linear accelerator model within a commercial treatment planning system (Pinnacle3 v8.0m). The method was developed for a 6MV photon beam on the Varian Clinac 21-EX, at a nominal dose rate of 400MU/min. The Varian aS1000 EPID was unmounted from the linear accelerator and scanned to acquire CT images of the EPID. The CT images were imported into Pinnacle3 and were used as a quality assurance phantom to calculate dose on the EPID setup at a source to detector distance of 105cm. The best match of the dose distributions was obtained considering the image plane located at 106cm from the source to detector plane. The EPID was calibrated according to the manufacturer procedure and corrections were made for output factors. Arm-backscattering effect, based on profile correction curves, has been introduced. Five low-modulated and three high-modulated clinical planned treatments were predicted and measured with the method presented here and with MatriXX (IBA Dosimetry, Schwarzenbruck, Germany). A portal dose prediction method based on Pinnacle3 was developed without modifying the commissioned parameters of the model in use in the clinic. CT images of the EPID were acquired and used as a quality assurance phantom. The CT images indicated a mean density of 1.16g/cm3 for the sensitive area of the EPID. Output factor measured with the EPID were lower for small fields and larger for larger fields (beyond 10×10cm2 ). Arm-backscatter correction showed a better agreement at the target side of the EPID. Analysis of Gamma index comparison (3%, 3mm) indicated a minimum of 97.4% pass rate for low modulated and 98.3% for high modulated treatments. Pass rates were similar for MatriXX measurements. The method developed here can be easily implemented into clinic, as neither additional modeling of the clinical energy nor an independent image prediction algorithm are necessary. The main advantage of this method is that portal dose prediction is calculated with the same algorithm and beam model used for patient dose distribution calculation. This method was independently validated with an ionization chamber matrix.

Dosimetric Validation of Volumetric Modulated Arc Therapy (VMAT) Using AAPM TG-119 Benchmark Plans in an Upgraded CLINAC 2100CD for Flattening Filter Free (FFF) Photon Beams

Background:Recently we have upgraded our Varian Clinac 2100CD with a 6MV FFF beam, this upgrade being the first of its kind in our country. Even though the dosimetric characteristics of FFF beams have been reported both in experimental and Monte Carlo studies, application in planning and delivery is complex. The aim of this study was to validate the commissioning of upgraded FFF beams dosimetrically using AAPM TG-119 bench mark plans for VMAT and to make a comparison with IMRT plans for both flattened filtered and FFF beams.Materials and Methods:AAPM TG-119 proposes a set of test clinical cases for testing the accuracy of IMRT planning and delivery systems. For these clinical cases we generated four treatment plans using IMRT FF, IMRT FFF, VMAT FF and VMAT FFF on a Varian Clinac 2100CD machine equipped with a millennium 120 MLC in Eclipse treatment planning system. Dose prescription and planning objectives were set according to the TG-119 goals and plans were scored based on planning objectives. Plans were compared using dose coverage, the conformity index and the homogeneity index. Point doses were measured at points recommended by TG-119 using a CC13 ion chamber. Planar dosimetry was accomplished using Imatrix and gamma evaluation was conducted using Omnipro IMRT software.Results:Dose distributions of FFF beam based plans were comparable to FF plans for both IMRT and VMAT. Our planning results matched TG-119 planning results. Measured point doses were within ±2% of planned doses and planar dosimetry gamma values were <1 for >95% of data points for all plans.Conclusion:We found a reduction of 40% treatment time for FFF against FF beams for sliding window IMRT. Upgraded FFF beams were in good agreement with TG-119 benchmark plans and goals.