PDD Curves Research Articles

TOPAS simulation of photoneutrons in radiotherapy: accuracy and speed with variance reduction

Objective. We provide optimal particle split numbers for speeding up TOPAS Monte Carlo simulations of linear accelerator (linac) treatment heads while maintaining accuracy. In addition, we provide a new TOPAS physics module for simulating photoneutron production and transport. Approach. TOPAS simulation of a Siemens Oncor linac was used to determine the optimal number of splits for directional bremsstrahlung splitting as a function of the field size for 6 MV and 18 MV x-ray beams. The linac simulation was validated against published data of lateral dose profiles and percentage depth-dose curves (PDD) for the largest square field (40 cm side). In separate simulations, neutron particle split and the custom TOPAS physics module was used to generate and transport photoneutrons, called ‘TsPhotoNeutron’. Verification of accuracy was performed by comparing simulations with published measurements of: (1) neutron yields as a function of beam energy for thick targets of Al, Cu, Ta, W, Pb and concrete; and (2) photoneutron energy spectrum at 40 cm laterally from the isocenter of the Oncor linac from an 18 MV beam with closed jaws and MLC. Main results. The optimal number of splits obtained for directional bremsstrahlung splitting enhanced the computational efficiency by two orders of magnitude. The efficiency decreased with increasing beam energy and field size. Calculated lateral profiles in the central region agreed within 1 mm/2% from measured data, PDD curves within 1 mm/1%. For the TOPAS physics module, at a split number of 146, the efficiency of computing photoneutron yields was enhanced by a factor of 27.6, whereas it improved the accuracy over existing Geant4 physics modules. Significance. This work provides simulation parameters and a new TOPAS physics module to improve the efficiency and accuracy of TOPAS simulations that involve photonuclear processes occurring in high-Z materials found in linac components, patient devices, and treatment rooms, as well as to explore new therapeutic modalities such as very-high energy electron therapy.

Characterization of the radiation beam of a tomotherapy equipment with MCNP

This work aims to model and characterize the radiation beam of one Accuray tomotherapy equipment using the Monte Carlo Code MCNP5 (Monte Carlo N-Particle). This tomotherapy equipment is used for delivering high doses of radiation in tumor regions to kill cancer cells and shrink the tumor during radiation therapy of cancer patients, however, the radiation can damage surrounding areas and nearby organs at risk (OAR) if the radiation field is not well delimited. In particular, intensity-modulated radiotherapy treatments (IMRT) with tomotherapy equipment offer great benefits to patients allowing treatment of tumor regions without affecting surrounding areas and OAR. Nowadays, it is well known that a correct simulation of transport of radiation in tomotherapy equipment facilitates considerably the estimation of ideal doses in the tumor, surrounding regions, and OAR. For that reason, in this work, we simulated the geometry of the 6 MV ACCURAY Tomotherapy equipment of the CECAN using the MCNP5. The model includes a TomoLINAC consisting of an electron source that emits Gaussian distribution particles with an average energy of 5.7 MeV and width of 0.3 MeV. The emitted particles impact the tungsten target and pass through primary collimators and jaws that define the irradiation field in the isocenter. To validate the geometry and radiation transport in the TomoLINAC the curves of depth dose percentage (PDD) estimated by simulation and the curves measured experimentally were tuned. In the same way, the simulated transverse and longitudinal profiles were compared with the experimental results. In addition, a comparison between the qualities of the radiation beam characterized with MCNP and measured experimentally in CECAN showed a deviation of 1%. For the simulations, cylindrical detectors located inside a water phantom were considered and it was employed the tally *F8. A good agreement was observed between the PDD's curves obtained from the simulation and those measured experimentally for a field of 5 × 10 cm2 in the isocenter and SSD (distance from the source to the surface) of 85 cm. Also, the comparison between the simulated and experimental transverse profiles obtained at 1.5 cm, 10 cm and 15 cm depth with a radiation field of 5 × 40 cm2 showed very good agreement. The longitudinal profiles were estimated with the same depths as the transverse ones, but for each of them, the openings of the jaws were 5.0 cm, 2.5 cm and 1.0 cm in the longitudinal direction, which corresponds to the direction in which the patient's table moves. The comparison between the simulated and experimental longitudinal profiles showed good concordance too. Once the radiation beam of the ACCURAY tomotherapy equipment had been characterized, experimental dose measurements were made using a Cheese phantom and two A1SL ionization chambers. These results obtained experimentally were compared with those estimated with MCNP for a field of 5 × 40 cm2 at the isocenter and SAD of 85 cm and, it was concluded that both results were similar considering the regions of uncertainty. Finally, we must highlight that the modeling and characterization of the radiation beam of CECAN's ACCURAY tomotherapy equipment can be a key tool for dose estimations in different cancer treatment plans and future research.

Radiological and Dosimetric Evaluation of Biomaterial Composite Phantoms with High Energy Photons and Electrons.

The current study was undertaken to investigate the radiological and dosimetric parameters of natural product-based composite (SPI/NaOH/IA-PAE/ Rhizophora spp .) phantoms. The radiological properties of the phantoms were measured at different gamma energies from Compton scatter of photons through angles of 0, 30, 45, 60, 75, and 90 degrees. Ionization chamber (IC) and Gafchromic EBT3 film dosimeters were employed to evaluate the dosimetric characteristics for photons (6-10 MV) and electrons (6-15 MeV). Radiological property results of the composite phantoms were consistent with good quality compared to those of solid water phantoms and theoretical values of water. Photon beam quality index of the SPI15 phantom with p-values of 0.071 and 0.073 exhibited insignificant changes. In addition, good agreement was found between PDD curves measured with IC and Gafchromic EBT3 film for both photons and electrons. The computed therapeutic and half-value depth ranges matched within the limits and are similar to those of water and solid water phantoms. Therefore, the radiological and dosimetric parameters of the studied composite phantom permit its use in the selection of convenient tissue- and water-equivalent phantom material for medical applications.

Performance of the HYPERSCINT scintillation dosimetry research platform for the 1.5 T MR-linac

Objective. Adaptive radiotherapy techniques available on the MR-linac, such as daily plan adaptation, gating, and dynamic tracking, require versatile dosimetric detectors to validate end-to-end workflows. Plastic scintillator detectors (PSDs) offer great potential with features including: water equivalency, MRI-compatibility, and time-resolved dose measurements. Here, we characterize the performance of the HYPERSCINT RP-200 PSD (MedScint, Quebec, CA) in a 1.5 T MR-linac, and we demonstrate its suitability for dosimetry, including in a moving target. Approach. Standard techniques of detector testing were performed using a Beamscan water tank (PTW, Freiburg, DE) and compared to microDiamond (PTW, Freiburg, DE) readings. Orientation dependency was tested using the same phantom. An RW3 solid water phantom was used to evaluate detector consistency, dose linearity, and dose rate dependence. To determine the sensitivity to motion and to MRI scanning, the Quasar MRI4D phantom (Modus, London, ON) was used statically or with sinusoidal motion (A = 10 mm, T = 4 s) to compare PSD and Semiflex ionization chamber (PTW, Freiburg, DE) readings. Conformal beams from gantry 0° and 90° were used as well as a 15-beam 8 × 7.5 Gy lung IMRT plan. Main results. Measured profiles, PDD curves and field-size dependence were consistent with the microDiamond readings with differences well within our clinical tolerances. The angular dependence gave variations up to 0.8% when not irradiating directly from behind the scintillation point. Experiments revealed excellent detector consistency between repeated measurements (SD = 0.06%), near-perfect dose linearity (R 2 = 1) and a dose rate dependence <0.3%. Dosimetric effects of MRI scanning (≤0.3%) and motion (≤1.3%) were minimal. Measurements were consistent with the Semiflex (differences ≤1%), and with the treatment planning system with differences of 0.8% and 0.4%, with and without motion. Significance. This study demonstrates the suitability of the HYPERSCINT PSD for accurate time-resolved dosimetry measurements in the 1.5 T MR-linac, including during MR scanning and target motion.

Dosimetric Characterization of DSF/NaOH/IA-PAE/R. spp. Phantom Material for Radiation Therapy

Background: Different compositions of DSF/NaOH/IA-PAE/R. spp. composite particleboard phantoms were constructed. Methods: Photon attenuation characteristics were ascertained using gamma rays from 137Cs and 60Co. Absorbed doses at the location of an ionization chamber and Gafchromic EBT3 radiochromic films were calculated for high-energy photons (6 and 10 MV) and electrons (6, 9, 12, and 15 MeV). Results: The calculated TPR20,10 values indicate that the percentage discrepancy for 6 and 10 MV was in the range of 0.29-0.72% and 0.26-0.65%. It was also found that the relative difference in the dmax to water and solid water phantoms was between 1.08-1.28% and 5.42-6.70%. The discrepancies in the determination of PDD curves with 6, 9, 12, and 15 MeV, and those of water and solid water phantoms, ranged from 2.40-4.84%. Comparable results were found using the EBT3 films with variations of 2.0-7.0% for 6 and 10 MV photons. Likewise, the discrepancies for 6, 9, 12, and 15 MeV electrons were within an acceptable range of 2.0-4.5%. Conclusions: On the basis of these findings, the DSF/NaOH/IA-PAE/R. spp. particleboard phantoms with 15 wt% IA-PAE addition level can be effectively used as alternative tissue-equivalent phantom material for radiation therapy applications.

Dosimetry of small photon fields in the presence of bone heterogeneity using MAGIC polymer gel, Gafchromic film, and Monte Carlo simulation.

BackgroundThe presence of heterogeneity within the radiation field increases the challenges of small field dosimetry. In this study, the performance of MAGIC polymer gel was evaluated in the dosimetry of small fields beyond bone heterogeneity.Materials and methodsCircular field sizes of 5, 10, 20 and 30 mm were used and Polytetrafluoroethylene with density of 2.2 g/cm3 was used as the bone equivalent material. The PDD curves, beam profiles, and penumbra widths were measured using MAGIC polymer gel, EBT2 film, and Monte Carlo simulation.ResultsThe maximum differences between MAGIC and EBT2 are 6.1, 4.7, 2.4, and 2.2 for PDD curves at 5, 10, 20, and 30 mm circular fields, respectively. The dose differences and distance to agreement between MAGIC and MC were within 1.89%/0.46 mm, 1.66%/0.43 mm, 1.28%/0.77 mm, and 1.31%/0.81 mm for beam profile values behind bone heterogeneity at 5, 10, 20, and 30 mm field sizes, respectively.ConclusionThe results presented that the MAGIC polymer gel dosimeter is a proper instrument for dosimetry beyond high density heterogeneity.

ANALISA DISTRIBUSI DOSIS PADA FANTOM HOMOGEN DAN INHOMOGEN DENGAN SIMULASI MONTE CARLO

Monte Carlo simulation with electron gamma shower (EGSnrc) code can produce 3-dimensional dose distribution data. The percent depth dose curve (PDD), dose profile, and isodose curve can be extracted through this 3-dimensional data. In this study, a photon source with an energy of 2 MeV is placed at the source to surface distance (SSD) from the phantom surface by adjusting the collimator aperture which is used to control the size of the exposure field. The SSD distance was varied at a distance of 50 cm, 70 cm, 80 cm, 90 cm, and 100 cm. The size of the exposure area is also varied by 2 × 2 cm 2 , 5 × 5 cm 2 , 7 × 7 cm 2 , and 10 × 10 cm 2 . The dose distribution analysis was carried out on homogeneous phantoms containing water and inhomogeneous phantoms containing tissue/bone/lung/bone/tissue material. The PDD curve and dose profile represent dose changes with depth and x or y direction, respectively. The PDD curve on a homogeneous phantom shows an increase in the value on the surface of the phantom to the depth with the maximum dose. This curve then decreases gradually with increasing depth. Inhomogeneous ghosts show spikes in the border region of the two media with different densities. This is due to the contribution of the backscattered electrons generated by the bone media and into the tissue media. This phenomenon is also seen in the isodose curve for inhomogeneous phantoms.

Monte Carlo Modeling and Verification of 6 MV Linear Accelerator

Purpose: To model the ELEKTA COMPACT accelerator head by using EGSnrc/BEAMnrc/DOSXYZnrc and to validatethe simulation according to the depth-dose and lateral profiles of different radiation fields measured by the water phantom. Methods: IBA Blue Water Phantom2 and CC13 Ionization Chamber were used to measure the depth-dose curves at 10 cm × 10 cm field and profile curves at 10 cm depth underwater. In BEAMnrc, the main components of accelerator head and the initial electron beam are established based on the specifications file, and the phase space file containing the photon beam information is generated. In DOXYZnrc, phase space files were used to irradiate a homogeneous water phantom of the same size as the IBA water phantom, and the simulated percentage depth dose curves and lateral profiles were outputted. The accuracy of the model was evaluated by mean square error (MSE) compared with the measured data. PDD curves are used to determine the energy of the initial electron beam. Dose profile curves are used to adjust the flattening filter. The penumbra on lateral profiles is used to adjust the full-width half-maximum (FWHM) of the electron source. Result: The electron energy of 5.8 MeV was considered the best match after comparing the PDD curves of 5.6 - 6.2 MeV electron beams. The flattening filter can only be adjusted by trial. In the final result, the maximum fluctuation of profile curve within 80% of the maximum field size is less than 3%, which meets the requirements of field flatness. The optimum FWHM for different fields is not consistent due to the Transmission penumbra. But a match can be approached by adjusting the FWHM every 10 cm field size.

Improved reconstruction methodology of clinical electron energy spectra based on Tikhonov regularization and generalized simulated annealing

Electron beam radiotherapy is the most widespread treatment modality todeal with superficial cancers. In electron radiotherapy, the energy spectrum isimportant for electron beam modelling and accurate dose calculation. Since thepercentage depth-dose (PDD) is a function of the beam’s energy, the reconstruction of the spectrum from the depth-dose curve represents an inverse problem.Thus, the energy spectrum can be related to the depth-dose by means of anappropriate mathematical model as the Fredholm equation of the first kind.Since the Fredholm equation of the first kind is ill-posed, some regularizationmethod has to be used to achieve a useful solution. In this work the Tikhonovregularization function was solved by the generalized simulated annealing optimization method. The accuracy of the reconstruction was verified by thegamma index passing rate criterion applied to the simulated PDD curves forthe reconstructed spectra compared to experimental PDD curves. Results showa good coincidence between the experimental and simulated depth-dose curvesaccording to the gamma passing rate better than 95% for 1% dose difference(DD)/1 mm distance to agreement (DTA) criteria. Moreover, the results showimprovement from previous works not only in accuracy but also in calculationtime. In general, the proposed method can help in the accuracy of dosimetryprocedures, treatment planning and quality control in radiotherapy.

Empirical method for modeling the percent depth dose curves of electron beam in radiation therapy

Abstract Introduction: This study presents an empirical method to model the electron beam percent depth dose curve (PDD) using the primary and tail functions in radiation therapy. The modeling parameters N and n can be used to derive the depth relative stopping power of the electron energy in radiation therapy. Methods and Materials: The electrons PDD curves were modeled with the primary-tail function in this study. The primary function included exponential function and main parameters of N, µ while the tail function was composed by a sigmoid function with the main parameter of n. The PDD for five electron energies were modeled by the primary and tail function by adjusting the parameters of N, µ and n. The R50 and Rp can be derived from the modeled straight line of 80% to 20% region of PDD. The same electron energy with different cone sizes was also modeled with the primary-tail function. The stopping power for different electron energies at different depths can also be derived from the parameters of N, µ and n. Percent ionization depth curve can then be derived from the percent depth dose by dividing its depth relevant stopping power for comparing with the original water phantom measurement. Results: The main parameters N, n increase, but µ decreases in primary-tail function when electron energy increased. The relationship of parameters n, N and LN(-µ) with electron energy are n = 31.667 E0 - 88, N = 0.9975 E0 - 2.8535, LN(-µ) = -0.1355 E0 - 6.0986, respectively. Stopping power of different electron energy can be derived from n and N with the equation: stopping power = (−0.042 ln N E 0 + 1.072)e(−n−E0·5·10−5+0.0381·d), where d is the depth in water. Percent depth dose was derived from the percent reading curve by multiplying the stopping power relevant to the depth in water at certain electron energy. Conclusion: The PDD of electrons at different energies and field sizes can be modeled with an empirical model to deal with the stopping power calculation. The primary-tail equation provides a uncomplicated solution than a pencil beam or other numerical algorism for investigators to research the behavior of electron beam in radiation therapy.

The influence of various vascular access ports on MV photon beam uniformity examined on the PMMA phantom

The presence of a high-density material object such as the vascular access port made of titanium, can affect the homogeneity of dose distribution in underlying tissues. This influence depends on numerous factors but in the first place on the composition material of such an object and its geometry. In this work an influence of the various titanium-made vascular ports, placed in the 6 MV photon field, have been analyzed. The vascular ports of various sizes were placed on the top of thepolymethyl-methacrylate (PMMA) phantom which is then scanned on a computed tomography (CT) simulator to generate the digitized 3-dimensional images for the purpose of treatment planning. The treatment plans were prepared in matRad treatment planning toolkit. The beam profiles and the percentage depth doses have been analyzed. The observed maximum dose values, for ports A, B, C and D, relatively to the maximum dose value in PMMA phantom alone, were 102.25%, 100.62%, 101.78% and 102.48%, respectively. The titanium edges of the ports reduce the dose below them in amount of 8.52%, 8.64%, 10.01% and 10.04% observed for ports A, B, C and D, respectively, in comparison to the central axis dose value obtained in PMMA phantom for the port-free case. The established changes in PDD curves and beam profiles depend on the vascular access port dimensions, reservoir volumes as well as of the amount of titanium content.

Commissioning and quality assurance of Euromechanics add-on multileaf collimator

In this study, the beam characteristics of a Euromechanics add-on MLC that has been installed on a Varian CLINAC 2100 C/D linear accelerator are presented. This was the first installation of 60-leaf PMLC from Euromechanics Company worldwide and all mechanical and dosimetric parameters were measured before clinical use of this kind of MLC. Mechanical tests were executed for different gantry and collimator angles. Leaf position accuracy and leaf gap reproducibility were checked with four different tests. The leaf transmissions, collimator (Sc), phantom (Sp), total (Sc,p) scatter factors, output of the machine, beam profiles for off-axis ratios, central axis depth dose, flatness, symmetry and penumbra have been measured for different field sizes pre and post MLC installation in 6 and 18 MV-mode. To evaluate the effect of new data on clinical plans, different beam setup configurations conformed with MLC and custom blocks were planned on CT images of thorax a CIRS phantom model 002LFC in the same treatment planning system. Leaf position in picket fence test found to be in range between 4.89–5.02 cm instead of nominal 5 cm, however the results of this test with EPIDs image and PIPSpro software showed the higher deviation rather than the results reported from the tests with EBT3 films. The measured data showed that on average Sc,p and Sc were increased 0.22% (P = 0.86) and 0.34% (P = 0.86) for 6 MV and 0.37% (P = 0.84) and 0.42% (P = 0.88) for 18 MV beams for different field sizes, respectively. Good agreement was observed between the PDD and profile curves pre and post MLC installation that was expected based on no changes in beam energy and geometry of the collimators. Based on the mechanical and dosimetry results which have been achieved from our different standard tests, it was found no significant differences between pre and post MLC installation values. This indicates, installation and using this system is clinically acceptable.

Effect of an integral quality monitor on 4-, 6-, 10-MV, and 6-MV flattening filter-free photon beams.

PurposeTo investigate the effect of an integral quality monitor (IQM; iRT Systems GmbH, Koblenz, Germany) on 4, 6, 10, and 6‐MV flattening filter‐free (FFF) photon beams.MethodsWe assessed surface dose, PDD20,10, TPR20,10, PDD curves, inline and crossline profiles, transmission factor, and output factor with and without the IQM. PDD, transmission factor, and output factor were measured for square fields of 3, 5, 10, 15, 20, 25, and 30 cm and profiles were performed for square fields of 3, 5, 10, 20, and 30 cm at 5‐, 10‐, and 30‐cm depth.ResultsThe differences in surface dose of all energies for square fields of 3, 5, 10, 15, 20, and 25 cm were within 3.7% whereas for a square field of 30 cm, they were 4.6%, 6.8%, 6.7%, and 8.7% for 4‐MV, 6‐MV, 6‐MV‐FFF, and 10‐MV, respectively. Differences in PDD20,10, TPR20,10, PDD, profiles, and output factors were within ±1%. Local and global gamma values (2%/2 mm) were below 1 for PDD beyond d max and inline/crossline profiles in the central beam region, respectively. The gamma passing rates (10% threshold) for PDD curves and profiles were above 95% at 2%/2 mm. The transmission factors for 4‐MV, 6‐MV, 6‐MV‐FFF, and 10‐MV for field sizes from 3 × 3 to 30 × 30 cm2 were 0.926–0.933, 0.937–0.941, 0.937–0.939, and 0.949–0.953, respectively.ConclusionsThe influence of the IQM on the beam quality (in particular 4‐MV X‐ray has not verified before) was tested and introduced a slight beam perturbation at the surface and build‐up region and the edge of the crossline/inline profiles. To use IQM in pre‐ and intra‐treatment quality assurance, a tray factor should be put into treatment planning systems for the dose calculation for the 4‐, 6‐, 10‐, and 6‐MV flattening filter‐free photon beams to compensate the beam attenuation of the IQM detector.

Construction and performance evaluation of a buildup bolus for breast intraoperative electron radiotherapy

The under-dosage of the target surface in breast intraoperative electron radiotherapy (IOERT) is one of the main concerns which can be compensated through employing a buildup bolus. The aim of this study is to design and introduce a new buildup bolus for breast IOERT to improve both surface dose and dose uniformity within the target.PLA (Polylactic Acid) material was selected to construct the flat panel bolus. Two different infill percentages of 60% and 100% were considered for bolus printing and their dosimetric characteristics were evaluated through Monte Carlo simulation and based on the closer matching to those of water, the best filling percentage was selected. Then, the bolus was constructed by a Diane K36-S 3D printer and its dosimetric performance in intraoperative electron beam was evaluated through ionometric dosimetry inside the developed bolus. All of the irradiations were performed by LIAC12 dedicated IOERT accelerator and the dosimetric data were obtained by Advanced Markus dosimeter.The results showed that the PLA bolus with 100% infill percentage would have the best performance as a buildup bolus. The measured PDD curves inside the constructed PLA bolus at different energies had desirable accordance with those measured inside the water.Based on the obtained results, it can be concluded that the dosimetric characteristics of designed PLA bolus would be satisfactorily in breast IOERT. This toxic-free PLA based bolus can be considered for surface dose enhancement and dose uniformity improvement during breast IOERT.

Polyvinyl alcohol (PVA) based bolus material for high energy photons and electrons

The study focused on the use of fabricated bolus materials based on the polyvinyl alcohol (PVA) to increase the surface dose on high energy photons and electrons. The PVA based bolus materials were fabricated at 2.5, 5, 7.5 and 10% PVA in water at target density of 1.0 g/cm3. The PVA based boluses were scanned by using the computed tomography (CT) scanner along with the solid water phantoms. The percentage depth (PDD) dose of the solid water phantoms and the PVA based bolus materials were measured by using the treatment planning software (TPS) at 6 and 10 MV photons and 6 and 9 MeV electrons. The PDD curved obtained were compared to that of the standard bolus material and the surface dose were measured. The results showed that the PDD curves of solid water phantoms with PVA based bolus materials were consistent with that in standard bolus material within 1.4% percentage of difference for both 6 and 10 MV photons and within 2.9 and 1.4% percentage of differences in 6 and 9 MeV electrons respectively. The increment of surface dose in PVA based bolus materials were in excellent agreement to the standard bolus material and the percentage of PVA did not significantly affect the surface dose. The overall results showed the potential use of the PVA to be used as bolus material for clinical photons and electrons.

Monte Carlo dose calculations of shielding disks with different material combinations in intraoperative electron radiation therapy (IOERT)

PurposeShielding disks play an important role in intraoperative electron radiation therapy, and different designs are currently used in clinical practice. This paper investigates the dosimetric impact of the shielding disk used during intraoperative electron radiation therapy (IOERT). Materials and MethodsThis paper focuses on the study of four shielding disks types that have been used in our clinic: Aluminum (Al)/Lead (Pb), PMMA/Copper(Cu)/PMMA, Aluminum (Al)/Copper (Cu) and Aluminum (Al)/Steel with their specific thicknesses. The theoretical study was conducted with the EGSnrc Monte Carlo (MC) code. On the other hand, the measurements were carried out with gafchromic films for the four shielding disks for the same setup inside the water phantom. Finally, a comparison of the simulated and measured PDD curves was performed for the four material combinations. ResultsMC simulation and gafchromic measurements illustrated that dose values under the four shielding disks types were close to 0, whereas the backscattering enhancement of the disks were 103% with Al/Pb shielding disk, 102% with Al/Steel shielding disk, 102% with Al/Cu shielding disk, 95% with PMMA/Cu/PMMA shielding disk. The PDDs values of the gafchromic films in front of the disks were: 107%, 105%, 104%, and 94% for the Al/Pb, Al/Steel, Al/Cu, and PMMA/Cu/PMMA disks respectively. ConclusionsThe dose values above and under the shielding disks were acceptable for the four studied shielding types. Demonstrated it is possible to use any of them clinically, while the best shielding disk was the Al/Pb since it has minimum thickness and a small backscatter enhancement.

Monte carlo calculation of the energy spectrum of a 6 MeV electron beam using penetration and energy loss of positrons and electrons code

Background:The limited bibliographic existence of research works on the use of Monte Carlo simulation to determine the energy spectra of electron beams compared to the information available regarding photon beams is a scientific task that should be resolved.Aims:In this work, Monte Carlo simulation was performed through the PENELOPE code of the Sinergy Elekta accelerator head to obtain the spectrum of a 6 MeV electron beam and its characteristic dosimetric parameters.Materials and Methods:The central-axis energy spectrum and the percentage depth dose curve of a 6 MeV electron beam of an Elekta Synergy linear accelerator were obtained by using Monte Carlo PENELOPE code v2014. For this, the linear accelerator head geometry, electron applicators, and water phantom were simplified. Subsequently, the interaction process between the electron beam and head components was simulated in a time of 86.4x104 s.Results:From this simulation, the energy spectrum at the linear accelerator exit window and the surface of the phantom was obtained, as well as the associated percentage depth dose curves. The validation of the Monte Carlo simulation was performed by comparing the simulated and the measured percentage depth dose curves via the gamma index criterion. Measured percentage depth- dose was determined by using a Markus electron ionization chamber, type T23343. Characteristic parameters of the beam related with the PDD curves such as the maximum dose depth (R100), 90% dose depth (R90), 90% dose depth or therapeutic range (R85), half dose depth (R50), practical range (Rp), maximum range (Rmax), surface dose (Ds), normalized dose gradient (G0) and photon contamination dose (Dx) were determined. Parameters related with the energy spectrum, namely, the most probable energy of electrons at the surface (Ep,0) and electron average energy (E– 0) were also determined.Conclusion:It was demonstrated that PENELOPE is an attractive and accurate tool for the obtaining of dosimetric parameters of a medical linear accelerator since it can reliably reproduce important clinical data such as the energy spectrum, depth dose, and dose profile.

Evaluating the performance of a 3D PLA buildup bolus in breast intraoperative radiotherapy.

Introduction: The received dose to the surface of tumor bed is one of the main issues in beast intraopeative radiotherapy (IORT). Totally, the administered dose to the surface is usually lower than that of radiotherapy reference point (distal end of tumor bed). Employing a buildup bolus can effectively increase the surface dose and therefore, eliminate such created cold points during IORT. The aim of this study is to evaluate the performance of a 3D buildup bolus, made of PLA (polylactic acid), for breast IORT. Materials and Methods: A cubic PLA bolus with different infill percentage of 20% to 100%, with 10% increments, was simulated through BEAMnrc Monte Carlo code and irradiated by intraoperative electron energies of 6, 8, 10 and 12 MeV produced from LIAC IORT accelerator. The LIAC head was also modeled using BEAMnrc code. Then, the PDD curves for different infill percentages of studied bolus were calculated by the DOSXYZnrc code and compared with those obtained in water at various energies. All of comparisons were performed by gamma analysis and gamma index calculation. The dose difference and distance to agreement in gamma index calculations were considered as 2% and 2 mm, respectively. Results: The results showed that high infill percentage corresponds to the high density and desirable homogeneity of bolus material. The bolus modeling in BEAMnrc code as a homogeneous material is satisfying for infill percentages greater than 20%. With increasing the bolus infill percentage, the accordance of obtained PDDs with those of water increments too. The PDD curves in the presence of PLA bolus with 100% infill percentage has the best agreement with the obtained PDDs inside water at different energies. Conclusion: Based on the obtained results, it can be concluded that the dosimetric characteristics of high infill percentages of PLA material are compatible with those of water. Therefore, PLA can be considered as an effective buildup bolus for surface dose enhancement in breast IORT. By tuning the infill percentage of PLA material, one can change the dosimetric properties of obtained bolus. The best dosimetric results are obtained when the PLA bolus is made with 100% infill percentage.

13 Response to high energy photons of the redesigned PTW T31022 PinPoint 3D ion chamber

Introduction The aim of this study was to investigate the dosimetric performances of the new PTW T31022 PinPoint 3D which replaces the PTW T31016 PinPoint 3D. Methods Experimental data were collected using a Varian TrueBeam linear accelerator (Varian Medical Systems, Palo Alto) at nominal energies of 6 and 10 MV with (WFF) and without a flattening filter (FFF) (6WFF, 10WFF and 6FFF, 10FFF, respectively). Measurements of absolute and relative dosimetry were undertaken. Ion collection efficiency associated with Jaffe plots for different dose per pulse, polarity effect, beam profiles, depth dose curves (PDD) and output factors associated to small field output correction factors were investigated. Results The T31022 Jaffe plots were found to be more linear than those of the T31016. The use of the two-voltage technique is sufficient for clinical practice and is consistent with the accuracy of the Jaffe plots technique. Less polarity effect was found for T31022 compared to T31016. The polarity correction factor of the T31022 was dependent on the field size, although it was at a lesser extent than for the T31016. The largest deviation of polarity factors was obtained for the largest field size and for WFF beams. From 3x3 cm2 to 40x40 cm2 field sizes, excellent agreement was found between T31022 output ratios corrected from polarity and T31010 Semiflex results, exhibiting maximum deviations of 0.2% for all energies. Without polarity correction maximum deviations are also relatively low reaching 0.7%, −0.4%, 0.7% and −0.4% for 6WFF, 6FFF, 10WFF and 10FFF respectively. The use of the chamber in small fields ( ⩽ 1x1 cm2) leads to an underestimation of output factors but presents lower field output correction factors than its predecessor. PDD curves present good agreement but discrepancies around 2% were observed for the largest field size and have been identified as a polarity dependence. This effect was moreover emphasized by keeping the electrometer at a distance less than 5 m from the water phantom. Conclusions The T31022 PinPoint 3D ion chamber is an improvement over its predecessor, the T31016, in terms of polarity, ion recombination and in small field dosimetry. The T31022 polarity correction factor has to be considered when measuring output factors and PDD for large field size.

OA240] Comparison between four routine control devices detecting photon beam energy changes

Purpose The quality assurance (QA) program of a LINAC includes the control of the stability of the photon beam energy, through a control variable, the beam quality BQ. Our department owns four devices for this task. In this work a method to compare the performance of these QA devices is designed and applied. Methods Two photon beams with energies 6 and 15 MV, produced by a Siemens Artiste LINAC, were used to analyse the response of the following QA devices: StarTrack (Iba Dosimetry), QuickCheck and StarCheck (PTW) and IC Profiler (Sun Nuclear). They determine the BQ indirectly through their own control variable, BQ dev which shall be related to the BQ measured in reference conditions. To determine this relationship, the photon energy was modified by varying the bending magnet intensity I BM For each value of I BM , three PDD curves in water and three measurements for each device were taken. Through the measurements taken before and assuming that they follow a normal distribution, pairs of values of BQ and BQ dev were generated using Monte Carlo simulation methods. The pairs of simulated values are related between them by the relationship obtained previously. By comparing if the pair of values are inside or outside the tolerance interval, each simulation is classified as true positive TP, false positive FP, true negative TN or false negative FN. The tolerance interval chosen was 0.5%. Summing all the TP, FP, TN and FN, two parameters can be calculated: sensitivity and specificity. These parameters will allow to compare the performance between devices. Results For all devices, the relationship between BQ dev and BQ is linear, with a determination coefficient r 2 ≥ 0.9 . The QuickCheck and IC Profiler devices showed the best performance, with a sensitivity and a specificity higher than 95%. The StarTrack device had a good sensitivity, but a specificity of around 85%, for both energies. The StarCheck device showed a decent sensitivity, but a specificity of 67.7% and 78.7% for each energy, respectively. Conclusions The QuickCheck and IC Profiler devices showed the best performance, followed by StarTrack. The StarCheck device had a specificity much lower than the other three devices.