D Czarnecki

Monte Carlo calculated correction factors for diodes and ion chambers in small photon fields

The application of small photon fields in modern radiotherapy requires the determination of total scatter factors Scp or field factors Ω(f(clin), f(msr))(Q(clin), Q(msr)) with high precision. Both quantities require the knowledge of the field-size-dependent and detector-dependent correction factor k(f(clin), f(msr))(Q(clin), Q(msr)). The aim of this study is the determination of the correction factor k(f(clin), f(msr))(Q(clin), Q(msr)) for different types of detectors in a clinical 6 MV photon beam of a Siemens KD linear accelerator. The EGSnrc Monte Carlo code was used to calculate the dose to water and the dose to different detectors to determine the field factor as well as the mentioned correction factor for different small square field sizes. Besides this, the mean water to air stopping power ratio as well as the ratio of the mean energy absorption coefficients for the relevant materials was calculated for different small field sizes. As the beam source, a Monte Carlo based model of a Siemens KD linear accelerator was used. The results show that in the case of ionization chambers the detector volume has the largest impact on the correction factor k(f(clin), f(msr))(Q(clin), Q(msr)); this perturbation may contribute up to 50% to the correction factor. Field-dependent changes in stopping-power ratios are negligible. The magnitude of k(f(clin), f(msr))(Q(clin), Q(msr)) is of the order of 1.2 at a field size of 1 × 1 cm(2) for the large volume ion chamber PTW31010 and is still in the range of 1.05-1.07 for the PinPoint chambers PTW31014 and PTW31016. For the diode detectors included in this study (PTW60016, PTW 60017), the correction factor deviates no more than 2% from unity in field sizes between 10 × 10 and 1 × 1 cm(2), but below this field size there is a steep decrease of k(f(clin), f(msr))(Q(clin), Q(msr)) below unity, i.e. a strong overestimation of dose. Besides the field size and detector dependence, the results reveal a clear dependence of the correction factor on the accelerator geometry for field sizes below 1 × 1 cm(2), i.e. on the beam spot size of the primary electrons hitting the target. This effect is especially pronounced for the ionization chambers. In conclusion, comparing all detectors, the unshielded diode PTW60017 is highly recommended for small field dosimetry, since its correction factor k(f(clin), f(msr))(Q(clin), Q(msr)) is closest to unity in small fields and mainly independent of the electron beam spot size.

The influence of linac spot size on scatter factors

The use of small photon fields in modern radiotherapy requires the determination of total scatter factors Scp or field factorswith high precision. Therefore, chamber-dependent correction factors for dose measurements in small fields are necessary. In this study Monte Carlo simulations were used to calculate the field factorand chamber response-related correction factors for four different types of detectors in a clinical 6 MV photon beam for a square field size of 1 cm × 1 cm. As a beam source a Monte Carlo-based model of a Siemens KD linear accelerator was applied. The calculations aimed at the investigation of the influence of electron beam spot size on correction factors for small field dosimetry. The results confirm that accurate Monte Carlo calculations of the field factorcan only be carried out when the exact electron spot size is known. On the other hand no dependence of the electron beam spot size on the correction factors for the field size of 1 cm × 1 cm was observed.

Effective point of measurement for parallel plate and cylindrical ion chambers in megavoltage electron beams.

The presence of an air filled ionization chamber in a surrounding medium introduces several fluence perturbations in high energy photon and electron beams which have to be accounted for. One of these perturbations, the displacement effect, may be corrected in two different ways: by a correction factor pdis or by the application of the concept of the effective point of measurement (EPOM). The latter means, that the volume averaged ionization within the chamber is not reported to the chambers reference point but to a point within the air filled cavity. Within this study the EPOM was determined for four different parallel plate and two cylindrical chambers in megavoltage electron beams using Monte Carlo simulations. The positioning of the chambers with this EPOM at the depth of measurement results in a largely depth independent residual perturbation correction, which is determined within this study for the first time. For the parallel plate chambers the EPOM is independent of the energy of the primary electrons. Whereas for the Advanced Markus chamber the position of the EPOM coincides with the chambers reference point, it is shifted for the other parallel plate chambers several tenths of millimeters downstream the beam direction into the air filled cavity. For the cylindrical chambers there is an increasing shift of the EPOM with increasing electron energy. This shift is in upstream direction, i.e. away from the chambers reference point toward the focus. For the highest electron energy the position of the calculated EPOM is in fairly good agreement with the recommendation given in common dosimetry protocols, for the smallest energy, the calculated EPOM positions deviate about 30% from this recommendation.

Monte Carlo-based investigations on the impact of removing the flattening filter on beam quality specifiers for photon beam dosimetry

Purpose The impact of removing the flattening filter in clinical electron accelerators on the relationship between dosimetric quantities such as beam quality specifiers and the mean photon and electron energies of the photon radiation field was investigated by Monte Carlo simulations. The purpose of this work was to determine the uncertainties when using the well‐known beam quality specifiers or energy‐based beam specifiers as predictors of dosimetric photon field properties when removing the flattening filter. Methods Monte Carlo simulations applying eight different linear accelerator head models with and without flattening filter were performed in order to generate realistic radiation sources and calculate field properties such as restricted mass collision stopping power ratios Symbol, mean photon and secondary electron energies. To study the impact of removing the flattening filter on the beam quality correction factors kQ, this factor for detailed ionization chamber models was calculated by Monte Carlo simulations. Symbol. No caption available. Stopping power ratios Symbol and kQ values for different ionization chambers as a function of Symbol and %dd(10)x were calculated. Moreover, mean photon energies in air and at the point of measurement in water as well as mean secondary electron energies at the point of measurement were calculated. Symbol. No caption available. Symbol. No caption available. Results The results revealed that removing the flattening filter led to a change within 0.3% in the relationship between %dd(10)x and Symbol, whereby the relationship between Symbol and Symbol changed up to 0.8% for high energy photon beams. However, Symbol was a good predictor of Symbol for both types of linear accelerator with energies Symbol 10 MeV with a maximal deviation between both types of accelerators of 0.23%. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. According to the results, the mean photon energy below the linear accelerators head as well as at the point of measurement may not be suitable as a predictor of Symbol and kQ to merge the dosimetry of both linear accelerator types. It was possible to derive Symbol using the mean secondary electron energy at the point of measurement as a predictor with an accuracy of 0.17%. A bias between kQ for linear accelerators with and without flattening filter within 1.1% and 1.6% was observed for Symbol and %dd(10)x respectively. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Conclusion The results of this study have shown that removing the flattening filter led to a change in the relationship between the well‐known beam quality specifiers and dosimetric quantities at the point of measurement, namely Symbol, mean photon and electron energy. Furthermore, the results show that a beam profile correction is important for dose measurements with large ionization chambers in flattening filter free beams. Symbol. No caption available.

Impact of new ICRU Report 90 recommendations on calculated correction factors for reference dosimetry

In 2016 the ICRU published a new report dealing with key data for ionizing radiation dosimetry (ICRU Report 90). New recommendations have been made for the mean excitation energies I for air, graphite and liquid water as well as for the graphite density to use when evaluating the density effect. In addition, the ICRU Report 90 discusses renormalized photoelectric cross sections, but refuses to give a recommendation on the use of renormalization factors. However, the Consultative Committee for Ionizing Radiation recommends to use renormalized photoeffect cross sections. Goal of the present work is to evaluate the impact of these new recommendations on clinical reference dosimetry for high energy photon and electron beams. The beam quality correction factor k Q was calculated by Monte Carlo simulations for compact and parallel plate ionization chambers. In case of photons seven phase space files from clinical accelerators and twelve spectra taken from literature from 4 MV to 24 MV and additionally a 60Co source were applied. As electron source thirteen electron spectra available in literature were used in the range of 4 MeV-21 MeV. The new ICRU recommendations have a small impact on Monte Carlo calculated k Q values for the chosen ionization chambers in the range of 0.1%-0.35% only-the difference increases for higher photon energies. The impact of the ICRU Report 90 recommendations on Monte Carlo calculated stopping power ratios s w,a , perturbation factors p and beam quality correction factors k Q was investigated and confirmed a decrese of s w,a by a fraction of a percent for photon and electron beams. This study indicates that the impact of the new ICRU recommendation is within 0.35%. The determined deviations should be taken into account, when widely published Monte Carlo calculated values are examined.

EURADOS intercomparison exercise on Monte Carlo modelling of a medical linear accelerator

BACKGROUND In radiotherapy, Monte Carlo (MC) methods are considered a gold standard to calculate accurate dose distributions, particularly in heterogeneous tissues. EURADOS organized an international comparison with six participants applying different MC models to a real medical linear accelerator and to one homogeneous and four heterogeneous dosimetric phantoms. AIMS The aim of this exercise was to identify, by comparison of different MC models with a complete experimental dataset, critical aspects useful for MC users to build and calibrate a simulation and perform a dosimetric analysis. RESULTS Results show on average a good agreement between simulated and experimental data. However, some significant differences have been observed especially in presence of heterogeneities. Moreover, the results are critically dependent on the different choices of the initial electron source parameters. CONCLUSIONS This intercomparison allowed the participants to identify some critical issues in MC modelling of a medical linear accelerator. Therefore, the complete experimental dataset assembled for this intercomparison will be available to all the MC users, thus providing them an opportunity to build and calibrate a model for a real medical linear accelerator.

SU-F-T-261: Reconstruction of Initial Photon Fluence Based On EPID Images

PURPOSE Verifying an algorithm to reconstruct relative initial photon fluence for clinical use. Clinical EPID and CT images were acquired to reconstruct an external photon radiation treatment field. The reconstructed initial photon fluence could be used to verify the treatment or calculate the applied dose to the patient. METHODS The acquired EPID images were corrected for scatter caused by the patient and the EPID with an iterative reconstruction algorithm. The transmitted photon fluence behind the patient was calculated subsequently. Based on the transmitted fluence the initial photon fluence was calculated using a back-projection algorithm which takes the patient geometry and its energy dependent linear attenuation into account. This attenuation was gained from the acquired cone-beam CT or the planning CT by calculating a water-equivalent radiological thickness for each irradiation direction. To verify the algorithm an inhomogeneous phantom consisting of three inhomogeneities was irradiated by a static 6 MV photon field and compared to a reference flood field image. RESULTS The mean deviation between the reconstructed relative photon fluence for the inhomogeneous phantom and the flood field EPID image was 3% rising up to 7% for off-axis fluence. This was probably caused by the used clinical EPID calibration, which flattens the inhomogeneous fluence profile of the beam. CONCLUSION In this clinical experiment the algorithm achieved good results in the center of the field while it showed high deviation of the lateral fluence. This could be reduced by optimizing the EPID calibration, considering the off-axis differential energy response. In further progress this and other aspects of the EPID, eg. field size dependency, CT and dose calibration have to be studied to realize a clinical acceptable accuracy of 2%.

SU-G-TeP1-03: Beam Quality Correction Factors for Linear Accelerator with and Without Flattening Filter

PURPOSE The impact of removing the flattening filter on absolute dosimetry based on IAEAs TPR-398 and AAPMs TG-51 was investigated in this study using Monte Carlo simulations. METHODS The EGSnrc software package was used for all Monte Carlo simulations performed in this work. Five different ionization chambers and nine linear accelerator heads have been modeled according to technical drawings. To generate a flattening filter free radiation field the flattening filter was replaced by a 2 mm thick aluminum layer. Dose calculation in a water phantom were performed to calculate the beam quality correction factor kQ as a function of the beam quality specifiers %dd(10)x , TPR20,10 and mean photon and electron energies at the point of measurement in photon fields with (WFF) and without flattening filter (FFF). RESULTS The beam quality correction factor as a function of %dd(10)x differs systematically between FFF and WFF beams for all investigated ionization chambers. The largest difference of 1.8% was observed for the largest investigated Farmer-type ionization chamber with a sensitive volume of 0.69 cm3 . For ionization chambers with a smaller nominal sensitive volume (0.015 - 0.3 cm3 ) the deviation was less than 0.4% between WFF and FFF beams for %dd(10)x > 62%. The specifier TPR20,10 revealed only a good correlation between WFF and FFF beams (< 0.3%) for low energies. CONCLUSION The results confirm that %dd(10)x is a suitable beam quality specifier for FFF beams with an acceptable bias. The deviation depends on the volume of the ionization chamber. Using %dd(10)x to predictkQ for a large volume chamber in a FFF photon field may lead to not acceptable errors according to the results of this study. This bias may be caused by the volume effect due to the inhomogeneous photon fields of FFF linear accelerators.

SU‐F‐T‐72: Experimental Determination of the Positionuncertainties for ROOS Ionization Chambers in Clinical Electron Beams

PURPOSE National and international dosimetry protocols assume a position accuracy for ionization chambers of less than 0.2mm. To follow this precept the manufacturer PTW-Freiburg introduced a positioning assistance system (TRUFIX) for their particular ion chambers. Aim of this study is an experimental investigation of the positioning uncertainties for ROOS-type ionization chambers. METHODS For all measurements a linear accelerator Elekta Synergie was used. The experiments were performed in a water-phantom. To collimate the electron beam a 10×10cm2 applicator was installed. All measured depth dose curves were normalized to their maximum. In all cases the TRUFIX system was applied for chamber positioning. For the first measurement series, to determine the positioning reproducibility of a ROOS chamber, one person placed the chamber three times in a 6 MeV electron beam. The mean value of this three measurements was the reference for further six random persons who repeated this procedure. The results were compared for different depths (R50 , zref and Rp ). To investigate the impact of different individual chambers of the same type 10 different ROOS chambers were placed by the same person in a 6, 12 and 18MeV electron beam and the measured reference depths zref were compared. RESULTS The absolute positioning reproducibility is less than 0.1mm for the same person. The positioning uncertainties are increasing up to +/-0.3mm if different persons perform the chambers positioning within the water phantom. The comparison of the 10 different ROOS chambers resulted in reference depths zref with deviations in the range of +/-0.45mm for all energies. CONCLUSION The position accuracy of 0.2mm can be fulfilled with the TRUFIX system. The comparison of the 10 different ROOS ionization chambers showed noticeable deviations in the determined reference depth. The impact of a positioning uncertainty of about 0.3-0.4mm on the total perturbation correction will be considered.

SU‐E‐T‐350: Effective Point of Measurement and Total Perturbation Correction P for Parallel‐Plate Ion Chambers in High‐Energy Photon Beams

Purpose: This paper aims to determine the effective point of measurement and the total perturbation correction p of parallel-plate chambers for clinical photon dosimetry. Methods: The effective point of measurement (EPOM) was calculated using the EGSnrc Monte Carlo code system with the EGSnrc user code egs_ chamber. Depth dose curves of the ionization chambers were calculated in a water phantom for several high energy photon spectra (4, 6, 10, 15, 18 MV-X). Different normalization criterions (normalization to the maximum of the depth dose curve and normalization to the value in 10 cm depth) have been applied. The EPOM was determined by shifting the normalized depth dose curve of a small water voxel against the depth ionization curve until the disagreement (calculated by the root mean square deviation) reaches a minimum. In addition, the total perturbation correction p was calculated by the ratio of the dose to water and the product of the dose determined in the chamber and the water to air stopping power ratio. Results: The EPOM varied slightly depending on the chosen normalization criterion. For all chambers the necessary shift of the EPOM decreased linearly with increasing beam quality specifier TPR20/10. For the Roos and NACP chamber, the results were positive suggesting that the chambers need to be shifted towards the focus. For the Markus chamber, the required shift was negative and for the Advanced Markus chamber partly negative and partly positive. The total perturbation correction p was almost independent of the depth. Only for regions below 1 cm the perturbation correction deviated significantly from unity. Conclusion: In the present study, the effective point of measurement and the total perturbation correction p was determined for four parallel-plate ionization chambers and five clinical relevant photon spectra. Applying the calculated EPOM, the residual perturbation correction p was mostly depth independent.