IAEA TRS-398 Code Of Practice Research Articles

Monte Carlo calculated ionization chamber correction factors in clinical proton beams – deriving uncertainties from published data

For the update of the IAEA TRS-398 Code of Practice (CoP), global ionization chamber factors (fQ) and beam quality correction factors (kQ) for air-filled ionization chambers in clinical proton beams have been calculated with different Monte Carlo codes. In this study, average Monte Carlo calculated fQ and kQ factors are provided and the uncertainty of these factors is estimated. Average fQ factors in monoenergetic proton beams with energies between 60 MeV and 250 MeV were derived from Monte Carlo calculated fQ factors published in the literature. Altogether, 195 fQ factors for six plane-parallel and three cylindrical ionization chambers calculated with penh, fluka and geant4 were incorporated. Additionally, a weighted standard deviation of fQ factors was calculated, where the same weight was assigned to each Monte Carlo code. From average fQ factors, kQ factors were derived and compared to the values from the IAEA TRS-398 CoP published in 2000 as well as to the values of the upcoming version. Average Monte Carlo calculated fQ factors are constant within 0.6% over the energy range investigated. In general, the different Monte Carlo codes agree within 1% for low energies and show larger differences up to 2% for high energies. As a result, the standard deviation of fQ factors increases with energy and is ∼0.3% for low energies and ∼0.8% for high energies. kQ factors derived from average Monte Carlo calculated fQ factors differ from the values presented in the IAEA TRS-398 CoP by up to 2.4%. The overall estimated uncertainty of Monte Carlo calculated kQ factors is ∼0.5%–1% smaller than the uncertainties estimated in IAEA TRS-398 CoP since the individual ionization chamber characteristics (e.g. fluence perturbations) are considered in detail in Monte Carlo calculations. The agreement between Monte Carlo calculated kQ factors and the values of the upcoming version of IAEA TRS-398 CoP is better with deviations smaller than 1%.

A portable primary-standard level graphite calorimeter for absolute dosimetry in clinical pencil beam scanning proton beams

Objective. To report the use of a portable primary standard level graphite calorimeter for direct dose determination in clinical pencil beam scanning proton beams, which forms part of the recommendations of the proposed Institute of Physics and Engineering in Medicine (IPEM) Code of Practice (CoP) for proton therapy dosimetry. Approach. The primary standard proton calorimeter (PSPC) was developed at the National Physical Laboratory (NPL) and measurements were performed at four clinical proton therapy facilities that use pencil beam scanning for beam delivery. Correction factors for the presence of impurities and vacuum gaps were calculated and applied, as well as dose conversion factors to obtain dose to water. Measurements were performed in the middle of 10 × 10 × 10 cm3 homogeneous dose volumes, centred at 10.0, 15.0 and 25.0 g·cm−2 depth in water. The absorbed dose to water determined with the calorimeter was compared to the dose obtained using PTW Roos-type ionisation chambers calibrated in terms of absorbed dose to water in 60Co applying the recommendations in the IAEA TRS-398 CoP. Main results. The relative dose difference between the two protocols varied between 0.4% and 2.1% depending on the facility. The reported overall uncertainty in the determination of absorbed dose to water using the calorimeter is 0.9% (k = 1), which corresponds to a significant reduction of uncertainty in comparison with the TRS-398 CoP (currently with an uncertainty equal or larger than 2.0% (k = 1) for proton beams). Significance. The establishment of a purpose-built primary standard and associated CoP will considerably reduce the uncertainty of the absorbed dose to water determination and ensure improved accuracy and consistency in the dose delivered to patients treated with proton therapy and bring proton reference dosimetry uncertainty in line with megavoltage photon radiotherapy.

Current best estimates of beam quality correction factors for reference dosimetry of clinical proton beams

Objective. To review the currently available data on beam quality correction factors, for ionization chambers in clinical proton beams and derive their current best estimates for the updated recommendations of the IAEA TRS-398 Code of Practice. Approach. The reviewed data come from 20 publications from which values can be derived either directly from calorimeter measurements, indirectly from comparison with other chambers or from Monte Carlo calculated overall chamber factors, For cylindrical ionization chambers, a distinction is made between data obtained in the centre of a spread-out Bragg peak and those obtained in the plateau region of single-energy fields. For the latter, the effect of depth dose gradients has to be considered. To this end an empirical model for previously published displacement correction factors for single-layer scanned beams was established, while for unmodulated scattered beams experimental data were used. From all the data, chamber factors, and chamber perturbation correction factors, were then derived and analysed. Main results. The analysis showed that except for the beam quality dependence of the water-to-air mass stopping power ratio and, for cylindrical ionization chambers in unmodulated beams, of the displacement correction factor, there is no remaining beam quality dependence of the chamber perturbation correction factors Based on this approach, average values of the beam quality independent part of the perturbation factors were derived to calculate values consistent with the data in the literature. Significance. The resulting data from this analysis are current best estimates of values for modulated scattered beams and single-layer scanned beams used in proton therapy. Based on this, a single set of harmonized values is derived to be recommended in the update of IAEA TRS-398.

Experimental determination of k Q factors for two types of ionization chambers in scanned proton beams

Objective. Experimental determination of beam quality k Q factors for two types of Farmer ionization chambers, NE2571 and IBA FC65-G, in a scanned proton beam for three nominal energies (140 MeV, 180 MeV and 220 MeV) based on water calorimetry. Approach. Beam quality correction factors were determined comparing the results obtained with water calorimetry and ionometry. Water calorimetry was performed to determine the absorbed dose at a depth of measurement in water of 5 g cm−2, limited by the extension of the calorimeter glass vessel used. For the ionometry, two chambers of each type were included in the study. The ionization chambers were calibrated in terms of absorbed dose to water in 60Co at the Swedish Secondary Standard Dosimetry Laboratory, directly traceable to the BIPM, and were used according to the IAEA TRS-398 Code of Practice. Main results. The k Q values determined in the present work have been compared with the values tabulated in TRS-398 and its forthcoming update and also with those obtained in previous water calorimetric measurements and Monte Carlo calculations. All results were found to agree within the combined uncertainties of the different data. Significance. It is expected that the present work will serve as an experimental contribution to k Q -factors for the two chamber types and three scanned proton beam qualities used.

Monte Carlo calculation of quality correction factors based on air kerma and absorbed dose to water in medium energy x-ray beams

Clinical dosimetry is typically performed using ion chambers calibrated in terms of absorbed dose to water. As primary measurement standards for this quantity for low and medium energy x-rays are available only since a few years, most dosimetry protocols for this photon energy range are still based on air kerma calibration. For that reason, data for beam quality correction factors , necessary for the application of dose to water based protocols, are scarce in literature. Currently the international IAEA TRS-398 Code of Practice is under revision and new factors for a large number of ion chambers will be introduced in the update of this protocol. Several international groups provided the IAEA with experimental and Monte Carlo based data for this revision. Within the European Community the EURAMET 16NRM03 RTNORM project was initiated for that purpose. In the present study, Monte Carlo based results for the beam quality correction factors in medium energy x-ray beams for six ion chambers applying different Monte Carlo codes are presented. Additionally, the perturbation factor p Q , necessary for the calculation of dose to water from an air kerma calibration coefficient, was determined.The beam quality correction factor for the chambers varied in the investigated energy range by about 4%–5%, and for five out of six chambers the data could be fitted by a simple logarithmic function, if the half-value-layer was used as the beam quality specifier. Corresponding data using different Monte Carlo codes for the same ion chamber agreed within 0.5%. For the perturbation factor p Q , the data did not obey a comparable simple relationship with the beam quality specifier. The variation of p Q for all ion chambers was in the range of 3%–4%. Compared to recently published data, our p Q data is around 1% larger, although the same Monte Carlo code has been used. Compared to the latest experimental data, there are even deviations in the range of 2%.

Monte Carlo calculation of beam quality correction factors in proton beams using TOPAS/GEANT4

To provide Monte Carlo calculated beam quality correction factors (kQ) for monoenergetic proton beams using , a toolkit based on the Monte Carlo code .Monte Carlo simulations of six plane-parallel and four cylindrical ionization chambers were carried out. The latest ICRU 90 recommendations on key data for ionizing-radiation dosimetry were used to calculate the electronic stopping powers and to select the mean energy necessary to create an ion pair in air (). factors were calculated for a 60Co spectrum at a depth of 5 g cm−2. f Q factors and ratios as well as kQ factors were calculated at the entrance region of monoenergetic proton beams with energies between 60 MeV and 250 MeV.Additionally, perturbation correction factors for the Exradin A1SL ionization chamber at an energy of 250 MeV were calculated. factors agreed within 0.7% or better, f Q factors within 1.7% or better and ratios within 2.2% or better with Monte Carlo calculated values provided in the literature. Furthermore, kQ factors calculated in this work were found to agree within 1% or better with experimentally determined kQ factors provided in the literature, with only two exceptions with deviations of 1.4% and 2.4%.The total perturbation correction factor for the Exradin A1SL chamber was 0.969(7) and hence significantly different than unity in contrast to the assumption from the IAEA TRS-398 code of practice (CoP). can be used to calculate kQ factors in clinical proton beams. kQ factors for six plane-parallel and four cylindrical ionization chambers were calculated and provided for the upcoming update of the IAEA TRS-398 CoP.

Dose intercomparison at Italian hadrontherapy centers

PurposeTo perform the first dosimetric intercomparison for proton beams in Italy using ionization chambers, according to the IAEA TRS-398 code of practice. MethodsMeasurement sites included: National Center for Oncological Hadron Therapy (CNAO, Pavia), Center for Proton Therapy (CTP, Trento) and Center for Hadron Therapy and for advanced Nuclear Applications (CATANA, Catania). For comparison we also included a 6 MV photon beam produced at Istituti Clinici Scientifici Maugeri (ICSM, Pavia). For proton beams, both single pseudo-monoenergetic layers (in order to obtain a planned dose of 2 Gy at the reference depth of 2 cm in a water phantom) and Spread-out Bragg peaks (SOBP) have been delivered. Measurements were performed with a PTW Farmer 30010-1 and a PTW Advanced Markus type 34,045 ionization chamber. ResultsData obtained at CATANA, CNAO and CPT in terms of absorbed dose to water depth show good consistency within the experimental uncertainties, with a weighted mean of 1.99 ± 0.01 Gy and a standard error of 0.003 Gy, with reference to a nominal dose of 2 Gy as designed by the treatment planning system. ConclusionsThe results showed a standard deviation of less than 1% for single layer and SOBP beams, for all chambers and a percent deviation less than 1.5% for single layer measurements. The weighted means of the absorbed doses for clinical proton beams (118.19 MeV and 173.61 MeV) are consistent within less than 1%. These results agree within the 1.5% difference considered acceptable for national dose intercomparison.

41. Dose intercomparison at European hadrotherapy centers

Purpose To perform the first intercomparison study on nominal doses delivered by the Treatment Planning System in various European hadrontherapy centers, using an ionization chamber for reference dosimetry in water, according to IAEA TRS398 code of practice. Methods Measurement sites include the following clinical therapy centers: CNAO (Pavia) and HIT (Heidelberg) for protons and carbon ions; TPC - (Trento) and CATANA (Catania) for protons; Fondazione Maugeri (Pavia) for 6 MV photons. For ion beams, both single monoenergetic layer and SOBP, have been delivered to obtain a nominal dose of 2 Gy at the reference depth zref = 2 cm in a water phantom. The energies used were: in CNAO for protons (118.19 MeV, 173.61 MeV) and for carbon ions (208.58 MeV/u, 332.15 MeV/u), in Fondazione Maugeri 6 MV and in TPC - for protons (118 MeV, 173 MeV) [1] . Measurements were made with a PTW Farmer 30010-1 ionization chamber. Results CNAO, Fondazione Maugeri and TPS data show consistency within the experimental uncertainties, with a maximum relative deviation of 2.8%. The full set of measurements is planned to be completed in 2018. Conclusions Absorbed dose for clinical proton beams (118.19 MeV and 173.61 MeV) is consistent within less than 3% between CNAO and TPS, as well as for 6 MV photons. This work was performed within INFN projects RDH and MC-INFN.

TH-CD-BRA-05: First Water Calorimetric Dw Measurement and Direct Measurement of Magnetic Field Correction Factors, KQ,B, in a 1.5 T B-Field of An MRI Linac

Purpose: Reference dosimetry in MR-guided radiotherapy is performed in the presence of a B-field. As a consequence the response of ionization chambers changes considerably and depends on parameters not considered in traditional reference dosimetry. Therefore future Codes of Practices need ionization chamber correction factors to correct for both the change in beam quality and the presence of a B-field. The objective was to study the feasibility of water calorimetric absorbed-dose measurements in a 1.5 T B-field of an MRLinac and the direct measurement of kQ,B calibration of ionization chambers. Methods: Calorimetric absorbed dose to water Dw was measured with a new water calorimeter in the bore of an MRLinac (TPR20,10 of 0.702). Two waterproof ionization chambers (PTW 30013, IBA FC-65G) were calibrated inside the calorimeter phantom (ND,w,Q,B). Both measurements were normalized to a monitor ionization chamber. Ionization chamber measurements were corrected for conventional influence parameter. Based on the chambers’ Co-60 calibrations (ND,w,Q0), measured directly against the calorimeter. In this study the correction factors kQ,B was determined as the ratio of the calibration coefficients in the MRLinac and in Co-60. Additionally, kB was determined based on kQ values obtained with the IAEA TRS-398 Code of Practice. Results: The kQ,B factors of the ionization chambers mentioned above were respectively 0.9488(8) and 0.9445(8) with resulting kB factors of 0.961(13) and 0.952(13) with standard uncertainties on the least significant digit(s) between brackets. Conclusion: Calorimetric Dw measurements and calibration of waterproof ionization chambers were successfully carried out in the 1.5 T B-field of an MRLinac with a standard uncertainty of 0.7%. Preliminary kQ,B and kB factors were determined with standard uncertainties of respectively 0.8% and 1.3%. The kQ,B agrees with an alternative method within 0.4%. The feasibility of water calorimetry in the presence of B-fields was demonstrated by the direct determination of Dw and kQ,B. This work was supported by EMRP grant HLT06. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.

SU-E-T-146: Reference Dosimetry for Protons and Light-Ion Beams Based on Graphite Calorimetry.

The IAEA TRS-398 code of practice can be applied for the measurement of absorbed dose to water under reference conditions with an ionization chamber. For protons, the combined relative standard uncertainty on those measurements is less than 2% while for light-ion beams, it is considerably larger, i.e. 3.2%, mainly due to the higher uncertainty contributions for the water to air stopping power ration and the W air-value on the beam quality correction factors kQ,Q0 . To decrease this uncertainty, a quantification of kQ,Q0 is proposed using a primary standard level graphite calorimeter. This work includes numerical and experimental determinations of dose conversion factors to derive dose to water from graphite calorimetry. It also reports on the first experimental data obtained with the graphite calorimeter in proton, alpha and carbon ion beams. Firstly, the dose conversion has been calculated with by Geant4 Monte-Carlo simulations through the determination of the water to graphite stopping power ratio and the fluence correction factor. The latter factor was also derived by comparison of measured ionization curves in graphite and water. Secondly, kQ,Q0 was obtained by comparison of the dose response of ionization chambers with that of the calorimeter. Stopping power ratios are found to vary by no more than 0.35% up to the Bragg peak, while fluence correction factors are shown to increase slightly above unity close to the Bragg peak. The comparison of the calorimeter with ionization chambers is currently under analysis. For the modulated proton beam, preliminary results on W air confirm the value recommended in TRS-398. Data in both the non-modulated proton and light-ion beams indicate higher values but further investigation of heat loss corrections is needed. The application of graphite calorimetry to proton, alpha and carbon ion beams has been demonstrated successfully. Other experimental campaigns will be held in 2012. This work is supported by the BioWin program of the Wallon Government.

Possibility of using cylindrical ionization chambers for percent depth‐dose measurements in clinical electron beams

This study investigated the possibility of using cylindrical ionization chambers for percent depth-dose (PDD) measurements in high-energy clinical electron beams. The cavity correction factor, P(cav), for cylindrical chambers with various diameters was calculated as a function of depth from the surface to R50, in the energy range of 6-18 MeV electrons with the EGSnrc C(++) -based user-code CAVITY. The results were compared with those for IBA NACP-02 and PTW Roos parallel-plate ionization chambers. The effective point of measurement (EPOM) for the cylindrical chamber and the parallel-plate chamber was positioned according to the IAEA TRS-398 code of practice. The overall correction factor, P(Q), and the percent depth-ionization (PDI) curve for a PTW30013 Farmer-type chamber were also compared with those of NACP-02 and Roos chambers. The P(cav) values at depths between the surface and R50 for cylindrical chambers were all lower than those with parallel-plate chambers. However, the variation in depth for cylindrical chambers equal to or less than 4 mm in diameter was equivalent to or smaller than that for parallel-plate chambers. The P(Q) values for the PTW30013 chamber mainly depended on P(cav), and for parallel-plate chambers depended on the wall correction factor, P(waII), rather than P(cav). P(Q) at depths from the surface to R50 for the PTW30013 chamber was consequently a lower value than that with parallel-plate chambers. However, the variation in depth was equivalent to that of parallel-plate chambers at electron energies equal to or greater than 9 MeV. The shift to match calculated PDI curves for the PTW30013 chamber and water (perturbation free) varied from 0.65 to 0 mm between 6 and 18 MeV beams. Similarly, the shifts for NACP-02 and Roos chambers were 0.5-0.6 mm and 0.2-0.3 mm, respectively, and were nearly independent of electron energy. Calculated PDI curves for PTW30013, NACP-02, and Roos chambers agreed well with that of water by using the optimal EPOM. Therefore, the possibility of using cylindrical ionization chambers can be expected for PDD measurements in clinical electron beams.

Total skin electron therapy (TSET): A reimplementation using radiochromic films and IAEA TRS‐398 code of practice

The aim of this work is to present an updated implementation of total skin electron therapy (TSET) using IAEA TRS-398 code of practice for absolute dosimetry and taking advantage of the use of radiochromic films. The optimization of quality control tests is also included. A Varian 2100 C/D linear accelerator equipped with the special procedure HDTSe- (high dose rate total skin electron mode, E=6 MeV) was employed to perform TSET irradiations using the modified Stanford technique. The commissioning was performed following the AAPM report 23 recommendations. In particular, for dual-field beams irradiation, the optimal tilt angle was investigated and the dose distribution in the treatment plane was measured. For a complete six dual-field beams irradiation, the treatment skin dose on the surface of a cylindrical phantom was evaluated by radiochromic films and the B factor which relates the single dual-field skin dose to the six dual-field skin dose was assessed. Since the TRS-398 reference conditions do not meet the requirements of TSET absolute dosimetry, GafChromic EBT films were also employed to check and validate the application of the protocol. Simplified procedures were studied to verify beam constancy in PMMA phantoms without the more difficult setup of total skin irradiation. The optimized geometrical setup for dual-field beams was: Tilt angle = +/- 19 degrees, SSD=353 cm, and the beam degrader (200 x 100 X 1 cm3) placed at 320 cm from the source. As regards to dose homogeneity in the treatment plane, for dual-field beams irradiation, the mean relative dose value was 97% +/- 5% (normalizing to 100% at the calibration point level). For six dual-field beams irradiation, the multiplication factor B was 2.63. In addition, beam quality, dose rate, and bremsstrahlung contribution were also suitable for TSET treatments. The TRS-398 code of practice was used for TSET dosimetry, as dose measurements performed by ionization chamber and radiochromic film agreed within 2.5%. Simplified quality control tests and baseline values were presented in order to check flatness, symmetry, and field size with radiochromic films and output and beam quality constancy with ionization chamber. Short-term reproducibility and MU linearity tests were also included. Commissioning parameters met the requirements of TSET treatments and the matching of AAPM guidelines with the IAEA code of practice was successful. Frequent beam performance controls can be easily performed through the presented quality assurance tests. Radiochromic dosimetry facilitated the TSET commissioning and played a major role to validate the application of TRS-398.

Implementation of water calorimetry in a 180 MeV scanned pulsed proton beam including an experimental determination of kQ for a Farmer chamber

Water calorimetric measurements have been performed in a 180 MeV scanned pulsed proton beam and the absorbed dose determined has been compared with the results obtained using two NE2571 Farmer chambers and the IAEA TRS-398 code of practice. The depth of measurement in water corresponded to a residual range of Rres = 16.5 cm, corresponding to a mean energy of about 150 MeV. Ionization chambers were calibrated in terms of the absorbed dose to water in 60Co at the Swedish Secondary Standard Dosimetry Laboratory, directly traceable to Bureau International des Poids et Mesures. The present experimental investigation has shown that water calorimetry is feasible in a high-energy scanned pulsed proton beam. When comparing the results obtained with water calorimetry and ionometry, the beam quality correction factor, kQ, could be determined for the two NE2571 ionization chambers used. The kQ-factor was found to be 1.032 ± 0.013, which is in good agreement with the factor tabulated in IAEA TRS-398 for this chamber type (1.039 ± 0.018). The present result has also been compared with a previously obtained result in a passively scattered proton beam having similar energy. This comparison yielded a 1.1% deviation, which is not significant considering the combined uncertainties of the two experimental determinations of kQ. The dominating contribution to the combined uncertainty stems from the correction factor for ion recombination in the scanned proton beam (1%), and further studies are required in order to reduce this uncertainty and reveal any possible differences in the kQ-factor between these two proton beam delivery techniques.

The replacement correction factors for cylindrical chambers in high-energy photon beams

The values of the replacement correction factors (Prepl, or in the IAEA's notation pdispcav) used for cylindrical chambers in high-energy photon beams represent one of the most significant differences between the AAPM and the IAEA dosimetry protocols. In a previous study (Wang L L W and Rogers D W O 2008 Med. Phys. 35 1747–55), we found that the AAPM protocol adopted incorrect values of Prepl for cylindrical chambers in photon beams. For a 60Co beam, the calculated Prepl value is 0.5% higher than the AAPM value and about 1% higher than the IAEA value. It was still not clear why the IAEA values, which are based on measurements by Johansson et al, are incorrect. In this study, EGSnrc Monte Carlo simulation codes are used to simulate Johansson et al's experimental procedures for determining Prepl values. The simulation results agree well with the measurements if the chamber responses versus depth are normalized at dmax as was apparently done in the experiments as it was believed that the chambers of different radii gave the same maximum reading at the respective dmax. However, if the chamber responses are not normalized, then the simulated experimental results lead to a result which agrees well with the Prepl values calculated by the standard Monte Carlo methods. This demonstrates that the normalization procedure used in the experiments is incorrect as is based on an incorrect assumption, and thus the interpretation of Johansson et al's experimental values as Prepl (pdis) in the IAEA TRS-398 Code of Practice is wrong. The values of Prepl for cylindrical chambers of different radii in various high-energy photon beams are calculated and an empirical formula is given.

Chamber-quality factors in 60Co for three plane-parallel chambers for the dosimetry of electrons, protons and heavier charged particles: PENELOPE Monte Carlo simulations

The IBA-Scanditronix NACP-02, IBA-Wellhöfer PPC-40 and PPC-05 plane-parallel ionization chambers have been simulated with the Monte Carlo code PENELOPE to obtain their chamber- and quality-dependent factors fc,Qo for a 60Co gamma beam. These are applicable to the determination of kQ beam-quality factors for the dosimetry of electron, protons and heavier charged particles beams based on standards of absorbed dose to water. The factor fc,Q is equivalent to the product sw,airp, but it is not subject to the assumed independence of perturbation factors and stopping power (Sempau et al 2004 Phys. Med. Biol. 49 4427–44). The calculations have been carried out using three different 60Co source models: a monoenergetic point source, a point source with a realistic 60Co spectrum and the simulated phase space from a radiotherapy 60Co unit. Both the detailed geometries of the ionization chambers and of the 60Co unit have been obtained from the manufacturers. In the case of the NACP-02 chamber, values of fc,Qo have been compared with those in the IAEA TRS-398 Code of Practice and from other authors, results being in excellent agreement. The PPC-05 and PPC-40 chambers are of relatively new design, and their values have not been calculated before. Within the estimated uncertainty, computed at the 2σ level (95% confidence limit), the results for each of the three chambers appear to be independent of the degree of sophistication of the 60Co source model used. For the NACP-02 chamber this assumption is justified by the excellent agreement between the various models, which occurs at the level of one standard uncertainty. This suggests the possibility of adopting the mean value of the three source models, weighted with the inverse of their corresponding uncertainties, as a better estimate of fc,Qo. A consequence of the above conclusions is that the estimated uncertainty of kQ beam-quality factors of all charged particles referred to 60Co can potentially be decreased considerably using our approach. For example, the estimated relative standard uncertainty of the denominator of kQ, given in TRS-398 as 1.6% for plane-parallel ionization chambers, can be reduced to 0.06% for a NACP chamber using the mean value of fc,Qo given in this work. Similar reductions could be obtained for the combined standard uncertainty of the kQ beam-quality factors of all charged particles, notably electrons.

Experimental determination of beam quality factors, kQ, for two types of Farmer chamber in a 10 MV photon and a 175 MeV proton beam

Absorbed doses determined with a sealed water calorimeter operated at 4 °C are compared with the results obtained using ionization chambers and the IAEA TRS-398 code of practice in a 10 MV photon beam (TPR20,10 = 0.734) and a 175 MeV proton beam (at a depth corresponding to the residual range, Rres = 14.7 cm). Three NE 2571 and two FC65-G ionization chambers were calibrated in terms of absorbed-dose-to-water in 60Co at the Swedish secondary standard dosimetry laboratory, directly traceable to the BIPM. In the photon beam quality, calorimetry was found to agree with ionometry within 0.3%, confirming the kQ values tabulated in TRS-398. In contrast, a 1.8% deviation was found in the proton beam at 6 g cm−2 depth, suggesting that the TRS-398 tabulated kQ values for these two ionization chamber types are too high. Assuming no perturbation effect in the proton beam for the ionization chambers, a value for (wair/e)Q of 33.6 J C−1 ± 1.7% (k = 1) can be derived from these measurements. An analytical evaluation of the effect from non-elastic nuclear interactions in the ionization chamber wall indicates a perturbation effect of 0.6%. Including this estimated result in the proton beam would increase the determined (wair/e)Q value by the same amount.

CSnrc: Correlated sampling Monte Carlo calculations using EGSnrc

CSnrc, a new user-code for the EGSnrc Monte Carlo system is described. This user-code improves the efficiency when calculating ratios of doses from similar geometries. It uses a correlated sampling variance reduction technique. CSnrc is developed from an existing EGSnrc user-code CAVRZnrc and improves upon the correlated sampling algorithm used in an earlier version of the code written for the EGS4 Monte Carlo system. Improvements over the EGS4 version of the algorithm avoid repetition of sections of particle tracks. The new code includes a rectangular phantom geometry not available in other EGSnrc cylindrical codes. Comparison to CAVRZnrc shows gains in efficiency of up to a factor of 64 for a variety of test geometries when computing the ratio of doses to the cavity for two geometries. CSnrc is well suited to in-phantom calculations and is used to calculate the central electrode correction factor Pcel in high-energy photon and electron beams. Current dosimetry protocols base the value of Pcel on earlier Monte Carlo calculations. The current CSnrc calculations achieve 0.02% statistical uncertainties on Pcel, much lower than those previously published. The current values of Pcel compare well with the values used in dosimetry protocols for photon beams. For electrons beams, CSnrc calculations are reported at the reference depth used in recent protocols and show up to a 0.2% correction for a graphite electrode, a correction currently ignored by dosimetry protocols. The calculations show that for a 1 mm diameter aluminum central electrode, the correction factor differs somewhat from the values used in both the IAEA TRS-398 code of practice and the AAPM's TG-51 protocol.

251 poster workshop Determination of fluence scaling factors for plastic water for high-energy electron beams using IAEA TRS-398 code of practice

251 poster workshop Determination of fluence scaling factors for plastic water for high-energy electron beams using IAEA TRS-398 code of practice

Ionization chamber dosimetry of small photon fields: a Monte Carlo study on stopping-power ratios for radiosurgery and IMRT beams

Absolute dosimetry with ionization chambers of the narrow photon fields used in stereotactic techniques and IMRT beamlets is constrained by lack of electron equilibrium in the radiation field. It is questionable that stopping-power ratio in dosimetry protocols, obtained for broad photon beams and quasi-electron equilibrium conditions, can be used in the dosimetry of narrow fields while keeping the uncertainty at the same level as for the broad beams used in accelerator calibrations. Monte Carlo simulations have been performed for two 6 MV clinical accelerators (Elekta SL-18 and Siemens Mevatron Primus), equipped with radiosurgery applicators and MLC. Narrow circular and Z-shaped on-axis and off-axis fields, as well as broad IMRT configured beams, have been simulated together with reference 10 × 10 cm2 beams. Phase-space data have been used to generate 3D dose distributions which have been compared satisfactorily with experimental profiles (ion chamber, diodes and film). Photon and electron spectra at various depths in water have been calculated, followed by Spencer-Attix (Δ = 10 keV) stopping-power ratio calculations which have been compared to those used in the IAEA TRS-398 code of practice. For water/air and PMMA/air stopping-power ratios, agreements within 0.1% have been obtained for the 10 × 10 cm2 fields. For radiosurgery applicators and narrow MLC beams, the calculated sw,air values agree with the reference within ±0.3%, well within the estimated standard uncertainty of the reference stopping-power ratios (0.5%). Ionization chamber dosimetry of narrow beams at the photon qualities used in this work (6 MV) can therefore be based on stopping-power ratios data in dosimetry protocols. For a modulated 6 MV broad beam used in clinical IMRT, sw,air agrees within 0.1% with the value for 10 × 10 cm2, confirming that at low energies IMRT absolute dosimetry can also be based on data for open reference fields. At higher energies (24 MV) the difference in sw,air was up to 1.1%, indicating that the use of protocol data for narrow beams in such cases is less accurate than at low energies, and detailed calculations of the dosimetry parameters involved should be performed if similar accuracy to that of 6 MV is sought.