IAEA TRS-398 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.

PRECISION AND RELIABILITY: CALIBRATION COEFFICIENTS AND LONG-TERM STABILITY ANALYSIS OF RADIOTHERAPY DOSIMETERS CALIBRATED BY SSDL, NUKLEAR MALAYSIA

One of the main objectives of the Secondary Standard Dosimetry Laboratory (SSDL) in radiotherapy dosimetry is verifying the radiation dose provided to patients is accurate and meets globally recognised standards. Properly calibrating the therapy dosimeters used in the radiotherapy centres is the first step the SSDL must take to ensure this goal is attained. This study analyses the calibration coefficients and long-term stability of therapy dosimeters calibrated by Nuklear Malaysia's SSDL. The dosimeters were calibrated in the absorbed dose to water using 60Co gamma rays, following the procedure described in IAEA TRS No. 398 and IAEA TRS No. 469. Two hundred therapy dosimeters from 33 radiotherapy centres were evaluated for the percentage deviation of the calibration coefficients provided by the SSDL and the manufacturer. Furthermore, the variation in calibration coefficient and the long-term stability of therapy dosimeters from 2011 to 2021 were examined. The percentage deviation of calibration coefficients between the SSDL and manufacturers found that most (82%) user dosimeters were within the IAEA's acceptance limit of ± 1.5%. Overall, the stability of calibration coefficient values ranged between 33.25% and -27.24%, with an average of 0.03%. As predicted, only 15% of the therapy dosimeters fulfill the criteria for long-term stability of 0.5%. In conclusion, proper maintenance and annual calibration of therapy dosimeters are very important to improve accuracy, minimise measurement uncertainty, and thus reduce the likelihood of errors in radiotherapy dosimetry.

PETRA: A pencil beam trimming algorithm for analytical proton therapy dose calculations with the dynamic collimation system.

The Dynamic Collimation System (DCS) has been shown to produce superior treatment plans to uncollimated pencil beam scanning (PBS) proton therapy using an in-house treatment planning system (TPS) designed for research. Clinical implementation of the DCS requires the development and benchmarking of a rigorous dose calculation algorithm that accounts for pencil beam trimming, performs monitor unit calculations to produce deliverable plans at all beam energies, and is ideally implemented with a commercially available TPS. To present an analytical Pencil bEam TRimming Algorithm (PETRA) for the DCS, with and without its range shifter, implemented in the Astroid TPS (.decimal, Sanford, Florida, USA). PETRA was derived by generalizing an existing pencil beam dose calculation model to account for the DCS-specific effects of lateral penumbra blurring due to the nickel trimmers in two different planes, integral depth dose variation due to the trimming process, and the presence and absence of the range shifter. Tuning parameters were introduced to enable agreement between PETRA and a measurement-validated Dynamic Collimation Monte Carlo (DCMC) model of the Miami Cancer Institute's IBA Proteus Plus system equipped with the DCS. Trimmer position, spot position, beam energy, and the presence or absence of a range shifter were all used as variables for the characterization of the model. The model was calibrated for pencil beam monitor unit calculations using procedures specified by International Atomic Energy Agency Technical Report Series 398 (IAEA TRS-398). The integral depth dose curves (IDDs) for energies between 70 MeV and 160 MeV among all simulated trimmer combinations, with and without the ranger shifter, agreed between PETRA and DCMC at the 1%/1mm 1-D gamma criteria for 99.99% of points. For lateral dose profiles, the median 2-D gamma pass rate for all profiles at 1.5%/1.5mm was 99.99% at the water phantom surface, plateau, and Bragg peak depths without the range shifter and at the surface and Bragg peak depths with the range shifter. The minimum 1.5%/1.5mm gamma pass rates for the 2-D profiles at the water phantom surface without and with the range shifter were 98.02% and 97.91%, respectively, and, at the Bragg peak, the minimum pass rates were 97.80% and 97.5%, respectively. The PETRA model for DCS dose calculations was successfully defined and benchmarked for use in a commercially available TPS.

Validation of RayStation Monte Carlo dose calculation algorithm for multiple LINACs

PurposeA photon Monte Carlo (MC) model was commissioned for flattened (FF) and flattening filter free (FFF) 6 MV beam energy. The accuracy of this model, as a single model to be used for three beam matched LINACs, was evaluated. MethodsMultiple models were created in RayStation v.10A for three linacs equipped with Elekta “Agility” collimator. A clinically commissioned collapsed cone (CC) algorithm (GoldCC), a MC model automatically created from the CC algorithm without further optimization (CCtoMC) and an optimized MC model (GoldMC) were compared with measurements. The validation of the model was performed by following the recommendations of IAEA TRS 430 and comprised of basic validation in a water tank, validation in a heterogeneous phantom and validation of complex IMRT/VMAT paradigms using gamma analysis of calculated and measured dose maps in a 2D-Array. ResultsDose calculation with the GoldMC model resulted in a confidence level of 3% for point measurements in water tank and heterogeneous phantom for measurements performed in all three linacs. The same confidence level resulted for GoldCC model. Dose maps presented an agreement for all models on par to each other with γ criteria 2%/2mm. ConclusionsThe GoldMC model showed a good agreement with measured data and is determined to be accurate for clinical use for all three linacs in this study.

Experimental and Monte Carlo-based determination of magnetic field correction factors in high-energy photon fields for two ionization chambers.

The integration of magnetic resonance tomography into clinical linear accelerators provides high-contrast, real-time imaging during treatment and facilitates online-adaptive workflows in radiation therapy treatments. The associated magnetic field also bends the trajectories of charged particles via the Lorentz force, which may alter the dose distribution in a patient or a phantom and affects the dose response of dosimetry detectors. To perform an experimental and Monte Carlo-based determination of correction factors , which correct the response of ion chambers in the presence of external magnetic fields in high-energy photonfields. The response variation of two different types of ion chambers (Sun Nuclear SNC125c and SNC600c) in strong external magnetic fields was investigated experimentally and by Monte Carlo simulations. The experimental data were acquired at the German National Metrology Institute, PTB, using a clinical linear accelerator with a nominal photon energy of 6 MV and an external electromagnet capable of generating magnetic flux densities of up to 1.5 T in opposite directions. The Monte Carlo simulation geometries corresponded to the experimental setup and additionally to the reference conditions of IAEA TRS-398. For the latter, the Monte Carlo simulations were performed with two different photon spectra: the 6 MV spectrum of the linear accelerator used for the experimental data acquisition and a 7MV spectrum of a commercial MRI-linear accelerator. In each simulation geometry, three different orientations of the external magnetic field, the beam direction and the chamber orientation were investigated. Good agreement was achieved between Monte Carlo simulations and measurements with the SNC125c and SNC600c ionization chambers, with a mean deviation of 0.3% and 0.6%, respectively. The magnitude of the correction factor strongly depends on the chamber volume and on the orientation of the chamber axis relative to the external magnetic field and the beam directions. It is greater for the SNC600c chamber with a volume of 0.6cm3 than for the SNC125c chamber with a volume of 0.1 cm3 . When the magnetic field direction and the chamber axis coincide, and they are perpendicular to the beam direction, the ion chambers exhibit a calculated overresponse of less than 0.7(6)% (SNC600c) and 0.3(4)% (SNC125c) at 1.5 T and less than 0.3(0)% (SNC600c) and 0.1(3)% (SNC125c) for 0.35 T for nominal beam energies of 6 MV and 7MV. This chamber orientation should be preferred, as may increase significantly in other chamber orientations. Due to the special geometry of the guard ring, no dead-volume effects have been observed in any orientation studied. The results show an intra-type variation of 0.17% and 0.07% standard uncertainty (k=1) for the SNC125c and SNC600c, respectively. Magnetic field correction factors for two different ion chambers and for typical clinical photon beam qualities were presented and compared with the few data existing in the literature. The correction factors may be applied in clinical reference dosimetry for existing MRI-linearaccelerators.

ACPSEM position paper: dosimetry for magnetic resonance imaging linear accelerators

Consistency and clear guidelines on dosimetry are essential for accurate and precise dosimetry, to ensure the best patient outcomes and to allow direct dose comparison across different centres. Magnetic Resonance Imaging Linac (MRI-linac) systems have recently been introduced to Australasian clinics. This report provides recommendations on reference dosimetry measurements for MRI-linacs on behalf of the Australiasian College of Physical Scientists and Engineers in Medicine (ACPSEM) MRI-linac working group. There are two configurations considered for MRI-linacs, perpendicular and parallel, referring to the relative direction of the magnetic field and radiation beam, with different impacts on dose deposition in a medium. These recommendations focus on ion chambers which are most commonly used in the clinic for reference dosimetry. Water phantoms must be MR safe or conditional and practical limitations on phantom set-up must be considered. Solid phantoms are not advised for reference dosimetry. For reference dosimetry, IAEA TRS-398 recommendations cannot be followed completely due to physical differences between conventional linac and MRI-linac systems. Manufacturers’ advice on reference conditions should be followed. Beam quality specification of TPR20,10 is recommended. The configuration of the central axis of the ion chamber relative to the magnetic field and radiation beam impacts the chamber response and must be considered carefully. Recommended corrections to delivered dose are {k}_{{Q}_{msr}{Q}_{0}}^{{f}_{msr}{f}_{ref}}, a correction for beam quality and {k}_{overrightarrow{B},{Q}_{msr}}^{{f}_{msr}}, for the impact of the magnetic field on dosimeter response in the magnetic field. Literature based values for {k}_{overrightarrow{B},{Q}_{msr}}^{{f}_{msr}} are given. It is important to note that this is a developing field and these recommendations should be used together with a review of current literature.

Measuring and Determining the Output Factor of 6 and 10 MV WFF Photon Beams in Small Square and Rectangular Fields using Semiflex TM 31010 and Pintpoint TM 31014 Ionization Chamber

Research has been carried out on Measuring and Determining the Output Factors of 6 and 10 MV WFF photon beams in Small Square and Rectangular fields using Semiflex TM 31010 and PintPoint TM 31014 Ionization Chamber. This study aims to determine the value of the small field output factor. Measurements were carried out in a water phantom at a depth of 10 cm with a 100 cm SSD technique. The area of ​​the irradiation field used is (2 × 2; 3 × 3; 4 × 4; 2.5 × 1.6; 3.6 × 2.5; 4.5 × 2; and 5 × 3.2) cm2. Field output factor calculations were performed using the IAEA TRS. 483 dosimetry protocol. The results of measurements and calculations obtained for the two detectors used show that a decrease in the value of the output factor occurs when the field size is getting smaller. The output factor value in a small rectangular field looks smaller when compared to the output factor value in a small square field.

Uniformity calibration for large area XY strip parallel plate ionization chamber

Demand for large area parallel plate ionization chamber (PPIC) or large area ionization chamber (LAIC) has risen in recent years due to several advantages of the large effective area in monitoring therapeutic radiation beams. PPICs are designed for the measurements of beam profile and dosimetry in radiation therapy quality assurance (QA) procedures. Objective. Heterogeneous responses over the large sensitive area pose an undeniable concern for the straightforward applications of PPICs in clinical dosimetry. Uniformity calibration for the detector response is thus essential for the accurate performance of each PPIC unit. Approach. A large area XY strip PPIC, characterized by a large effective area of 345.44 × 345.44 mm2 and 256 readout channels, was investigated in this study. A new systematic uniformity calibration is developed to improve the lateral response of the PPIC over the measurements for both narrow beams and large square field beams. A 2D response map of the PPIC was obtained by a spot-scanning method using a compact x-ray tube (mini x-ray). The mini x-ray, providing stable radiation (uncertainty <0.1%), was moved with a step size of 20 mm in 2 dimensions across the entire PPIC surface to collect a complete spot scan. Different uniformity calibration methods were introduced for the measurement of the PPIC by adopting the information from the detector 2D response map. Main results. Deviation of the detector response, before calibration, was observed to reach about 7% for the testing PPIC unit which is much higher than the recommended uniformity response of 1% (IAEA TRS-398). The uniformity response of the PPIC improved significantly to less than 1% across the detector surface after calibration. Significance. The proposed methods enable the practical application of PPIC in routine clinical dosimetry and can be reliably adopted by any radiation facility to perform daily and monthly QA.

Application of a portable primary standard level graphite calorimeter for absolute dosimetry in a clinical low-energy passively scattered proton beam

Objective. A calibration service based on a primary standard calorimeter for the direct determination of absorbed dose for proton beams does not exist. A new Code of Practice (CoP) for reference dosimetry of proton beams is being developed by a working party of the UK Institute of Physics and Engineering in Medicine (IPEM), which will recommend that ionisation chambers are calibrated directly in their clinical beams against the proposed Primary Standard Proton Calorimeter (PSPC) developed at the National Physical Laboratory (NPL). The aim of this work is to report on the use of the NPL PSPC to directly calibrate ionisation chambers in a low-energy passively scattered proton beam following recommendations of the upcoming IPEM CoP. Approach. A comparison between the dose derived using the proposed IPEM CoP and the IAEA TRS-398 protocol was performed, and values were determined experimentally for three types of chambers. In total, 9 plane-parallel and 3 cylindrical chambers were calibrated using the two protocols for two separate visits. Main results. The ratio of absorbed dose to water obtained with the PSPC and with ionisation chambers applying TRS-398 varied between 0.98 and 1.00, depending on the chamber type. The new procedure based on the PSPC provides a significant improvement in uncertainty where absorbed dose to water measured with a user chamber is reported with an uncertainty of 0.9% (1), whereas the TRS-398 protocol reports an uncertainty of 2.0% and 2.3% (1) for cylindrical and plane-parallel chambers, respectively. The values found agree within uncertainties with those from TRS-398 and Monte Carlo calculations. Significance. The establishment of a primary standard calorimeter for the determination of absorbed dose in proton beams combined with the introduction of the associated calibration service following the IPEM recommendations will reduce the uncertainty and improve consistency in the dose delivered to patients.

Measurement of Absorbed Dose in Water Using Co-60 gamma source Teletherapy Units According to IAEA Dosimetry Protocols

Modern radiotherapy facilities like 3-Dimensional conformal radio therapy (3DCRT), Intensity Modulated Radiotherapy (IMRT), were recently suggested in Co-60 machine with Multi-leaf Collimator (MLC). In this study, two reference chambers NE-2571#1205 and NE2581#537 were used for absolute dose measurement in Equinox accelerator. A comparison of dose measurement by two different IAEA protocols TRS-277 and TRS-398 has been studied. Analyzing TRS-398, a common shaped empirical formula was developed for the four Co-60 units of four Medical Colleges in Bangladesh with fitting parameters. It was found that an average discrepancy in the determination of absorbed dose in water among the two different protocols TRS-277 and TRS-398 were 1.33 % for the chamber NE-2571#1205 and 0.65 % for the chamber NE2581#537 with combined uncertainty ±1.59 % (k=1). A good convergence has been obtained in the concepts and methods in this study.

IMPLEMENTATION OF TRS-398 PROTOCOL IN ROUTINE CALIBRATION OF LINAC BY DETERMINATION OF SLAB PHANTOM ON WATER PHANTOM CORRECTION FACTOR

The water phantom is used for LINAC calibration to measure absorbed dose radiation. Practically, it requires a long preparation time and is considered less efficient. To increase efficiency, the medical physics team in a hospital uses slab phantom as the calibration tool. Consequently, the correction factor is crucial to define the equivalence of the absorbed doses resulted from slab phantom. The absorbed dose measurement was performed according to the IAEA TRS-398 dosimetry protocol with a cylindrical ionization chamber detector for 6 MV photon beam and electron beams from Elekta Synergy Platform 154029 LINAC with 6 MeV, 8 MeV, 10 MeV, and 12 MeV energy variations. The field size for slab and water phantom is 30 cm x 30 cm x 30 cm. Based on the TRS-398 protocol, the correction factor of the slab phantom calculated based on absolute dosimetry for 6 MV photons beam, the electron beam of 6 MeV, 8 MeV, 10 MeV, and 12 MeV are 1.0018; 0.9995; 0.9979; 1.0041 and 1.0068, respectively. As a result, the absorbed dose radiation measured by the calibrated slab phantom using the resulted correction factor has an equivalent amount to the water phantom.

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.

Determination of scaling factors for a new plastic phantom at 6–15 MeV electron beams

Water is the recommended reference material for electron-beam dosimetry in protocols for external radiation therapy. However, water increases the difficulty of accurately positioning the ionization-chamber, which complicates the dosimetry process. To avoid such problems, the dosimetry protocol admits plastic phantoms for electron beams with beam qualities R50＜4.0 g cm-2. When using a plastic phantom, the depth scaling factor cpl and the fluence scaling factor hpl must be obtained in advance. Recently, a new plastic phantom with improved uniformity and durability, called Solid Water High Equivalency (SW557), has been developed. The present study determines the cpl and hpl of SW557 at 6–15 MeV electron beam irradiation in Monte Carlo simulations using the EGSnrc toolkit. The determined cpl and hpl values of SW557 were 0.954 and 1.006, respectively. The RMI-457 and WT1 phantoms were also evaluated in the same simulations for comparison. The determined cpl and hpl values of those phantoms by this study agreed within 0.1% and 0.3% of the published values in IAEA TRS-398. As the errors were within statistical uncertainty, the calculation procedure in this study was deemed appropriate. Furthermore, the absorbed doses at the reference depth and reproducible phantom thickness of SW557 with cpl and hpl corrections were in good agreement with the absorbed doses in water. The accurately determined cpl and hpl of SW557 at 6–15 MeV electron beams will enable accurate measurement in clinical use of absorbed dose in water using the SW557.

The Measurement of the Surface Dose in Regular and Small Radiation Therapy Fields Using Cherenkov Imaging.

Purpose: The aim of this study is to measure the output factor (OF) and profile of surface dose in regular and small radiation therapy fields using Cherenkov imaging (CI). Methods: A medical linear accelerator (linac) was employed to generate radiation fields, including regular open photon field (ROPF), regular wedge photon field (RWPF), regular electron field (REF) and small photon field (SPF). The photon beams consisted of two filter modes including flattening filter (FF) and flattening filter free (FFF). All fields were delivered to a solid water phantom. Cherenkov light was captured using a charge-coupled device system during phantom irradiation. The OF and profile of surface dose measured by CI were compared with those determined by film measurement, ionization chamber measurement and treatment planning system calculation in order to examine the feasibility of measuring surface dose OF and profile using CI. Results: The discrepancy between surface dose OF measured by CI and that determined by other methods is less than 6% in ROPFs with size less than 10 × 10 cm2, REFs with size less than 10 × 10 cm2, and SPFs except for 1 × 1 cm2 field. In the flat profile region, the discrepancy between surface dose profile measured by CI and that determined by other methods is less than 4% in REFs and less than 3% in ROPFs, RWPFs, and SPFs except for 1 × 1 cm2 field. The discrepancy of the surface dose profile is in compliance with the recommendation by IAEA TRS 430 reports. The discrepancy between field width measured by CI and that determined by film measurement is equal to or less than 2 mm, which is within the tolerance recommend by the guidelines of linac quality assurance in regular open FF photon fields, SPFs, and REFs with cone size of 10 × 10 cm2 in area. Conclusion: CI can be used to quantitatively measure the OF and profile of surface dose. It is feasible to use CI to measure the surface dose profile and field width in regular open FF photon fields and SPFs except for 1 × 1 cm2 field.

Comparison of Dosimetry Protocols for Electron Beam Radiotherapy Calibrations and Measurement Uncertainties.

This paper presents guidelines for the calibration of radiation beams that were issued by the International Atomic Energy Agency (IAEA TRS 398), the American Association of Physicists in Medicine (AAPM TG 51) and the German task group (DIN 6800-2). These protocols are based on the use of an ionization chamber calibrated in terms of absorbed dose to water in a standard laboratory’s reference quality beam, where the previous protocols were based on air kerma standards. This study aims to determine uncertainties in dosimetry for electron beam radiotherapy using internationally established high-energy radiotherapy beam calibration standards. Methods: Dw was determined in 6-, 12- and 18 MeV electron energies under reference conditions using three cylindrical and two plane-parallel ion chambers in concert with the IAEA TRS 398, AAPM TG 51 and DIN 6800-2 absorbed dose protocols. From mean measured Dw values, the ratio TRS 398/TG 51 was found to vary between 0.988 and 1.004, while for the counterpart TRS 398/DIN 6800-2 and TG 51/DIN 6800-2, the variation ranges were 0.991–1.003 and 0.997–1.005, respectively. For the cylindrical chambers, the relative combined uncertainty (k = 1) in absorbed dose measurements was 1.44%, while for the plane-parallel chambers, it ranged from 1.53 to 1.88%. Conclusions: A high degree of consistency was demonstrated among the three protocols. It is suggested that in the use of the presently determined dose conversion factors across the three protocols, dose intercomparisons can be facilitated between radiotherapy centres.

Technical note: Experimental determination of the effective point of measurement of the PTW-31010 ionization chamber in proton and carbon ion beams.

The accurate knowledge of the effective point of measurement (Peff ) is particularly important for measurements in proximity to high dose gradients such as in the distal fall-off of particle beams. For plane-parallel ionization chambers (ICs), Peff is well known and located at the center of the inner surface of the entrance window. For cylindrical ICs, Peff is shifted from the chamber's center toward the beam source. According to IAEA TRS-398, this shift can be calculated as 0.75·rcyl for light ions with rcyl being the radius of the cavity. For proton beams and in absence of a dose gradient, no shift is recommended. We have experimentally determined Peff for the 0.125cc Semiflex IC in both proton and carbon ion beams. The first method consisted of simultaneous irradiation of a plane-parallel IC and the Semiflex in a 4-cm wide spread-out Bragg peak. In the second method, a single-energy beam was used, and both ICs were positioned successively at the same measurement depths. For both approaches, the shift of the distal edge of the depth ionization distributions recorded by the two chambers at different reference points was used to calculate Peff of the Semiflex. Both methods were applied in carbon ion beams, and only the latter was applied in proton beams. Both methods yielded a similar Peff for carbon ions, 0.88·rcyl , and 0.84·rcyl , which results in a difference of only 0.1mm. The difference to the recommended value of 0.75·rcyl is 0.4 and 0.3mm, respectively, which is larger than the positioning uncertainty. In the proton beam, a Peff of 0.92·rcyl was obtained. The Peff for the 0.125ccSemiflex IC is shifted further from the cavity center as recommended by IAEA TRS-398 for light ions, with the shift for proton beams being even larger than for carbon ion beams.

Comparing the dosimeter-specific corrections for influence quantities of some parallel-plate ionization chambers in conventional electron beam dosimetry

The performance characteristics of some widely employed parallel-plate ionization chambers in dosimetry of conventional high energy electron beams were evaluated and compared in the present study following the recommendations of the IAEA TRS-398 reference dosimetry protocol.Three different types of PTW-made parallel-plate ionization chambers including Roos (TM34001), Markus (TM23343), and Advanced Markus (TM34045) were employed, and correction factors for polarity (kpol), recombination (ks), and quality conversion factor (kQ,Q0) were determined at different nominal electron energies of 4, 6, 9, 12, 16, and 20 MeV produced by a Varian Trilogy clinical Linac. All measurements were performed inside a MP3-M automatic water phantom in the reference condition of 100 cm SSD (source to surface distance), reference measurement depth (zref), and 10 × 10 cm2 field size at the phantom surface.The maximum and minimum range of kpol deviations from unity were respectively found for Markus and Roos ionization chambers. The maximum ks values also belonged to the Markus ionization chamber, while the minimum ks values were observed for the Advanced Markus chamber. The measured ks values through recommendations of the TRS-398 dosimetry protocol were in good accordance with those obtained by Jaffe-plot analysis for all considered ionization chambers. The type of employed ionization chamber can minimally affect the measured electron beam quality index (R50), while it can have a more considerable impact on kQ,Q0 value, especially in the case of the Markus chamber.From the results, it can be concluded that the Roos and Advanced Markus ionization chambers have a superior performance in the case of electron beam dosimetry, although all considered ionization chambers fulfilled the criteria requested by relevant reference dosimetry protocols.

Relative output factors of different collimation systems in truebeam STx medical linear accelerator

The IAEA TRS483 and TRS398 Code of Practices (CoP) were used to calculate relative output factors for small photon beams of 6X, 6XFFF energies shaped by High Definition Multileaf Collimator (HDMLC), jaws and cones mounted on TrueBeam STx medical linear accelerator (Varian Medical Systems), respectively. A comparison between these results were made. The results show a large discrepancy in relative output factor curves found among different collimation systems of the same equivalent field sizes and between the CoPs. Therefore, the specific beam modelling in treatment planning system for each type of the collimation system to be used for small fields maybe required for better computational accuracy.