Absorbed Dose Calibration Factors Research Articles

Output dose investigation and analysis of external beam radiotherapy in Beijing

Objective To study the output dose of external beam radiotherapy in Beijing and analyze the cause of output dose deviation. Guidance is given on the selection of measurement methods, calculation formulas and parameters. Methods During 2020, the output doses of medical electron accelerators, tomotherapy, cyberknife, and γ-knife were measured based on the water absorbed dose calibration factor combined with the requirements in the IAEA TRS398 and TRS483 reports, and low-energy X-rays were measured based on the air kerma calibration factor combined with the requirements in the AAPM protocal. The output dose was measured and compared with the preset irradiation value. Results A total of 59 external beam radiation therapy equipments’s output dose were investigated. 2 of the 49 accelerators’s absolute value of relative deviation between the system indication value and the measurement exceeded 3%; the deviation of other external beam radiotherapy equipments was less than 3%. Conclusion A unified method and multi-party quality control can ensure the accuracy of the output dose. In the routine operation of the equipment, the hospital should strictly implement periodic inspections to ensure accurate and reliable radiotherapy. 摘要： 目的 调查北京地方所属外照射放射治疗设备输出剂量的情况, 分析输出剂量相对偏差原因, 明确测量方法、 计算公式及参数的选取。 方法 2020 年期间, 采用水中吸收剂量校准因子结合国际原子能机构 (IAEA) TRS398 和 TRS483 号报告中的要求对医用电子加速器、螺旋断层放射治疗装置、机械臂放射治疗装置、 γ 射线立体定向放射治疗 系统的输出剂量进行测量, 采用空气比释动能校准因子结合 AAPM 方案中的要求对低能 X 射线治疗机的输出剂量进 行测量, 并与预置照射值进行比较分析输出剂量相对偏差。 结果共调查了 59 台外照射放射治疗设备的输出剂量, 49 台医用电子加速器中 2 台输出剂量相对偏差绝对值超过 3%; 其他外照射放射治疗设备的输出剂量相对偏差绝对值 均小于 3%。 结论 统一明确的测量方法和多方的质量控制检测可确保放射治疗设备输出剂量的准确, 在设备常规运 行中医院应严格执行周期性检验保证实施精确可靠的放射治疗。

画像誘導放射線治療におけるkV-cone beam CTの被ばく線量の評価

Image-guided radiotherapy (IGRT) is an increasingly commonly adopted technique. As a result, however, total patient dose is increasing rapidly, especially when kV-cone beam computed tomography (CBCT) is applied. This study investigated the dosimetry of kV-CBCT using a Farmer ionization chamber with a (60)Co absorbed-dose calibration factor. The absorbed-dose measurements were performed using an I'mRT phantom (RW3, IBA) which is employed for dose verification of intensity-modulated radiotherapy (IMRT). The I'mRT phantom was used as a substitute for head and pelvis phantoms. The kV-CBCT absorbed dose was evaluated from a beam quality conversion factor of kV to (60)Co and the ionization ratio of the I'mRT phantom and water, calculated using the Monte Carlo method. The dose distribution in the I'mRT phantom was also measured using a radiophotoluminescent glass dosimeter (RGD). The absorbed doses for the pelvis phantom (full scan) ranged from 2.5-4 cGy for kV-CBCT and 4-8 cGy for MV-CBCT. TomoTherapy resulted in a lower dose of approximately 1.3 cGy due to fan-beam. For the head phantom (half scan), the doses ranged from 0.1-0.7 cGy for kV-CBCT and 3-5 cGy for MVCBCT. The results for RGD were similar to ion chamber measurements. It is necessary to decrease the absorbed dose of the organs at risk every time IGRT is applied.

SU-FF-T-257: Gamma Knife Absolute Dose Rate Calibration Using the AAPM TG-51 Protocol and Comparison with the AAPM TG-21 Protocol

Purpose: In 1999 the American Association of Physicists in Medicine published a new calibration protocol for high energyphoton and electron dosimetry, the Task Group 51 protocol, which has, for the most part, replaced the former Task Group 21 protocol. However, the AAPM Task Group 21 protocol continues to be widely used for the absolute dose calibration of Gamma Knife units. The TG‐51 protocol is based on an absorbed dose calibration factor, whereas the TG‐21 protocol is based on an air kerma factor. The purpose of this study is to compare the TG‐51 and TG‐21 protocols for absorbed dose calibration of the Leksell Gamma Knife. Method and Materials: An ionization chamber was placed in the center of a spherical water phantom of 10 cm in diameter and measurements necessary for application of the TG‐51 protocol were taken. This data was used, following TG‐51 Worksheet A, to calculate the dose rate to water at 5 cm depth. A dose rate measurement was also performed following the TG‐21 protocol using the standard Elekta polystyrene phantom. Results: The dose rate measured using the TG‐21 protocol was 2.412 Gy/min at 8 cm depth and, using an exponential attenuation approximation (μ = 0.0063 mm−1)5 for the attenuation of the pencil beam radiation from the Gamma Knife, the dose rate measured at 5 cm using the TG‐51 protocol equates to a value of 2.435 Gy/min at 8 cm depth. Conclusion: The ratio of dose rate calculated using the TG‐51 protocol to that measured using the TG‐21 protocol was 1.01. The estimated overall uncertainty in each calibration result is less than 2%. Since the same ion chamber was used in both calibrations, the uncertainty in the comparison between the two calibration results is estimated to be 1%.

WE‐C‐T‐617‐02: Direct Calibration of Ionization Chambers in Linac Photon Beams

Purpose: Currently, the starting point for dosimetry in the radiotherapy clinic is an ion chamber calibrated at the standards laboratory in a beam. Conversion factors obtained from a protocol such as TG‐51 are then required to derive the dose in a linac beam. This paper details the measurement of these kQ factors in photon beams from a clinical linac. Method and Materials: The NRC primary standard water calorimeter was used to calibrate a set of NE2571 Farmer‐type ion chambers in 6, 10 & 25 MV photon beams from an Elekta Precise linac installed at NRC. A number of influence quantities — water purity, temperature sensors, temperature measurement system — were investigated in detail. In addition, significant effort was put into monitoring the stability of the linac to ensure that output variations did not significantly affect the measurements. Results: The kQ factors obtained were in good agreement with previous measurements using the NRC Vickers research linac reported by Seuntjens et al in 2000. This indicates no dependence on accelerator type and validates the use of %dd10x as a beam quality specifier for megavoltage photon beams. The calculated factors given in TG‐51 also agree well with these measured kQ factors. The standard uncertainty in the calibration of an ion chamber is estimated to be 0.45%, a significant improvement over using the calculated values. Conclusion: A clinical linac can be successfully used for primary standards dosimetry. Absorbed dose calibration factors for a set of NE2571 chambers have been obtained for 6, 10 & 25 MV photon beams. Factors for other chambers can be determined by direct comparison with these reference chambers in a water phantom.

112. The role of the secondary standard dosimetry laboratory in the quality assurance in radiotherapy in poland

In Poland, in 1937, a special measurement unit was established in the Radium Institute (former name of the Institute of Oncology). The continuation of this unit is the SSDL, which was established in 1966 at the Medical Physics Department (MPD) of the Institute of Oncology. In 1988, the Laboratory was approved as a member of the IAEA/WHO SSDL network, and is periodically audited by the IAEA. The SSDL plays an important role in maintaining proper dosimetry standards in radiotherapy in Poland. The main activities of the SSDL are: - Collecting data concerning the intrastructure of radiotherapy in Poland. This activity started in 1993 and is continued on the yearly basis. - Calibration of dosimeters at the national level; it started as early as 1937. The SSDL calibrates over 20 ionisation chambers each year. The Co-60 exposure calibration factors are established, and then the air-kerma and the absorbed dose calibration factors are calculated. The system linearity and energy characteristics are determined in the orthovoltage range. In 2002 the measurement facility for calibrating the mammography dosimeters was set-up. - External quality audits of dosimetry in radiotherapy centres in Poland. During the period 1991–2001, 151 audits, based on TLD dose inter-comparisons were performed in reference conditions. The deviations above 3.5% (detined as acceptance level) were observed in 18 cases (nearly 12%). Since 2002 the audits in more complex, non standard conditions started. - Preparation of protocols and recommendations on dosimetry in radiotherapy. The QC recommendations for Co-60 units and accelerators were issued (brachytherapy has been taken care of by the Centre of Oncology in Cracow). The QC- recommendations for simulators and CT-scanners are in preparation. Training of physicists and radiotherapists (including the participants from other East European countries) in order to adhere to the increasing complexity of modern radiotherapy procedures. Trainings are performed individually or in a form of courses. This activity follows the recommendations of the Euroatom Directive 97/43 on health protection of individuals against the dangers of ionizing radiation in relation to medical exposure.

Evaluation of factors to convert absorbed dose calibrations from graphite to water for the NPL high-energy photon calibration service

The National Physical Laboratory (NPL) provides a high-energy photon calibration service using 4–19 MV x-rays and 60Co γ-radiation for secondary standard dosemeters in terms of absorbed dose to water. The primary standard used for this service is a graphite calorimeter and so absorbed dose calibrations must be converted from graphite to water. The conversion factors currently in use were determined prior to the launch of this service in 1988. Since then, it has been found that the differences in inherent filtration between the NPL LINAC and typical clinical machines are large enough to affect absorbed dose calibrations and, since 1992, calibrations have been performed in heavily filtered qualities. The conversion factors for heavily filtered qualities were determined by interpolation and extrapolation of lightly filtered results as a function of tissue phantom ratio 20,10 (TPR20,10). This paper aims to evaluate these factors for all mega-voltage photon energies provided by the NPL LINAC for both lightly and heavily filtered qualities and for 60Co γ-radiation in two ways. The first method involves the use of the photon fluence-scaling theorem. This states that if two blocks of different material are irradiated by the same photon beam, and if all dimensions are scaled in the inverse ratio of the electron densities of the two media, then, assuming that all photon interactions occur by Compton scatter the photon attenuation and scatter factors at corresponding scaled points of measurement in the phantom will be identical. The second method involves making in-phantom measurements of chamber response at a constant target–chamber distance. Monte Carlo techniques are then used to determine the corresponding dose to the medium in order to determine the chamber calibration factor directly. Values of the ratio of absorbed dose calibration factors in water and in graphite determined in these two ways agree with each other to within 0.2% (1σ uncertainty). The best fit to both sets of results agrees with values determined in previous work to within 0.3% (1σ uncertainty). It is found that the conversion factor is not sensitive to beam filtration.

Determination of absorbed dose calibration factors for therapy levelelectron beam ionization chambers

Over several years the National Physical Laboratory (NPL) has been developingan absorbed dose calibration service for electron beam radiotherapy. To testthis service, a number of trial calibrations of therapy level electron beamionization chambers have been carried out during the last 3 years. Thesetrials involved 17 UK radiotherapy centres supplying a total of 46 chambers ofthe NACP, Markus, Roos and Farmer types. Calibration factors were derived fromthe primary standard calorimeter at seven energies in the range 4 to 19 MeVwith an estimated uncertainty of ±1.5% at the 95% confidence level.Investigations were also carried out into chamber perturbation, polarityeffects, ion recombination and repeatability of the calibration process. Theinstruments were returned to the radiotherapy centres for measurements to becarried out comparing the NPL direct calibration with the 1996 IPEMB air kermabased Code of Practice.It was found that, in general, all chambers of a particular type showed thesame energy response. However, it was found that polarity and recombinationcorrections were quite variable for Markus chambers - differences in thepolarity correction of up to 1% were seen. Perturbation corrections wereobtained and were found to agree well with the standard data used in the IPEMBCode. The results of the comparison between the NPL calibration and IPEMB Codeshow agreement between the two methods at the ±1% level for the NACPand Farmer chambers, but there is a significant difference for the Markuschambers of around 2%. This difference between chamber types is most likelyto be due to the design of the Markus chamber.

Absorbed-dose beam quality conversion factors for cylindrical chambers in high energy photon beams.

Recent working groups of the AAPM [Almond et al., Med. Phys. 26, 1847 (1999)] and the IAEA (Andreo et al., Draft V.7 of "An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water," IAEA, 2000) have described guidelines to base reference dosimetry of high energy photon beams on absorbed dose to water standards. In these protocols use is made of the absorbed-dose beam quality conversion factor, kQ which scales an absorbed-dose calibration factor at the reference quality 60Co to a quality Q, and which is calculated based on state-of-the-art ion chamber theory and data. In this paper we present the measurement and analysis of beam quality conversion factors kQ for cylindrical chambers in high-energy photon beams. At least three chambers of six different types were calibrated against the Canadian primary standard for absorbed dose based on a sealed water calorimeter at 60Co [TPR10(20)=0.572, %dd(10)x=58.4], 10 MV [TPR10(20)=0.682, %dd(10)x=69.6), 20 MV (TPR10(20)=0.758, %dd(10)x= 80.5] and 30 MV [TPR10(20) = 0.794, %dd(10)x= 88.4]. The uncertainty on the calorimetric determination of kQ for a single chamber is typically 0.36% and the overall 1sigma uncertainty on a set of chambers of the same type is typically 0.45%. The maximum deviation between a measured kQ and the TG-51 protocol value is 0.8%. The overall rms deviation between measurement and the TG-51 values, based on 20 chambers at the three energies, is 0.41%. When the effect of a 1 mm PMMA waterproofing sleeve is taken into account in the calculations, the maximum deviation is 1.1% and the overall rms deviation between measurement and calculation 0.48%. When the beam is specified using TPR10(20), and measurements are compared with kQ values calculated using the version of TG-21 with corrected formalism and data, differences are up to 1.6% when no sleeve corrections are taken into account. For the NE2571 and the NE2611A chamber types, for which the most literature data are available, using %dd(10)x, all published data show a spread of 0.4% and 0.6%, respectively, over the entire measurement range, compared to spreads of up to 1.1% for both chambers when the kQ values are expressed as a function of TPR10(20). For the PR06-C chamber no clear preference of beam quality specifier could be identified. When comparing the differences of our kQ measurements and calculations with an analysis in terms of air-kerma protocols with the same underlying calculations but expressed in terms of a compound conversion factor CQ, we observe that a system making use of absorbed-dose calibrations and calculated kQ values, is more accurate than a system based on air-kerma calibrations in combination with calculated CQ (rms deviation of 0.48% versus 0.67%, respectively).

Absorbed dose beam quality correction factors for the NE2571 chamber in a 5 MV and a 10 MV photon beam

Dose to water determination in clinical high-energy photon beams with ionization chambers calibrated in terms of absorbed dose to water has been proposed as an alternative to ionization chamber dosimetry based on air kerma calibrations. in the clinical beam is derived using a factor that scales the absorbed dose calibration factor in the reference beam to the absorbed dose calibration factor in the user beam. In the present study values were determined for the NE2571 chamber in a 5 MV and a 10 MV high-energy photon beam generated at the 15 MeV high-intensity electron linac of the University of Gent. A set of three NE2571 chambers was calibrated relative to the Gent sealed water calorimeter both in and in the linac beam at a depth of 5 cm and a source to detector distance of 100 cm. Two high-purity chemical water systems were used in the detection vessel of the calorimeter, -saturated and Ar-saturated pure water, which are both supposed to give a zero heat defect. and %dd(10) have been evaluated as beam quality specifiers. Simulations using the BEAM/DOSXYZ Monte Carlo system were performed to evaluate potential corrections on the measured beam qualities. The average values measured for the three NE2571 chambers in the 5 MV and 10 MV photon beams are and respectively. For the three chambers used, the maximum deviation of individual values is 0.2%. The measured beam quality specifiers %dd(10) and are 67.0 and 0.705 for the 5 MV beam and 75.0 and 0.759 for the 10 MV beam. Although our beam design is very different from those used by other investigators for the measurement of values, the agreement with their results is satisfactory showing a slightly better agreement when %dd(10) is used as the beam quality specifier.

A new approach to electron-beam reference dosimetry.

A new approach is proposed for electron-beam dosimetry under reference conditions and data necessary to use this approach are presented. The approach has the following features; it uses ion chambers and starts from an absorbed-dose calibration factor for 60Co to be consistent with the present proposal for the new AAPM photon-beam protocol; it uses R50 to specify the beam quality and the reference depth, dref = 0.6R50 - 0.1 (all quantities in cm), recommended by Burns et al. [Med. Phys. 23, 383-388 (1996)]; it has a formalism which is parallel to the kQ formalism for photon-beam dosimetry; it fully accounts for the impact on stopping-power ratios of realistic electron beams; it allows an easy transition to using primary standards for absorbed dose to water in electron beams when these are available. The equation for dose to water under reference conditions is; DWQ = MPionPgrQk'R50kecalND,w60Co. The term PgrQ is not needed with plane-parallel chambers but corrects for gradient effects with cylindrical chambers and is measured in the user's beam. The parameter kecal is associated with converting the 60Co absorbed-dose calibration factor into one for an electron beam of quality Qe and contains most of the chamber to chamber variation. Calculated values of kecal are presented as well as Monte Carlo calculated Pwall values for plane-parallel chambers in a water phantom irradiated by a 60Co beam since these are needed to calculate kecal. The factor k'R50 is a function of R50 and converts the absorbed-dose calibration factor to that for the electron-beam quality of interest. Two analytical expressions are presented which are close to universal expressions for all cylindrical Farmer-like chambers and for well-guarded plane-parallel chambers respectively. Calculated values are presented graphically for electron beams with energies between 5 and 50 MeV.

A comparison of absorbed dose standards for high-energy X-rays

An indirect comparison of the absorbed dose to water standards of the PTB and the NRC was carried out for 18/20 MV X-rays using five ionization chambers as transfer instruments. The absorbed dose standard of the PTB is based on the total absorption of 5.6 MeV electrons in Fricke solution. The NRC standard uses Fricke solution whose calibration is based on measurements made with a water calorimeter and a calculation of the heat defect. For high-energy X-rays, the difference found between the standards of 0.4% is within the expected uncertainties. The comparison was linked to existing 60Co absorbed dose to water and air kerma standards by comparing measurements of those quantities at the PTB, NRC and BIPM. Agreement was better than 0.7% in all cases. For the ionization chambers used in this study, the absorbed dose calibration factors for 18/20 MV X-rays are about 2% lower than those for 60Co gamma-rays.

The advantages of absorbed-dose calibration factors.

A formalism for clinical external beam dosimetry based on use of ion chamber absorbed-dose calibration factors is outlined in the context and notation of the AAPM TG-21 protocol. It is shown that basing clinical dosimetry on absorbed-dose calibration factors ND leads to considerable simplification and reduced uncertainty in dose measurement. In keeping with a protocol which is used in Germany, a quantity kQ is defined which relates an absorbed-dose calibration factor in a beam of quality Q0 to that in a beam of quality Q. For 38 cylindrical ion chambers, two sets of values are presented for ND/NX and Ngas/ND and for kQ for photon beams with beam quality specified by the TPR20(10) ratio. One set is based on TG-21's protocol to allow the new formalism to be used while maintaining equivalence to the TG-21 protocol. To demonstrate the magnitude of the overall error in the TG-21 protocol, the other set uses corrected versions of the TG-21 equations and the more consistent physical data of the IAEA Code of Practice. Comparisons are made to procedures based on air-kerma or exposure calibration factors and it is shown that accuracy and simplicity are gained by avoiding the determination of Ngas from NX. It is also shown that the kQ approach simplifies the use of plastic phantoms in photon beams since kQ values change by less than 0.6% compared to those in water although an overall correction factor of 0.973 is needed to go from absorbed dose in water calibration factors to those in PMMA or polystyrene. Values of kQ calculated using the IAEA Code of Practice are presented but are shown to be anomalous because of the way the effective point of measurement changes for 60Co beams. In photon beams the major difference between the IAEA Code of Practice and the corrected AAPM TG-21 protocol is shown to be the Prepl correction factor. Calculated kQ curves and three parameter equations for them are presented for each wall material and are shown to represent accurately the kQ curve for all ion chambers in this study with a wall of that specified material and a thickness less than 0.25 g/cm2. Values of kQ can be measured using the primary standards for absorbed dose in photon beams.

The role of humidity and other correction factors in the AAPM TG-21 dosimetry protocol.

A detailed derivation is presented of the formulas required to determine Ngas and Dmed in the AAPM TG-21 dosimetry protocol. This protocol specifies how to determine the absorbed dose in an electron or photon beam when using exposure or absorbed dose calibrated ion chambers. It is shown that the expression given in TG-21's recent letter of clarification is incorrect. Accounting for humidity correctly increases, by 0.4%, all absorbed dose determinations using an exposure calibrated ion chamber. Taking into account other correction factors in the equation for exposure could also have varying, but significant effects (possibly over 1%). These are the stem scatter correction, the axial nonuniformity correction and the electrode correction for electrodes made of different materials from the wall. Attention is drawn to differences in the definitions of the exposure and absorbed dose calibration factors, Nx and ND, respectively, as supplied by the NBS and the NRCC.