Code Of Practice For Dosimetry Research Articles

Feasibility study of shielded diode detector for small field dosimetry.

Improved imaging techniques and modern radiotherapy treatment delivery in the treatment field are reduced to the precise size of the tumor, which necessitates the need for small-field dosimetry. Dosimetry in small-field dosimetry is challenging because most of the available code of practice for dosimetry is based on the cavity theory concept. Some small-sized detectors show good spatial resolution and sensitivity. Of the available small detectors, the diamond detector's performance is remarkably good. Most of the centers for radiotherapy lack diamond detectors. In this situation, if a diode detector is available, we can use it for small-field dosimetry by applying the Daisy Chaining method correction methods. In this study, the diode detector's response is not over-responding because of the defective diode. So this diode cannot be used for further measurements, and we have to regularly check the performance of the diode before using it for measurements.

Monte Carlo calculation of beam quality correction factors for PTW cylindrical ionization chambers in photon beams

The beam quality correction factor kQ for megavoltage photon beams has been calculated for eight PTW (Freiburg, Germany) ionization chambers (Farmer chambers PTW30010, PTW30011, PTW30012, and PTW30013, Semiflex 3D chambers PTW31021, PTW31010, and PTW31013, and the PinPoint 3D chamber PTW31016). Simulations performed on the widely used NE-2571 ionization chamber have been used to benchmark the results. The Monte Carlo code PENELOPE/penEasy was used to calculate the absorbed dose to a point in water and the absorbed dose to the active air volume of the chambers for photon beams in the range 4 to 24 MV. Of the nine ionization chambers analysed, only five are included in the current version of the International Code of Practice for dosimetry based on standards of absorbed dose to water (IAEA TRS 398). The values reported in this work agree with those in the literature within the uncertainty estimates and are to be included in the average values of the data obtained by different working groups for the forthcoming update of TRS 398.

Extending the IAEA-AAPM TRS-483 methodology for radiation therapy machines with field sizes down to 10 × 2 cm2.

The purpose of this study is to provide a calibration methodology for radiation therapy machines where the closest field to the conventional reference field may not meet the lateral charged particle equilibrium (LCPE) condition of the machine-specific reference (msr) field. We provided two methodologies by extending the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) TRS-483 code of practice (COP) (Palmans etal. TRS-483: Dosimetry of small static fields used in external beam radiotherapy: an international code of practice for reference and relative dose determination; 2017) methodology for the calibration of radiation therapy machines with 6MV flattening filter free (FFF) beam and with field sizes down to 10× 2cm2 . Two methods of calibration were provided following the TRS-483. In calibration Method I, the generic correction factors were calculated using Monte Carlo (MC) for seven detectors and rectangular physical field sizes ranging from 10×2cm2 to 10×10cm2 . In calibration Method II, we extended the methodology in TRS-483 for deriving the equivalent square msr field sizes for rectangular field sizes down to 10×2cm2 . The beam quality specifier for a hypothetical 10×10cm2 field was derived by extending the methodology provided in the TRS-483. Since the beam quality correction values for the conventional reference field ( ) tabulated in TRS-483 are provided only for large reference chambers, we calculated the values analytically for our beam quality specifier and chambers used, using interaction data in TRS-398 (Andreo, etal. TRS-398: Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water; 2001). The correction values calculated using the first method for chambers with an electrode made of C552 almost did not vary across the different field sizes studied (within 0.1%) while it varied by 1.6% for IBA CC01 with electrode made of steel. Extending the equivalent field and beam quality specifier determination methodology of TRS-483 resulted in a maximum error of 1.3% on the beam quality specifier for the 2×2cm2 field size. However, this had a negligible impact on the values (less than 0.1%). For chambers with C552 and Al electrode material, the correction factors determined using the two methods of calibration were in agreement to within 0.5%. However, for the chambers with electrode made of higher atomic number (Z), the difference between the two methodologies could be as large as 1.5%. It was shown that this difference can be reduced to less than 0.5% if central electrode perturbation effects and values introduced in TRS-483 were taken into account. In this study, applying the correction values calculated using the calibration Method I to the chamber reading improved the consistency on an absorbed dose determination from 0.5% to 0.1% standard deviation (except for the Exradin A16). For this reason we recommend using calibration Method I. If the values are not available for the user's detector, calibration Method II can be used to predict the correction factors. However, the second methodology should not be used for chambers with electrode made of high-Z material unless the electrode perturbation effects and values are taken into account.

A dosimetric evaluation of the IAEA-AAPM TRS483 code of practice for dosimetry of small static fields used in conventional linac beams and comparison with IAEA TRS-398, AAPM TG51, and TG51 Addendum protocols.

The International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) have jointly published a new code of practice (CoP), TRS483, for the dosimetry of small static photon fields used in external beam radiotherapy. It gave recommendations on how to perform reference dosimetry in nonstandard machine-specific reference (msr) fields and measure field output factors in small fields. The purpose of this work was to perform a dosimetric evaluation of the recommendations given in this CoP. All measurements were done in a Varian TrueBeam™ STx linear accelerator. Five ionization chambers were used for beam quality measurements, four Farmer type ionization chambers for performing reference dosimetry and two diodes for performing field output factor measurements. Field output factor measurements were done for fourteen field sizes (ranging from 0.5 cm × 0.5 cm to 10 cm × 10 cm). Beam energies used were: 6 MV WFF, 6 MV FFF, 10 MV WFF, and 10 MV FFF. Where appropriate, results from this study were compared with those obtained from the recommendations given in the IAEA TRS398 CoP, AAPM TG51 and TG51 Addendum protocols. Beam quality measurements show that for all beam energies and for equivalent square msr field sizes ranging from 4 cm × 4 cm to 10 cm × 10 cm, agreement between calculated and measured values of TPR20,10 (10) was within 0.6%. When %dd(10,10)X was used as beam quality specifier, the agreement was found to be within 0.8%. Absorbed dose to water per unit monitor unit at the depth of maximum dose zmax in a beam of quality Q, Dw,Qzmax/MU, was determined using both %dd(10,10)X and TPR20,10 (10) as beam quality specifiers. Measured ratios of Dw,Q (zmax )/MU, determined using the two approaches, ranged between 0.999 and 1.000 for all the beam energies investigated. Comparison with TRS398, TG51 and TG51 addendum protocols show that depending on beam energy, the mean values of the ratios TRS398/TRS483, TG51/TRS483, and TG51 Addendum/TRS483 of Dw,Q (zmax )/MU determined using both approaches show excellent agreement with TRS398 CoP (to within 0.05%); agreement with TG51 and TG51 addendum was to within 0.3% for all four beam energies investigated. Field output factors, determined using the two methods recommended in the TRS483 CoP, showed excellent agreement between the two methods. For the 1 cm collimator field size, the mean value of the field output factor obtained from an average of the two detectors investigated was found to be 2% lower than the mean value of the uncorrected ratio of readings. For beams with and without flattening filters, the values of Dw,Q (zmax )/MU obtained following the new CoP are found to be consistent with those obtained using TRS398, TG51 and TG51 addendum protocols to within 0.3%. Field output factors for small beams can be improved when the correction factors for different detectors included in TRS483 are appropriately incorporated into their dosimetry.

Assessment of patient dose and optimization levels in chest and abdomen CR examinations at referral hospitals in Tanzania.

The aim of this study was to evaluate the radiation doses to patient during chest and abdomen CR examinations, and assess the related level of optimization at five referral hospitals in Tanzania. The international code of practice for dosimetry in diagnostic radiology was applied to determine the entrance surface air kerma (ESAK) to patients. The level of optimization was assessed from low‐contrast objects scores of phantom images at different exposures. The results show that mean ESAK varied from 0.16 to 0.37 mGy for chest PA and from 2 to 6 mGy for abdomen AP. Assuming similar patient and phantom attenuations, the optimization performed at all facilities was consistent with phantom evaluations in terms of tube potential settings in use. However, all facilities seemed to operate at higher tube load values above 5 mAs for chest examination, which can lead to unnecessary patient doses. Inadequate initial training on CR technology explains in large proportion the inappropriate use of exposure parameters.PACS numbers: 87.50.up, 87.59bd

TH‐A‐213‐02: IAEA/AAPM Code of Practice for the Dosimetry of Static Small Photon Fields

Modern radiotherapy techniques such as SRS/SRT, SBRT, IMRT, VMAT as well as specialized machines such as Tomotherapy, CyberKnife and Gamma Knife use small photon fields with at least one dimension <3 cm. Dosimetry in such small fields is challenging because of large detector perturbations due to non‐equilibrium conditions, occlusion of the primary photon source and large dose gradients across the field. Many small‐size dosimeters have been proposed for use in small fields. However, their characteristics especially the volume averaging and fluence perturbations have only recently been adequately understood. An IAEA‐AAPM working group has provided a framework for reference dosimetry in non‐compliant beams and the measurement of field output factors small fields (1). The AAPM TG‐155 (2) has adopted this framework to provide guidelines on relative dosimetry. This course explains the code of practice for absolute dosimetry that is under review and discusses the availability of correction factors to convert detector readings to doses. TG‐155 defines small field conditions, provides recommendations for suitable detectors and recommendations for good working practice for relative dosimetry (PDD, TMR, output factor, etc.) and dose calculations based on the new formulation. It also discusses beam modeling and dose calculations as a critical step in clinical utilization of small field radiotherapy. Alfonso et al, Med Phys 35, 5179–5186 (2008). Das et al, Med Phys (under review, 2014) Learning Objectives: Concepts and recommended procedures in the IAEA‐AAPM code of practice for dosimetry of small fields. Physics of dosimetry in non‐equilibrium conditions and definition of small fields Recommended procedures for relative dosimetry in TG‐1554. Choice of detectors for small field dosimetry, perturbations and corrections for dosimetry Understand the detector properties required for small field measurements Understand the advantages and limitations of commercially available detectors for small field dosimetry Research supported with operating grants from CIHR, NSERC, and BIOWin, a program funded at the Universite Catholique Louvain by the Walloon Government (Belgium). Student stipends supported by Medical Physics Research Training Network funded by the Collaborative Research and Training Experience program of the NSERC.Seuntjens: support by Sun Nuclear Corporation

TH‐A‐213‐00: Small Field Dosimetry: Overview of AAPM TG‐155 and the IAEAAAPM Code of Practice

Modern radiotherapy techniques such as SRS/SRT, SBRT, IMRT, VMAT as well as specialized machines such as Tomotherapy, CyberKnife and Gamma Knife use small photon fields with at least one dimension <3 cm. Dosimetry in such small fields is challenging because of large detector perturbations due to non‐equilibrium conditions, occlusion of the primary photon source and large dose gradients across the field. Many small‐size dosimeters have been proposed for use in small fields. However, their characteristics especially the volume averaging and fluence perturbations have only recently been adequately understood. An IAEA‐AAPM working group has provided a framework for reference dosimetry in non‐compliant beams and the measurement of field output factors small fields (1). The AAPM TG‐155 (2) has adopted this framework to provide guidelines on relative dosimetry. This course explains the code of practice for absolute dosimetry that is under review and discusses the availability of correction factors to convert detector readings to doses. TG‐155 defines small field conditions, provides recommendations for suitable detectors and recommendations for good working practice for relative dosimetry (PDD, TMR, output factor, etc.) and dose calculations based on the new formulation. It also discusses beam modeling and dose calculations as a critical step in clinical utilization of small field radiotherapy. Alfonso et al, Med Phys 35, 5179–5186 (2008). Das et al, Med Phys (under review, 2014) Learning Objectives: Concepts and recommended procedures in the IAEA‐AAPM code of practice for dosimetry of small fields. Physics of dosimetry in non‐equilibrium conditions and definition of small fields Recommended procedures for relative dosimetry in TG‐1554. Choice of detectors for small field dosimetry, perturbations and corrections for dosimetry Understand the detector properties required for small field measurements Understand the advantages and limitations of commercially available detectors for small field dosimetry Research supported with operating grants from CIHR, NSERC, and BIOWin, a program funded at the Universite Catholique Louvain by the Walloon Government (Belgium). Student stipends supported by Medical Physics Research Training Network funded by the Collaborative Research and Training Experience program of the NSERC.Seuntjens: support by Sun Nuclear Corporation

TH‐A‐213‐01: Small Field Dosimetry: Overview of AAPM TG‐155

Modern radiotherapy techniques such as SRS/SRT, SBRT, IMRT, VMAT as well as specialized machines such as Tomotherapy, CyberKnife and Gamma Knife use small photon fields with at least one dimension <3 cm. Dosimetry in such small fields is challenging because of large detector perturbations due to non‐equilibrium conditions, occlusion of the primary photon source and large dose gradients across the field. Many small‐size dosimeters have been proposed for use in small fields. However, their characteristics especially the volume averaging and fluence perturbations have only recently been adequately understood. An IAEA‐AAPM working group has provided a framework for reference dosimetry in non‐compliant beams and the measurement of field output factors small fields (1). The AAPM TG‐155 (2) has adopted this framework to provide guidelines on relative dosimetry. This course explains the code of practice for absolute dosimetry that is under review and discusses the availability of correction factors to convert detector readings to doses. TG‐155 defines small field conditions, provides recommendations for suitable detectors and recommendations for good working practice for relative dosimetry (PDD, TMR, output factor, etc.) and dose calculations based on the new formulation. It also discusses beam modeling and dose calculations as a critical step in clinical utilization of small field radiotherapy. Alfonso et al, Med Phys 35, 5179–5186 (2008). Das et al, Med Phys (under review, 2014) Learning Objectives: Concepts and recommended procedures in the IAEA‐AAPM code of practice for dosimetry of small fields. Physics of dosimetry in non‐equilibrium conditions and definition of small fields Recommended procedures for relative dosimetry in TG‐1554. Choice of detectors for small field dosimetry, perturbations and corrections for dosimetry Understand the detector properties required for small field measurements Understand the advantages and limitations of commercially available detectors for small field dosimetry Research supported with operating grants from CIHR, NSERC, and BIOWin, a program funded at the Universite Catholique Louvain by the Walloon Government (Belgium). Student stipends supported by Medical Physics Research Training Network funded by the Collaborative Research and Training Experience program of the NSERC.Seuntjens: support by Sun Nuclear Corporation

A clinical audit programme for diagnostic radiology: the approach adopted by the International Atomic Energy Agency

The International Atomic Energy Agency (IAEA) has a mandate to assist member states in areas of human health and particularly in the use of radiation for diagnosis and treatment. Clinical audit is seen as an essential tool to assist in assuring the quality of radiation medicine, particularly in the instance of multidisciplinary audit of diagnostic radiology. Consequently, an external clinical audit programme has been developed by the IAEA to examine the structure and processes existent at a clinical site, with the basic objectives of: (1) improvement in the quality of patient care; (2) promotion of the effective use of resources; (3) enhancement of the provision and organisation of clinical services; (4) further professional education and training. These objectives apply in four general areas of service delivery, namely quality management and infrastructure, patient procedures, technical procedures and education, training and research. In the IAEA approach, the audit process is initiated by a request from the centre seeking the audit. A three-member team, comprising a radiologist, medical physicist and radiographer, subsequently undertakes a 5-d audit visit to the clinical site to perform the audit and write the formal audit report. Preparation for the audit visit is crucial and involves the local clinical centre completing a form, which provides the audit team with information on the clinical centre. While all main aspects of clinical structure and process are examined, particular attention is paid to radiation-related activities as described in the relevant documents such as the IAEA Basic Safety Standards, the Code of Practice for Dosimetry in Diagnostic Radiology and related equipment and quality assurance documentation. It should be stressed, however, that the clinical audit does not have any regulatory function. The main purpose of the IAEA approach to clinical audit is one of promoting quality improvement and learning. This paper describes the background to the clinical audit programme and the IAEA clinical audit protocol.

Dose rate influence on deep dose deposition using a 6 MV x-ray beam from a linear accelerator

Linear accelerators used in radiation therapy treatments usually provide the capability of irradiating with different dose rates. The medical physicist may choose arbitrary the dose rate to use, although the relative biological effectiveness (RBE) varies with this physical parameter. In this work the measurements of absorbed dose of the x-ray beams from a Varian 6 MV linear accelerator CLINAC 600C/D were made in a water phantom, using an appropriate dosimetry system, consisting of an electrometer and a waterproof ionization chamber. The code of practice for dosimetry used in this work is the IAEA (International Atomic Energy Agency) TRS-398, since water relates closely to the biological effects of radiation. The measurements were made in three depths, with three different field sizes and six different dose rates. The data were treated and the graphs show the obtained results. Different dose rates changes the deep dose deposition at the same depth and this must be considered in the planning.

Quality assurance for radiotherapy in prostate cancer: Point dose measurements in intensity modulated fields with large dose gradients

Purpose: We aimed to evaluate an optimization algorithm designed to find the most favorable points to position an ionization chamber (IC) for quality assurance dose measurements of patients treated for prostate cancer with intensity-modulated radiotherapy (IMRT) and fields up to 10 cm × 10 cm. Methods and Materials: Three cylindrical ICs (PTW, Freiburg, Germany) were used with volumes of 0.6 cc, 0.125 cc, and 0.015 cc. Dose measurements were made in a plastic phantom (PMMA) at 287 optimized points. An algorithm was designed to search for points with the lowest dose gradient. Measurements were made also at 39 nonoptimized points. Results were normalized to a reference homogeneous field introducing a dose ratio factor, which allowed us to compare measured vs. calculated values as percentile dose ratio factor deviations ΔF (%). A tolerance range of ΔF (%) of ±3% was considered. Results: Half of the ΔF (%) values obtained at nonoptimized points were outside the acceptable range. Values at optimized points were widely spread for the largest IC (i.e., 60% of the results outside the tolerance range), whereas for the two small-volume ICs, only 14.6% of the results were outside the tolerance interval. No differences were observed when comparing the two small ICs. Conclusions: The presented optimization algorithm is a useful tool to determine the best IC in-field position for optimal dose measurement conditions. A good agreement between calculated and measured doses can be obtained by positioning small volume chambers at carefully selected points in the field. Large chambers may be unreliable even in optimized points for IMRT fields ≤10 cm × 10 cm.

Ion recombination correction in the Clatterbridge Centre of Oncology clinical proton beam

Most codes of practice for dosimetry of proton beams do not give a clear recommendation on the determination of recombination correction factors for ionization chambers. In this work, recombination corrections were measured in the low-energy clinical proton beam of the Clatterbridge Centre of Oncology (CCO) using data collected at different dose rates and different polarizing voltages. This approach allows the separation of contributions from initial and volume recombination and was compared with results from extrapolation and two-voltage methods. A modified formulation of the method is presented for a modulated beam in which the ionization current is time dependent. Using a set-up with two identical chambers placed face-to-face yielded highly accurate data for plane-parallel ionization chambers. This method may also be used for high-energy photon and electron beam dosimetry. At typical dose rates of 26 Gy min−1 used clinically at the CCO, the recombination correction is 0.8% and thus is of importance for reference dosimetry. The proton beam should be treated as purely continuous given the high pulse repetition frequency of the cyclotron beam. The results show that the volume recombination parameter for protons is consistent with values measured for photon beams. Initial recombination was found to be independent of beam quality, except for a tendency to increase at the distal edge of the Bragg peak; this is only relevant for depth dose measurements. Using a general equation for recombination and generic values for the initial and volume recombination parameters (A = 0.25 V and m2 = 3.97 × 103 s cm−1 nC−1 V2), the experimental results are reproduced within 0.1% for all conditions met in this work. For the CCO beam and similar proton beams used for treating optical targets operating at high dose rates, the recombination correction factor can be overestimated by up to 2%, resulting in an overestimation of dose to water by the same amount, if the recommendation from IAEA TRS-398, which is only valid for pulsed beams, is followed without consideration.

Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry Based on Standards of Absorbed Dose to Water; Technical Reports Series No. 398,

Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry Based on Standards of Absorbed Dose to Water; Technical Reports Series No. 398,

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).

The selection of stopping power and mass energy absorption coefficient data for the HPA Code of Practice for dosimetry

All the recent protocols for high-energy X-ray dosimetry, introduced by the AAPM (1983), NACP (1980) and the HPA (1983), result in absorbed dose conversion factors significantly different from those that have been in use since 1969 when ICRU Report 14 was published. Clearly for all protocols, general or specific, it is necessary to select appropriate stopping power and mass absorption coefficient ratios for a range of X-ray beam qualities. Cunningham and Shultz (1984) have recently described a method of selecting these ratios, taking into account differences in spectral properties of radiation beams rather than the peak accelerating voltage which is not an adequate index of radiation quality. A table is given showing quality of X-ray or gamma -ray radiation versus narrow beam attenuation coefficient in water.