Comparison between TG‐51 and TG‐21: Calibration of photon and electron beams in water using cylindrical chambers

Cho, S. H.; Lowenstein, Jessica; Balter, Peter A; Wells, N. H.; Hanson, W. F.

doi:10.1120/jacmp.v1i3.2643

Cited by 11 publications

(13 citation statements)

References 5 publications

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“…A compilation is given in tables 1 and 2. References include the comparisons between TG-51 and TG-21 protocols by Ding et al (2000), Cho et al (2000), Huq and Andreo (2001), Huq et al (2001b) and Tailor and Hanson (2002); between TRS-398 and TRS-277, 381 by Andreo et al (2001Andreo et al ( , 2002; between TRS-398, TG-51, TRS-277, TRS-381, NCS report-2 and report-5 by Palmans et al (2002Palmans et al ( , 2003; between TRS-398, TG-51, TRS-277 and the Japanese protocol JARP by Araki and Kubo (2002) and between TG-51 and German DIN 6800-2 by Dohm et al (2001) graphical presentation of data published for all combinations of chamber types, beam types and energies and protocols is impractical, only selected combinations of such data will be presented to bring out the salient features of the differences between the N D,w and the N K based protocols.…”

Section: Comparison Between N Dw and N K Based Protocolsmentioning

confidence: 99%

Advances in the determination of absorbed dose to water in clinical high-energy photon and electron beams using ionization chambers

Huq

Andreo

2004

Phys. Med. Biol.

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During the last two decades, absorbed dose to water in clinical photon and electron beams was determined using dosimetry protocols and codes of practice based on radiation metrology standards of air kerma. It is now recommended that clinical reference dosimetry be based on standards of absorbed dose to water. Newer protocols for the dosimetry of radiotherapy beams, based on the use of an ionization chamber calibrated in terms of absorbed dose to water, N(D,w), in a standards laboratory's reference quality beam, have been published by several national or regional scientific societies and international organizations. Since the publication of these protocols multiple theoretical and experimental dosimetry comparisons between the various N(D,w) based recommendations, and between the N(D,w) and the former air kerma (NK) based protocols, have been published. This paper provides a comprehensive review of the dosimetry protocols based on these standards and of the intercomparisons of the different protocols published in the literature, discussing the reasons for the observed discrepancies between them. A summary of the various types of standards of absorbed dose to water, together with an analysis of the uncertainties along the various steps of the dosimetry chain for the two types of formalism, is also included. It is emphasized that the NK-N(D,air) and N(D,w) formalisms have very similar uncertainty when the same criteria are used for both procedures. Arguments are provided in support of the recommendation for a change in reference dosimetry based on standards of absorbed dose to water.

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Section: Comparison Between N Dw and N K Based Protocolsmentioning

confidence: 99%

Advances in the determination of absorbed dose to water in clinical high-energy photon and electron beams using ionization chambers

Huq

Andreo

2004

Phys. Med. Biol.

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“…The outputs measured using the circular cutouts were normalised to that in the absolute calibration condition. For the linac used in this study, the absolute dose point was calibrated at the reference depth ( d ref ) defined by the AAPM TG-51 protocol using the 10 × 10 cm 2 applicator and the standard 10 × 10 cm 2 cutout 13 , 14 . In this study, the reference dose point for the ROF measurements was set at d m .…”

Section: Methodsmentioning

confidence: 99%

Dosimetric changes induced by positional uncertainty of cutout in electron radiotherapy

2008

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Dosimetric changes caused by the positional uncertainty of centring a small electron cutout to the machine central beam axis (CAX) of the linear accelerator (linac) were investigated. First, six circular cutouts with 4 cm diameter were made with their centres shifted off from the machine CAX for 0, 2, 4, 6, 8 and 10 mm using the 6 · 6 cm 2 applicator. Then, the percentage depth doses (PDDs) at the machine CAX and cutout centre were measured using the 4, 9 and 16 MeV clinical electron beams produced by a Varian 21 EX linac. The cross-and in-line axis beam profiles were measured at depth of maximum dose (d m ) and source-to-surface distance equal to 100 cm using a scanning water tank system and diode detector. When the cutout centre was shifted away from machine CAX for the electron beam with low energy of 4 MeV, the d m , depths of the 80 (R 80 ) and 90% (R 90 ) depth dose at the machine CAX had no significant change (<0.1 mm). For higher energies of 9 and 16 MeV beams, the d m were reduced with 0.45 and 1.63 mm per mm off-axis shift between the cutout centre and the machine CAX, respectively. The R 80 and R 90 were reduced with 0.7 mm per mm off-axis shift for both energies. When there was a 4 mm off-axis shift, the relative output factors for the 4, 9 and 16 MeV beams were reduced with 0.8, 1.6 and 0.5%, respectively. The isodose coverage of the in-line axis beam profile was reduced when the cutout centre was shifted away from machine CAX. It is important for radiation oncologists, dosimetrists, therapists and physicists to note such dosimetric changes in the electron radiotherapy to the patient, because such positional uncertainty is unavoidable in fabricating an electron cutout in the mould room.

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“…The RPC has presented posters and papers on several aspects of TG‐51 3 , 4 and discussed a wide spectrum of questions from the medical physics community. The data provided by institutions on the RPC forms were reviewed and the participating physicists were contacted whenever the RPC questioned the data.…”

Section: Methodsmentioning

confidence: 99%

“…The RPC was involved in beta testing the TG‐51 protocol, 3 and implemented the protocol on January 1, 2000. The RPC also mailed a set of forms to the participating institutions.…”

Section: Introductionmentioning

confidence: 99%

TG‐51: Experience from 150 institutions, common errors, and helpful hints

Tailor

Hanson

2003

J Applied Clin Med Phys

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The Radiological Physics Center (RPC) is a resource to the medical physics community for assistance regarding dosimetry procedures. Since the publication of the AAPM TG‐51 calibration protocol, the RPC has responded to numerous phone calls raising questions and describing areas in the protocol where physicists have had problems. At the beginning of the year 2000, the RPC requested that institutions participating in national clinical trials provide the change in measured beam output resulting from the conversion from the TG‐21 protocol to TG‐51. So far, the RPC has received the requested data from ~ 150 of the ~ 1300 institutions in the RPC program. The RPC also undertook a comparison of TG‐21 and TG‐51 and determined the expected change in beam calibration for ion chambers in common use, and for the range of photon and electron beam energies used clinically. Analysis of these data revealed two significant outcomes: (i) a large number (~ 1/2) of the reported calibration changes for photon and electron beams were outside the RPC's expected values, and (ii) the discrepancies in the reported versus the expected dose changes were as large as 8%. Numerous factors were determined to have contributed to these deviations. The most significant factors involved the use of plane‐parallel chambers, the mixing of phantom materials and chambers between the two protocols, and the inconsistent use of depth‐dose factors for transfer of dose from the measurement depth to the depth of dose maximum. In response to these observations, the RPC has identified a number of circumstances in which physicists might have difficulty with the protocol, including concerns related to electron calibration at low energies false(R50<2.1emcmfalse), and the use of a cylindrical chamber at 6 MeV electrons. In addition, helpful quantitative hints are presented, including the effect of the prescribed lead filter for photon energy measurements, the impact of shifting the chamber depth for photon depth‐dose measurements, and the impact of updated stopping‐power data used in TG‐51versus that used in TG‐21, particularly for electron calibrations.PACS number(s): 87.53.–j, 87.66.–a

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Comparison between TG‐51 and TG‐21: Calibration of photon and electron beams in water using cylindrical chambers

Cited by 11 publications

References 5 publications

Advances in the determination of absorbed dose to water in clinical high-energy photon and electron beams using ionization chambers

Advances in the determination of absorbed dose to water in clinical high-energy photon and electron beams using ionization chambers

Dosimetric changes induced by positional uncertainty of cutout in electron radiotherapy

TG‐51: Experience from 150 institutions, common errors, and helpful hints

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