Status of radiation dosimetry in Germany using ionization chamber calibrated in terms of absorbed dose to water

To validate the calculated values of kQ for high-energy photon beams given in the International Code of Practice for radiotherapy dosimetry based on water-absorbed-dose standards, a comparison with experimental values derived in standards laboratories and in clinical beams has been made. The study includes a compilation of experimental values for ionization chambers of the type NE2561/2611, NE2571, PTW30001 and PR06. The energy dependence of the G(Fe3+) ratio of high-energy x-rays to 60Co gamma-rays by Klassen et al is taken into account for all the Fricke-derived values. For three of the chamber types analysed, the comparison shows that the calculated values are a very good estimate of the average values of kQ in the entire range of photon beam qualities available for clinical use. For the NE2571 chamber type a difference which increases with energy between calculated and experimental kQ factors has been observed; however, the largest difference with a fit describing the entire set of experimental data is always smaller than 0.4%. It is concluded that if the recommendation of the Code of Practice for an individual calibration of the user's chamber at a range of photon beam qualities is not available, the use of calculated kQ factors will yield absorbed dose to water determinations accurate within the uncertainty limits of the majority of experimental data available. The good agreement between calculated and measured values, obtained for practically all the experimental data using TPR(20,10) as photon beam quality specifier, is not satisfied in some cases for two high-energy soft beams used at the Canadian NRC. There appears to be no justification for a change to a different photon beam quality specifier solely on the grounds that such a limited set of data is not described by the same distributions as the rest of the experimental data.

Section: Formalismmentioning

confidence: 99%

“…In therapeutic electron and photon beams the general assumption of (W air ) Q = (W air ) Q 0 yields the simpler equation for k Q,Q 0 (Hohlfeld 1988)…”

Section: L27mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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A comparison between calculated and experimentalk_Qphoton beam quality correction factors

Andreo

2000

“…Furthermore, the new standards of absorbed dose to water offer the possibility of reducing the uncertainty in the dosimetry of radiotherapy beams, provide a more robust system of primary standards than air kerma based standards, and allow the use of a simple formalism 3 . Following the development of standards of absorbed dose to water and national dosimetry protocols, pioneered by the UK (Burns et al 1988, IPSM 1990 and Germany (DIN 1997, Hohlfeld 1988) more than ten years ago, new dosimetry protocols based on the use of an ionization chamber calibrated in terms of absorbed dose to water in a 60 Co γ -ray beam, N D,w , together with calculated beam quality correction factors k Q have been published in Russia (Gosstandart 1990), in North America, AAPM TG-51 (Almond et al 1999) and by the IAEA, TRS-398 . Many laboratories already provide calibrations at the radiation quality of 60 Co γ -rays and some have extended calibrations to high-energy photon and electron beams, modalities initiated in the UK by Burns et al 1988 andMcEwen et al (2001), respectively, and followed for photon beams by the laboratories of Australia, Canada, Belgium, France, Italy, Switzerland, etc (see references in Andreo (2000)).…”

Section: Introductionmentioning

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

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.

“…A beam-quality correction factor, denoted as k Q,Q 0 , is used to fit to the user-beam quality, Q. Direct measurements of k Q,Q 0 at the same quality as the user beam are not feasible in most standard laboratories, and therefore theoretical values (Hohlfeld 1988, Andreo 1992, Rogers 1992) are commonly used. Each protocol uses a different beam-quality index: TPR 20,10 in TRS-398 and %dd (10) x in TG-51.…”

Section: Introductionmentioning

confidence: 99%

Comparison between TG-51 and TRS-398: electron contamination effect on photon beam-quality specification

Medina¹,

Teijeiro²,

Salvador³

et al. 2003

Two dosimetry protocols based on absorbed dose to water have recently been implemented: TG-51 and TRS-398. These protocols use different beam-quality indices: %dd(10)x and TPR20,10. The effect of electron contamination in measurements of %dd(10)x has been proposed as a disadvantage of the TG-51. For actual measurements of %dd(10)x in five clinical beams (Primus 6-18 MV, SL-75/5 6 MV, SL-18 6-15 MV) a purging magnet was employed to remove the electron contamination. Also, %dd(10)x was measured in the different ways described in TG-51 for high-energy beams: with a lead foil at 50 cm from the phantom surface, at 30 cm, and for open beam. Moreover, TPR20,10 was determined. Also, periodic quality-control measurements were used for comparing both quality indices and variation over time, but D20,10 was used instead of TPR20,10 and measurements in open beam for the %dd(10)x determination. Considering both protocols, S(w,air) and kQ were calculated in order to compare the results with the experimental data. Significant differences (0.3% for kQ) were only found for the two high-energy beams, but when the electron contamination is underestimated by TG-51, the difference in kQ is lower. Differences in the other cases and variations over time were less than 0.1%.