Wall correction factors, , for thimble ionization chambers

Buckley, Lesley; Rogers, D. W. O.

doi:10.1118/1.2161403

Cited by 51 publications

(63 citation statements)

References 33 publications

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“…In order to obtain a consistent conversion of absorbed dose in the chamber cavity to absorbed dose in water ͑i.e., the product of all perturbation factors must be equal to the ratio of absorbed dose in water to absorbed dose in the chamber͒, a series of scoring volumes is defined starting at the fully modeled chamber and ending at a small volume of water placed at the centroid of the chamber. Similar to previous studies, [23][24][25] a series of cavity doses is defined as follows: with liquid water ͑density of 1.000 g / cm 3 ͒, and ͑7͒ D w,point : absorbed dose in a 1 mm radius sphere of water placed at the centroid of the chamber and representing absorbed dose at a point in water at the location of measurement.…”

Section: Iib Monte Carlo Methodsmentioning

confidence: 99%

“…For ionization chambers, this perturbation is caused by several factors, which can be classified as follows: ͑1͒ The presence of structural and electronic components, ͑2͒ a detection material with a different atomic composition than water, ͑3͒ a detection material with different density than water, and ͑4͒ a detector volume that is finite. As formalized in reference dosimetry protocols, 11-14 and more recent publications, [23][24][25] ionization chamber perturbation factors represent each of these effects individually ͑although in practice, they are correlated͒ when converting absorbed dose to the chamber to absorbed dose to water. The presence of the chamber stem, central electrode, and wall will cause the charged-particle fluence to change in terms of quality and quantity due to attenuation and scatter in these components.…”

Section: Iia Formalism Of Absorbed Dose At a Pointmentioning

confidence: 99%

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Ionization chamber gradient effects in nonstandard beam configurations

et al. 2009

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Purpose: For the purpose of nonstandard beam reference dosimetry, the current concept of reporting absorbed dose at a point in water located at a representative position in the chamber volume is investigated in detail. As new nonstandard beam reference dosimetry protocols are under development, an evaluation of the role played by the definition of point of measurement could lead to conceptual improvements prior to establishing measurement procedures. Methods: The present study uses the current definition of reporting absorbed dose to calculate ionization chamber perturbation factors for two cylindrical chamber models ͑Exradin A12 and A14͒ using the Monte Carlo method. The EGSnrc based user-code EGSគchamber is used to calculate chamber dose responses of 14 IMRT beams chosen to cause considerable dose gradients over the chamber volume as previously used by Bouchard and Seuntjens ͓"Ionization chamber-based reference dosimetry of intensity modulated radiation beams," Med. Phys. 31͑9͒, 2454-5465 ͑2004͔͒. Results: The study shows conclusively the relative importance of each physical effect involved in the nonstandard beam correction factors of 14 IMRT beams. Of all correction factors involved in the dosimetry of the beams studied, the gradient perturbation correction factor has the highest magnitude, on average, 11% higher compared to reference conditions for the Exradin A12 chamber and about 5% higher for the Extradin A14 chamber. Other perturbation correction factors ͑i.e., P wall , P stem , and P cel ͒ are, on average, less than 0.8% different from reference conditions for the chambers and beams studied. The current approach of reporting measured absorbed dose at a point in water coinciding with the location of the centroid of the chamber is the main factor responsible for large correction factors in nonstandard beam deliveries ͑e.g., intensity modulated radiation therapy͒ reported in literature. Conclusions: To reduce or eliminate the magnitude of perturbation correction factors in nonstandard beam reference dosimetry, two possible ways to report absorbed dose are suggested: ͑1͒ Reporting average dose to the sensitive volume of the chamber filled with water, combined with removing the reference field implicit gradient effect when measuring output factors, and ͑2͒ reporting average dose to the chamber itself during output factor verifications. The first option could be adopted if clinical beam correction factors are negligible. The second option could simplify quality assurance procedures when correction factors are not negligible and have to be calculated using Monte Carlo simulations.

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Section: Iib Monte Carlo Methodsmentioning

confidence: 99%

Section: Iia Formalism Of Absorbed Dose At a Pointmentioning

confidence: 99%

Ionization chamber gradient effects in nonstandard beam configurations

et al. 2009

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“…Therefore, the difference in the k Q factors for the three protocols appears due to the difference between the perturbation factors obtained by a semi‐analytic approach and MC calculations. Several studies14, 15, 16, 36 have compared individual perturbation correction factors obtained using MC calculations for cylindrical chambers with those obtained using a semi‐analytic approach, such as that used in IAEA TRS‐398 and AAPM TG‐51. For a 60 Co beam, the value of the replacement factor derived from MC calculations by Wang and Rogers17 was 0.5% higher than the AAPM TG‐51 value and approximately 1% higher than the IAEA TRS‐398 value.…”

Section: Discussionmentioning

confidence: 99%

“…For a 60 Co beam, the value of the replacement factor derived from MC calculations by Wang and Rogers17 was 0.5% higher than the AAPM TG‐51 value and approximately 1% higher than the IAEA TRS‐398 value. Buckley and Rogers16 and Muir and Rogers14 showed that the values of the wall correction factors derived from MC calculations for a wall material comprising PMMA, such as that found in PTW 30013, a wall material comprising C‐552, such as that found in Exradin A12, and a wall material comprising POM, such as that found in IBA FC65‐P, differed by −0.2%–0.4% from the AAPM TG‐51 value. However, in this study, the k Q factors for the three protocols agreed within a relative uncertainty of 1.0% in the k Q values for IAEA TRS‐398 and JSMP 12.…”

Section: Discussionmentioning

confidence: 99%

“…Further, since the publication of AAPM TG‐51 and IAEA TRS‐398, Monte Carlo (MC) simulation methods8, 9 have been developed and accurately benchmarked10, 11, 12 for the calculations of detailed chamber geometries 13, 14, 15. Moreover, studies in current literature have provided ionization chamber perturbation correction factors15, 16, 17 and beam quality conversion factors 14, 15, 18. Although the AAPM TG‐51 provided k Q factors for only cylindrical chambers, which represented the majority of reference chambers available at the time of publication, more than 30 different designs for ionization chambers have recently become available.…”

Section: Introductionmentioning

confidence: 99%

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Comparison of AAPM Addendum to TG‐51, IAEA TRS‐398, and JSMP 12: Calibration of photon beams in water

Kinoshita

Oguchi

Nishimoto

et al. 2017

J Applied Clin Med Phys

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The American Association of Physicists in Medicine (AAPM) Working Group on TG‐51 published an Addendum to the AAPM's TG‐51 protocol (Addendum to TG‐51) in 2014, and the Japan Society of Medical Physics (JSMP) published a new dosimetry protocol JSMP 12 in 2012. In this study, we compared the absorbed dose to water determined at the reference depth for high‐energy photon beams following the recommendations given in AAPM TG‐51 and the Addendum to TG‐51, IAEA TRS‐398, and JSMP 12. This study was performed using measurements with flattened photon beams with nominal energies of 6 and 10 MV. Three widely used ionization chambers with different compositions, Exradin A12, PTW 30013, and IBA FC65‐P, were employed. Fully corrected charge readings obtained for the three chambers according to AAPM TG‐51 and the Addendum to TG‐51, which included the correction for the radiation beam profile (P rp), showed variations of 0.2% and 0.3% at 6 and 10 MV, respectively, from the readings corresponding to IAEA TRS‐398 and JSMP 12. The values for the beam quality conversion factor k Q obtained according to the three protocols agreed within 0.5%; the only exception was a 0.6% difference between the results obtained at 10 MV for Exradin A12 according to IAEA TRS‐398 and AAPM TG‐51 and the Addendum to TG‐51. Consequently, the values for the absorbed dose to water obtained for the three protocols agreed within 0.4%; the only exception was a 0.6% difference between the values obtained at 10 MV for PTW 30013 according to AAPM TG‐51 and the Addendum to TG‐51, and JSMP 12. While the difference in the absorbed dose to water determined by the three protocols depends on the kQ and P rp values, the absorbed dose to water obtained according to the three protocols agrees within the relative uncertainties for the three protocols.

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Modified electron beam output calibration based on IAEA Technical Report Series 398

Pawiro

Mahfirotin

Assegab

et al. 2022

J Applied Clin Med Phys

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Purpose The recently worldwide standard measurement of electron beam reference dosimetry include the International Atomic Energy Agency (IAEA) Technical Report Series (TRS)‐398 and Association of Physicists in Medicine (AAPM) Task Group (TG)‐51 protocols. Muir et al. have modified calibration methods for electron beam calibration based on AAPM TG‐51. They found that the use of cylindrical chambers at low energy gave acceptable results. In this study, we propose and report a modified calibration for electron beam based on IAEA TRS‐398, the standard reference dosimetry protocol worldwide. Methods This work was carried out with energies of 6, 8, 10, 12, and 15 MeV. The electron beam is generated from Elektra Synergy Platform and Versa HD linear accelerator. The charge readings were measured with PTW 30013, IBA CC13, Exradin A1Sl, and Exradin A11 chambers connected to the electrometer. The dose calculation uses an equation of modified calibration for electron beam using the updated kQ${k_Q}$ factor in previous work. The absorbed dose to water for electron beam is expressed in dose per monitor unit (cGy/MU). Thus, we compared dose per monitor unit (D/MU) calculation using a modified calibration to TRS‐398. Results In this work, we have succeeded in implementing the modified calibration of electron beam based on TRS‐398 by applying a cylindrical chamber in all energy beams and using the updated kQ${k_Q}$ factor. The ratio of the absorbed dose to water between original and modified calibration protocols of TRS‐398 (Dw) for the cylindrical chamber was 1.002 on the Elekta Synergy Platform and 1.000 on the Versa HD while for the parallel‐plate chamber it was 1.013 on the Elekta Synergy Platform and 1.014 on the Versa HD. Based on these results, both the cylindrical and parallel‐plate chambers are still within the tolerance limit allowed by the TRS‐398 protocol, which is ±2%. Therefore, modified calibration based on TRS‐398 gives acceptable results and is simpler to use clinically.

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Wall correction factors, , for thimble ionization chambers

Cited by 51 publications

References 33 publications

Ionization chamber gradient effects in nonstandard beam configurations

Ionization chamber gradient effects in nonstandard beam configurations

Comparison of AAPM Addendum to TG‐51, IAEA TRS‐398, and JSMP 12: Calibration of photon beams in water

Modified electron beam output calibration based on IAEA Technical Report Series 398

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