Design of a phantom for the quality control of high dose rate 192Ir source used in brachytherapy

doi:10.1016/j.radonc.2007.01.005

Radiotherapy and Oncology

Volume 82, Issue 2, February 2007, Pages 222-228

https://doi.org/10.1016/j.radonc.2007.01.005 Get rights and content

Abstract

Background and purpose

A new phantom is proposed for measuring the strength of ¹⁹²Ir high dose rate sources and for verification of the dose calculated by the treatment planning system. The complete formalism and measurement procedure for this phantom is described, as well as the preliminary results obtained in a number of centers around Brazil.

Materials and methods

The measurements are performed using powder thermoluminescent dosimeter capsules; the source strength is measured in air and the verification of the dose calculation algorithm in water phantom.

The correction factors required to take into account the specificities related to the geometry and the phantom materials have been assessed using the PENELOPE Monte Carlo code and experimental methods.

The dedicated phantom, constructed to use as part of a QA program, in this case specifically for high dose rate ¹⁹²Ir brachytherapy sources, allows simultaneous irradiation of three thermoluminescent dosimeter capsules, requiring only one source stop (dwell positions).

Results

The phantom was mailed to seven radiotherapy institutions in Brazil, and the results show its usefulness in verifying the source air kerma and correctness of treatment planning dose calculation in water phantom.

Conclusions

The comparison made between the phantom measurements, the well-type ionization chamber, and source specifications stated by the hospital (most of the times provided by the source manufacturer) agreed within 3% showing the quality in the HDR dose delivery in Brazilian radiotherapy centers.

Section snippets

Phantom design and construction

The phantom was designed to measure two important dosimetric parameters needed to assure the desired level of accuracy in the dose delivered to the patient in brachytherapy: First, the source strength at 10 cm from the source in air [8]; second, the absorbed dose at 2 cm in water. The latter point was made coincident with the historical point A used in gynaecological brachytherapy [6], [10], [14], [18]. The phantom was designed to have three cylindrical polyethylene capsules (20 mm inner length, 3

Correction factors evaluated by Monte Carlo

The results of the gamma ray spectrum calculated by Monte Carlo inside the TLD powder inserted in the phantom for the sources used by the microSelectron and VariSource are presented in Fig. 3. The mean energy in air of the sources used in the MicroSelectron and VariSource systems (estimated from the calculated the spectra) is 357 and 349, and 360 and 352 keV in water. The results presented in Fig. 3 are normalized to the total fluence both at 10 cm in air and at 2 cm in water. The correction

Conclusion

The proposed dedicated phantom was developed to use TLD powder contained in cylindrical capsules in a quality control program involving high dose rate ¹⁹²Ir brachytherapy sources. This phantom is very simple to use, requiring only one source stop. It allows for the simultaneous irradiation of three TLD capsules equally distant from the source, yielding 15 readings per irradiation under the same geometry. This procedure has a lower uncertainty than that reported by the ESTRO program for ¹⁹²Ir

Acknowledgements

The authors thank the IAEA for providing fellowships to two of the authors, IVIC, UCV, and the hospitals (HCCAC, HSL, HSVP, IOJF, HCUSP, IAVC, CHG) that have spontaneously and generously agreed to participate in this study.

HCCAC – Hospital do Cancer (CA CAMARGO), HSL – Hospital Sírio Libanês, HSVP – Hospital Sao Vicente de Paulo, IOJF – Instituto Oncológico Juiz de Fora, HCUSP – Hospital de Clinicas, IAVC – Instituto Antonio Vieira de Carvalho, CHG 2 – Clinica Radiologica da Gavea.

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Determination of the correction factors used in Fricke dosimetry for HDR 192Ir sources employing the Monte Carlo method
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Different research groups have been striving to establish a reliable method of determining the Dw at a primary level through chemical dosimetry, using the Fricke method [4,5]. This method has been studied, demonstrating great reliability, at the Radiological Sciences Laboratory of the Rio de Janeiro State University (LCR/UERJ), where several works on this subject have already been published [6–12]. Briefly, Fricke dosimetry consists of measuring, by spectrophotometry, the conversion of ferrous ions present in the solution into ferric ions, caused by exposure to ionizing radiation [13,14].
Fricke dosimetry has shown great potential in the direct measurement of the absolute absorbed dose for ¹⁹²Ir sources used in HDR brachytherapy. This work describes the determination of the correction factors necessary to convert the absorbed dose in the Fricke solution to the absorbed dose to water. Methods: The experimental setup for Fricke irradiation using a ¹⁹²Ir source was simulated. The holder geometry used for the Fricke solution irradiation was modelled for MC simulation, using the PENELOPE. Results: The values of the factors determined for validation purposes demonstrated differences of less than 0.2% when compared to the published values. Four factors were calculated to correct: the differences in the density of the solution (1.0004 ± 0.0004); the perturbations caused by the holder (0.9989 ± 0.0004); the source anisotropy and the water attenuation effects (1.0327 ± 0.0012); and the distance from the center of the detection volume to the source (7.1932 ± 0.0065). Conclusion: Calculated corrections in this work show that the largest correction comes from the inverse squared reduction of the dose due to the point of measurement shift from the reference position of 1 cm. This situation also causes the correction due to volume averaging and attenuation in water to be significant. Future versions of the holder will aim to reduce these effects by having a position of measurement closer to the reference point thus requiring smaller corrections.
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Radiotherapy and Oncology

Abstract

Background and purpose

Materials and methods

Results

Conclusions

Section snippets

Phantom design and construction

Correction factors evaluated by Monte Carlo

Conclusion

Acknowledgements

References (27)

PENELOPE: an algorithm for Monte Carlo simulation of the penetration and energy loss of electrons and positrons in matter

Nucl Inst Meth B

The ESTRO-QUALity assurance network (EQUAL)

Radiother Oncol

The IAEA/WHO postal programme for radiotherapy hospitals

Radiother Oncol

Spectra and air-kerma strength for encapsulated ¹⁹²Ir sources

Med Phys

Absorbed dose calculations in a brachytherapy pelvic phantom using the Monte Carlo method

J Appl Clin Med Phys

Intercomparison of calibration procedures for ¹⁹²Ir HDR sources in Brazil

Phys Med Biol

Source strength and calibration of HDR/PDR sources

Brachytherapy physics

Clinical intracavitary systems for treatment of gynaecologic malignancies

Brachytherapy physics

Cited by (16)

Determination of the correction factors used in Fricke dosimetry for HDR <sup>192</sup>Ir sources employing the Monte Carlo method

Application of Monte Carlo calculations for validation of a treatment planning system in high dose rate brachytherapy

Experimental determination of the energy response of alanine pellets in the high dose rate <sup>192</sup>Ir spectrum

The design of an audit test for 60 Co brachytherapy treatment planning system

The effect of tandem-ovoid titanium applicator on points A, B, bladder, and rectum doses in gynecological brachytherapy using <sup>192</sup>Ir

Feasibility study of patient-specific quality assurance system for high-dose-rate brachytherapy in patients with cervical cancer

Quality assuranceDesign of a phantom for the quality control of high dose rate 192Ir source used in brachytherapy

Abstract

Background and purpose

Materials and methods

Results

Conclusions

Section snippets

Phantom design and construction

Correction factors evaluated by Monte Carlo

Conclusion

Acknowledgements

Nucl Inst Meth B

Radiother Oncol

Radiother Oncol

Spectra and air-kerma strength for encapsulated 192Ir sources

Med Phys

Absorbed dose calculations in a brachytherapy pelvic phantom using the Monte Carlo method

J Appl Clin Med Phys

Intercomparison of calibration procedures for 192Ir HDR sources in Brazil

Phys Med Biol

Source strength and calibration of HDR/PDR sources

Brachytherapy physics

Clinical intracavitary systems for treatment of gynaecologic malignancies

Brachytherapy physics

Quality assurance
Design of a phantom for the quality control of high dose rate ¹⁹²Ir source used in brachytherapy

Spectra and air-kerma strength for encapsulated ¹⁹²Ir sources

Intercomparison of calibration procedures for ¹⁹²Ir HDR sources in Brazil