Development of a TLD mailed system for remote dosimetry audit for (192)Ir HDR and PDR sources.

BACKGROUND AND PURPOSE: In the framework of an ESTRO ESQUIRE project, the BRAPHYQS Physics Network and the EQUAL-ESTRO laboratory have developed a procedure for checking the absorbed dose to water in the vicinity of HDR or PDR sources using a mailed TLD system. The methodology and the materials used in the procedure are based on the existing EQUAL-ESTRO external radiotherapy dose checks. MATERIALS AND METHODS: A phantom for TLD postal dose assurance service, adapted to accept catheters from different HDR afterloaders, has been developed. The phantom consists of three PMMA tubes supporting catheters placed at 120 degrees around a central TLD holder. A study on the use of LiF powder type DTL 937 (Philitech) has been performed in order to establish the TLD calibration in dose-to-water at a given distance from (192)Ir source, as well as to determine all correction factors to convert the TLD reading into absorbed dose to water. The dosimetric audit is based on the comparison between the dose to water measured with the TL dosimeter and the dose calculated by the clinical TPS. Results of the audits are classified in four different levels depending on the ratio of the measured dose to the stated dose. The total uncertainty budget in the measurement of the absorbed dose to water using TLD near an (192)Ir HDR source, including TLD reading, correction factors and TLD calibration coefficient, is determined as 3.27% (1s). RESULTS: To validate the procedures, the external audit was first tested among the members of the BRAPHYQS Network. Since November 2004, the test has been made available for use by all European brachytherapy centres. To date, 11 centres have participated in the checks and the results obtained are very encouraging. Nevertheless, one error detected has shown the usefulness of this audit. CONCLUSION: A method of absorbed dose to water determination in the vicinity of an (192)Ir brachytherapy source was developed for the purpose of a mailed TL dosimetry system. The accuracy of the procedure was determined. This method allows a check of the whole dosimetry chain for this type of brachytherapy afterloading system and can easily be performed by mail to any institution in the European area and elsewhere. Such an external audit can be an efficient QC method complementary to internal quality control as it can reveal some errors which are not observable by other means.

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

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.

The EQUAL-ESTRO audit on geometric reconstruction techniques in brachytherapy.

BACKGROUND AND PURPOSE: A geometric check procedure of the reconstruction techniques used in brachytherapy treatment planning systems was developed by the EQUAL (European Quality Laboratory) Laboratory in the framework of the ESTRO's (European Society for Therapeutic Radiology and Oncology) project 'ESQUIRE' (Education Science and QUality assurance In Radiotherapy in Europe [Baumann M, Brada M. Towards equity in turbulent Europe ESTRO, European cooperation and the European Commission. Radiother Oncol 2005;75:251-2. Heeren G. The bright but ephemeral life of a rainbow. A chronical of seventeen years of intensive ESTRO-EU cooperation. Radiother Oncol 2005;75:253-7]) by the task group Braphyqs (Brachytherapy physics quality system). PATIENTS AND METHODS: The check is performed by using the so-called 'Baltas' phantom, mailed to the participating centres in order to check the local technique of geometric reconstruction used in dose calculation. RESULTS: To validate the procedures, the check was first tested among the members of the Braphyqs Network. Since November 2002, the system is open to other centres. Until now 152 reconstructions have been checked. Eighty-six percent of the results were within an acceptance level after the first check. For the remaining 14%, a second check has been proposed. The results of the re-checks are in most cases within an acceptance level, except for 2% of the reconstructions. CONCLUSIONS: The geometric check is available from the EQUAL Laboratory for all the brachytherapy centres. The decrease of the deviations observed between the two checks demonstrates the importance of this kind of external audit as some errors were revealed, which were not discovered before with techniques used in clinical quality control routines.