Dosimetry and quality control of brachytherapy with low-energy photon sources (125I)

Dosimetry and quality control of brachytherapy with low-energy photon sources (125I)

In 2003 the NCS established a new subcommittee to propose quality assurance (QA) procedures on the use of low energy photon (LEP) sources in permanent prostate brachytherapy (PPBT). As for previous subcommittees, the work was focussed typically on the Dutch and Belgian situation. As a first activity, the subcommittee circulated an extensive questionnaire to all radiotherapy institutes performing PPBT with LEP sources in the Netherlands (12) and Belgium (22), requesting information on the quality control (QC) procedures in use at that moment. In a second stage, site visits to all institutes were organized to perform source strength measurements and to investigate the local source strength measurement procedure.

Only the use of 125I sources was reported, most often for PPBT, but also for eye and breast treatments. At the time of the survey, 7 different types of 125I sources were used in the Netherlands and Belgium and a large variety in QC instruments and methods for verifying the air kerma strength of the seeds was observed. In total 6 out of the 34 institutes reported not having a specific instrument to verify the source strength of the seeds used for PPBT. Of the 29 instruments available in the hospitals only 3 were calibrated by a Secondary Standard Dosimetry Laboratory (SSDL), 4 were calibrated by the manufacturer. The visiting teams conducted on-site source strength measurements in 30 institutes and compared results with data stated by the supplier of the sources and if available with measurements performed by the local medical physicist. In general, the results by the NCS visiting team agreed with the results stated by the supplier within ± 5%.

Four different treatment planning systems (TPSs) were in use in Belgian and Dutch radiotherapy institutes, all with the American Association of Physicists in Medicine Task Group-43 (AAPM TG-43) dose calculation formalism implemented. However, a large variability in calculation models was observed and in 9 institutes the AAPM TG-43 update (2004) was not followed. The TPS tests revealed variations in dose calculation results due to incorrect application of the anisotropy model and to the use of source data which were since 2004 no longer recommended. Only for a small number of combinations of TPS, institute and seed type the 1D dose calculation errors were smaller than ±1% in the calculation points at distances between 0.5 and 6 cm. Misinterpretation of the TPS test and/or misunderstanding of the TG-43 dose calculation formalism caused larger errors and missing data for the 2D calculation tests. Most institutes apply the 1D model to calculate the prostate brachytherapy dose distributions. Most TPSs proved to underestimate the volumes in dose-volume histogram (DVH) calculations. The accuracy of DVH calculation depended on dose grid resolution and number of sampling points. Dose summation and representation of isodose lines proved to be correct in all reports.

The data obtained from the questionnaires were compared with recommendations given in international reports on QC of brachytherapy and on QC of TPS. With this background, recommendations have been formulated for dosimetry and quality control of low-energy photon sources (125I) to be used in the clinical practice of PPBT in Belgium and the Netherlands.

https://doi.org/10.25030/ncs-020

The NCS frequently publishes reports for fellow professionals in which recommendations are given for various quality control procedures or otherwise. The members of the NCS board and the members of the concerning subcommittee do not claim any authority exceeding that of their professional expertise. Responsibility on how the NCS recommendations are implemented lies with the user, taking into account the practice in his/her institution.

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