Phantom investigations on CT seed imaging for interstitial brachytherapy

Background and purposeThe Braphyqs group (BRAchytherapy PHYsics Quality System, the brachytherapy physicist’s task group of GEC-ESTRO) investigated the quality of CT- and X-ray based seed reconstruction procedures using the Kiel-phantom. In this study systematic phantom investigations on CT post-planning and the results of a mailed multi-centre inter-comparison are presented.Materials and methodsThe phantom was equipped with a test configuration composed of 17 non-radioactive seeds. To investigate the quality of seed reconstruction CT measurements with varying CT parameters and different seed models were carried out.In a mailed multi-centre approach the phantom was sent to six European seed centres. The centres performed a typical CT- or X-ray based post-planning. The coordinates of the reconstructed sources were compared with the known positions in the phantom.ResultsIn the systematic study it was found for the used CT scanner and seed models that when the slice thickness or the table index (respectively, an appropriate pitch for helical scans) reaches 4 or 5 mm the accuracy of the CT seed reconstruction decreases in longitudinal direction. No influences of scanned field of view, tube current, kVp, or scan type (axial or spiral) on seed reconstruction accuracy were detected. This finding was confirmed by the multi-centre evaluation.It was demonstrated that the Kiel-phantom is a suitable quality assurance (QA) tool for the assessment of the seed reconstruction accuracy in post-implant procedures and that it is a feasible QA test tool for a mailed multi-centre approach.ConclusionsQA of seed post-planning is necessary. A trend was observed that when the slice thickness and table index is 4 or 5 mm the standard deviation of the reconstructed seeds increases for CT-based post-planning. Individual optimizations can be performed with dedicated phantoms.