title: Improved source path localisation in ring applicators and the clinical impact for gynecological brachytherapy.
abstract: OBJECTIVE
  The path of subsequent dwell positions of an afterloader source being moved through a ring applicator for cervix cancer brachytherapy deviates from an ideal circle and the position of marker wires. This can lead to deviations of several millimetres between real and assumed dwell positions for treatment planning with simplified source path models. The aim of this study was to test video- and autoradiography-based methods for source path determination, and to study the influence of dwell position accuracy on dose-volume histogram (DVH)-parameters.
  
  
  METHODS
  Videos of the exact motion of the source wire through three different (r = 26, 30, 34 mm) computed tomography/magnetic resonance (CT/MR) compatible plastic ring applicators were recorded. Observed dwell positions covering the whole length of each applicators channel were used to adjust the circular source path model. The agreement of the true source positions derived from video analysis with those of the corrected circular source path was verified using autoradiography. The impact of an accurate source path definition on dose planning was analysed by simulating clinically relevant uncertainties in 10 clinical treatment plans.
  
  
  RESULTS
  Depending on the ring size, source path diameters had to be increased by 0.5-1.0 mm in order to achieve acceptable maximum differences between observed and corrected dwell positions (1.3-2.0 mm). Autoradiography analysis showed a positional accuracy within ± 3 mm (extended standard deviation k = 2). For shifts of ± 2.5 mm for even all dwell positions, the systematic and random variation of the D2cm(3) for bladder, rectum, and sigmoid was within 3%, while the impact on DVH uncertainties was much smaller for clinical target volume (CTV)HR and gross tumour volume (GTV).
  
  
  CONCLUSIONS
  It is strongly advised to verify the real source path for ring applicators during acceptance testing in order to assure accurate source path definition and dose planning. Autoradiography can be used for source path verification with acceptable accuracy for treatment planning and dose reporting.
authors.affiliation: Department of Radiotherapy, Comprehensive Cancer Center, Medical University of Vienna.
authors.forename: Irene
authors.initials: I
authors.lastname: Humer
authors.affiliation: Department of Radiotherapy, Comprehensive Cancer Center, Medical University of Vienna ; Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria.
authors.forename: Christian
authors.initials: C
authors.lastname: Kirisits
authors.affiliation: Department of Radiotherapy, Comprehensive Cancer Center, Medical University of Vienna.
authors.forename: Daniel
authors.initials: D
authors.lastname: Berger
authors.affiliation: Department of Radiotherapy, Comprehensive Cancer Center, Medical University of Vienna.
authors.forename: Petra
authors.initials: P
authors.lastname: Trnková
authors.affiliation: Department of Radiotherapy, Comprehensive Cancer Center, Medical University of Vienna ; Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria.
authors.forename: Richard
authors.initials: R
authors.lastname: Pötter
authors.affiliation: Department of Radiotherapy, Comprehensive Cancer Center, Medical University of Vienna ; Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria.
authors.forename: Nicole
authors.initials: N
authors.lastname: Nesvacil
keywords: applicator geometry; brachytherapy; gynaecological; quality assurance; source path reconstruction; treatment planning