gms | German Medical Science

27th German Cancer Congress Berlin 2006

German Cancer Society (Frankfurt/M.)

22. - 26.03.2006, Berlin

Repositioning accuracy of two different mask systems 3D revisited: a comparison using true 3D-3D matching with cone-beam CT

Meeting Abstract

  • corresponding author presenting/speaker Judit Boda-Heggemann - Klinik für Strahlentherapie und Radioonkologie, Universitätsklinikum Mannheim, Deutschland
  • Cornelia Walter - Klinik für Strahlentherapie und Radioonkologie, Universitätsklinikum Mannheim
  • Hansjörg Wertz - Klinik für Strahlentherapie und Radioonkologie, Universitätsklinikum Mannheim
  • Beate Küpper - Klinik für Strahlentherapie und Radioonkologie, Universitätsklinikum Mannheim
  • Angelika Rahn - Klinik für Strahlentherapie und Radioonkologie, Universitätsklinikum Mannheim
  • Frank Lohr - Klinik für Strahlentherapie und Radioonkologie, Universitätsklinikum Mannheim
  • Frederik Wenz - Klinik für Strahlentherapie und Radioonkologie, Universitätsklinikum Mannheim

27. Deutscher Krebskongress. Berlin, 22.-26.03.2006. Düsseldorf, Köln: German Medical Science; 2006. DocOP379

The electronic version of this article is the complete one and can be found online at:

Published: March 20, 2006

© 2006 Boda-Heggemann et al.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( You are free: to Share – to copy, distribute and transmit the work, provided the original author and source are credited.



Reliable immobilisation and accurate interfractional repositioning are of major importance in modern radiotherapy. Systems used for fractionated stereotactic treatment that are usually mask based, can exhibit a significant daily variation in target alignment.

Accuracy of positioning systems has been assessed with ap/lateral X-ray films, electronic portal imaging (EPID), video images or repeated CT/MRI scans outside the treatment rooms, but these methods offer only a 2D or 2D/3D matching, or are not directly associated with actual treatments.

KV cone-beam CT (CBCT) on board of a linear accelerator enables daily assessment and correction of both translational and rotational errors with small dose exposition and minimal additional time requirements by acquiring true 3D-image data and comparison with planing image (CT) data sets. Positioning errors can then be determined in six degrees of freedom and be immediately corrected.

The objective of this study was to assess and compare the magnitude of the interfractional translational and rotational setup errors in two different established head mask systems based on volumetric data (true 3D-3D matching) acquired with a CBCT (Elekta Synergy).

11 patients treated for intracranial/head and neck tumors were enrolled. Six of these were positioned in rigid masks (Delta-Cast EliteTM); five in thermoplastic masks. Before delivering a treatment fraction, CBCT volume imaging was performed and was analysed based on matching that was limited either to the skull region or the neck region by different physicians and by an automatic bone matching algorithm. X, y and z components of translation of the target isocenter, the calculated absolute magnitude target isocenter translation vector and target isocenter rotation in x, y and z directions were analysed. Mean length of the displacement vector in stereotactic masks for intracranial regions was 0.312±0.152 cm, and 0.472±0.174 cm for thermoplastic masks for automatic matching, the results with manual matching were similar. Detailed are presented in Table 1 [Tab. 1].


a) stereotactic masks have a high intracranial repositioning accuracy per se.

b) thermoplastic masks are more precise than assumed and, because of the small rotational error and according to preliminary data regarding residual error after position correction and regarding intrafractional movement, may be used for precision treatments when combined with CBCT.

c) Without any special procedures, the precision in case of the neck region was in both cases worse than in the intracranial regions. Total length of the displacement vector, however is still acceptable for most situations, can be improved with body tattoos and may be further reduced by image guided position correction.