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Radiat Oncol. 2019 Dec 18;14(1):231. doi: 10.1186/s13014-019-1425-7.

Intrafractional 6D head movement increases with time of mask fixation during stereotactic intracranial RT-sessions.

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Department of Therapeutic Radiology and Oncology, Medical University of Innsbruck, Anichstraße 35, A-6020, Innsbruck, Austria.
Department of Therapeutic Radiology and Oncology, Medical University of Innsbruck, Anichstraße 35, A-6020, Innsbruck, Austria.
Department of Medical Statistics, Informatics and Health Economics, Medical University of Innsbruck, Innsbruck, Austria.



The present study investigates the intrafractional accuracy of a frameless thermoplastic mask used for head immobilization during stereotactic radiotherapy. Non-invasive masks cannot completely prohibit head movements. Previous studies attempted to estimate the magnitude of intrafractional inaccuracy by means of pre- and postfractional measurements only. However, this might not be sufficient to accurately map also intrafractional head movements.


Intrafractional deviation of mask-fixed head positions was measured in five patients during a total of 94 fractions by means of close-meshed repeated ExacTrac measurements (every 1.4 min) conducted during the entire treatment session. A median of six (range: 4 to 11) measurements were recorded per fraction, delivering a dataset of 453 measurements.


Random errors (SD) for the x, y and z axes were 0.27 mm, 0.29 mm and 0.29 mm, respectively. Median 3D deviation was 0.29 mm. Of all 3D intrafractional motions, 5.5 and 0.4% exceeded 1 mm and 2 mm, respectively. A moderate correlation between treatment duration and mean 3D displacement was determined (rs = 0.45). Mean 3D deviation increased from 0.21 mm (SD = 0.26 mm) in the first 2 min to a maximum of 0.53 mm (SD = 0.31 mm) after 10 min of treatment time.


Pre- and post-treatment measurement is not sufficient to adequately determine the range of intrafractional head motion. Thermoplastic masks provide both reliable interfractional and intrafractional immobilization for image-guided stereotactic hypofractionated radiotherapy. Greater positioning accuracy may be obtained by reducing treatment duration (< 6 min) and applying intrafractional correction.

TRIAL REGISTRATION:, NCT03896555, Registered 01 April 2019 - retrospectively registered.


ExacTrac; Frameless stereotactic radiation therapy; Intrafractional accuracy; Thermoplastic mask head fixation; X-ray position monitoring

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