Abstract
BackgroundThe 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.Materials and methodsIntrafractional 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.ResultsRandom 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.ConclusionPre- 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 registrationClinicaltrials.gov, NCT03896555, Registered 01 April 2019 - retrospectively registered.
Highlights
The present study investigates the intrafractional accuracy of a frameless thermoplastic mask used for head immobilization during stereotactic radiotherapy
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
A total of 551 ExacTrac measurements were made with thermoplastic masks during 96 fractions of facilitated positioning
Summary
The present study investigates the intrafractional accuracy of a frameless thermoplastic mask used for head immobilization during stereotactic radiotherapy. Several studies have shown that image guidance makes set-up and repositioning uncertainty with the non-invasive mask immobilization comparable to that of invasive stereotactic ring application [2, 4,5,6] This method may have less intrafractional accuracy due to the nonrigid construction, indirect immobilization of the skull, and unpredictable patient movement. Many studies reported this effect only by means of quantifying pre- and Mangesius et al Radiation Oncology (2019) 14:231 postfractional deviations of the patient’s head by either CBCT (cone beam computed tomography) or ExacTrac [2, 4, 7, 8]. Novel single-isocenter intracranial irradiation techniques for multiple metastases [12,13,14] demand for highest precision since even smallest-scale rotational deviations may lead to insufficient target coverage of more distant lesions
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