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Detecting MLC errors in stereotactic radiotherapy plans with a liquid filled ionization chamber array


Quality assurance of stereotactic radiotherapy demands the use of equipment with the highest resolution and sensitivity available. This study examines the sensitivity of a commercially available liquid-filled ionization chamber array—the Octavius 1000 SRS (PTW, Frieburg, Germany) for detecting small (sub-millimetre) multi-leaf collimator (MLC) alignment errors in static square fields (side length 16–40 mm). Furthermore, the effectiveness of detecting small MLC errors in clinical stereotactic radiotherapy patient plans using the device was also evaluated. The commonly used gamma pass rate metric (of the measurements compared with treatment planning system generated results) was used. The gamma pass rates were then evaluated as a function of MLC position error (MLC error size 0.1–2.5 mm). The detector array exhibited a drop in pass rate between plans without error and those which had MLC errors induced. For example a drop in pass rate of 4.5 % (gamma criteria 3 %, 1 mm) was observed when a 0.8 mm error was introduced into a 16 mm square field. Furthermore the drop in pass rate increased as the MLC position error increased. This study showed that the Octavius 1000 SRS array could be a useful tool for applications requiring the detection of small geometric delivery uncertainties.

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All PTW equipment used in this study was provided by Nucletron Pty Ltd (Newtown, Australia) on a trial basis to the medical physics department. Measurements taken during the trial period form the basis of this study. The authors would like to extend their gratitude to Nucletron for providing the equipment.

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Correspondence to Patrick O’Connor.

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O’Connor, P., Seshadri, V. & Charles, P. Detecting MLC errors in stereotactic radiotherapy plans with a liquid filled ionization chamber array. Australas Phys Eng Sci Med 39, 247–252 (2016).

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  • Octavius 1000 SRS
  • Stereotactic QA
  • Dosimetry
  • Patient specific QA