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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  1. 1.

    Low DA, Moran JM, Dempsey JF, Dong L, Oldham M (2011) Dosimetry tools and techniques for IMRT. Med Phys 38:1313–1338

    Article  PubMed  Google Scholar 

  2. 2.

    Alber M, Broggi S, De Wagter C et al (2008) Guidelines for the verfication of IMRT. ESTRO, Brussels

    Google Scholar 

  3. 3.

    O’Daniel J, Das S, Wu J, Fang-Fang Y (2010) Patient specific quality assurance: transition from IMRT to IMAT. J Phys 250:012050. doi:10.1088/1742-6596/250/1/012050

    Google Scholar 

  4. 4.

    Gloi AM, Buchanan RE, Zuge CL, Goettler AM (2011) RapidArc quality assurance through MapCHECK. J Appl Clin Med Phys 12:39–47

    Google Scholar 

  5. 5.

    Li G, Zhang Y, Jiang X, Bai S et al (2013) Evaluation of the ArcCHECK QA system for IMRT and VMAT verification. Phys Med 29:295–303

    Article  PubMed  Google Scholar 

  6. 6.

    Oliver M, Gagne I, Bush K, Zavgorodni S et al (2010) Clinical significance of multi-leaf collimator positional errors for volumetric modulated arc therapy. Radiother Oncol 97:554–560

    Article  PubMed  Google Scholar 

  7. 7.

    Heilemann G, Poppe B, Laub W (2013) On the sensitivity of common gamma-index evaluation methods to MLC misalignments in rapidarc quality assurance. Med Phys 40:031702

    CAS  Article  PubMed  Google Scholar 

  8. 8.

    Low DA, Harms WB, Mutic S, Purdy JA (1998) A technique for quantitative evaluation of dose distributions. Med Phys 25:656–661

    CAS  Article  PubMed  Google Scholar 

  9. 9.

    Markovic M, Stathakis S, Manroidis P, Jurkovic I et al (2014) Characterization of a two-dimensional liquid-filled ion chamber detector array used for verification of the treatments in radiotherapy. Med Phys 41:051704

    Article  PubMed  Google Scholar 

  10. 10.

    Poppe B, Stellje TS, Looe HK, Chofor N (2013) Performance parameters of a liquid filled ionization chamber array. Med Phys 40:082106

    CAS  Article  PubMed  Google Scholar 

  11. 11.

    Shaw E, Kline R, Gillin M, Souhami L et al (1993) Radiation therapy oncology group: radiosurgery quality assurance guidelines. Int J Radiat Oncol Biol Phys 27(5):1231–1239

    CAS  Article  PubMed  Google Scholar 

  12. 12.

    Kim J, Park S-Y, Hak JK, Kim JH et al (2014) The sensitivity of gamma-index method to the positioning errors of high-definition MLC in patient-specific VMAT QA for SBRT. Radiat Oncol 9:167

    Article  PubMed  PubMed Central  Google Scholar 

  13. 13.

    Nelms BE, Zhen H, Tome WA (2011) Per-beam, planar IMRT QA passing rates do not predict clinically relevant patient dose errors. Med Phys 38:1037–1044

    Article  PubMed  PubMed Central  Google Scholar 

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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