Reconstruction of high resolution MLC leaf positions using a low resolution detector for accurate 3D dose reconstruction in IMRT

Ruurd Visser, J. Godart, D. J. L. Wauben, J. A. Langendijk, A. A. van't Veld, E. W. Korevaar

Research output: Contribution to journalArticleAcademicpeer-review

Abstract

In pre-treatment dose verification, low resolution detector systems are unable to identify shifts of individual leafs of high resolution multi leaf collimator (MLC) systems from detected changes in the dose deposition. The goal of this study was to introduce an alternative approach (the shutter technique) combined with a previous described iterative reconstruction method to accurately reconstruct high resolution MLC leaf positions based on low resolution measurements. For the shutter technique, two additional radiotherapy treatment plans (RT-plans) were generated in addition to the original RT-plan; one with even MLC leafs closed for reconstructing uneven leaf positions and one with uneven MLC leafs closed for reconstructing even leaf positions. Reconstructed leaf positions were then implemented in the original RT-plan for 3D dose reconstruction. The shutter technique was evaluated for a 6 MV Elekta SLi linac with 5 mm MLC leafs (Agility (TM)) in combination with the MatriXX Evolution detector with detector spacing of 7.62mm. Dose reconstruction was performed with the COMPASS system (v2.0). The measurement setup allowed one row of ionization chambers to be affected by two adjacent leaf pairs. Measurements were obtained for various field sizes with MLC leaf position errors ranging from 1.0mm to 10.0mm. Furthermore, one clinical head and neck IMRT treatment beam with MLC introduced leaf position errors of 5.0mm was evaluated to illustrate the impact of the shutter technique on 3D dose reconstruction. Without the shutter technique, MLC leaf position reconstruction showed reconstruction errors up to 6.0mm. Introduction of the shutter technique allowed MLC leaf position reconstruction for the majority of leafs with sub-millimeter accuracy resulting in a reduction of dose reconstruction errors. The shutter technique in combination with the iterative reconstruction method allows high resolution MLC leaf position reconstruction using low resolution measurements with sub-millimeter accuracy.

Original languageEnglish
Pages (from-to)N642-N649
Number of pages8
JournalPhysics in Medicine and Biology
Volume61
Issue number23
DOIs
Publication statusPublished - 7-Nov-2016

Keywords

  • MLC
  • iterative reconstruction
  • 3D dose reconstruction
  • high resolution
  • 2D detector array
  • QUALITY-ASSURANCE
  • DOSIMETRY
  • VERIFICATION
  • FILM
  • BEAM
  • STEP

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