Reconstruction of high-resolution 3D dose from matrix measurements: error detection capability of the COMPASS correction kernel method

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

Research output: Contribution to journalArticleAcademicpeer-review


The COMPASS system (IBADosimetry) is a quality assurance (QA) tool which
reconstructs 3D doses inside a phantom or a patient CT. The dose is predicted
according to the RT plan with a correction derived from 2D measurements
of a matrix detector. This correction method is necessary since a direct
reconstruction of the fluence with a high resolution is not possible because
of the limited resolution of the matrix used, but it comes with a blurring of the
dosewhich creates inaccuracies in the dose reconstruction. This paper describes
the method and verifies its capability to detect errors in the positioning of a
MLC with 10 mm leaf width in a phantom geometry. Dose reconstruction was
performed forMLC position errors of various sizes at various locations for both
rectangular and intensity-modulated radiotherapy (IMRT) fields and compared
to a reference dose. It was found that the accuracy with which an error inMLC
position is detected depends on the location of the error relative to the detectors
in the matrix. The reconstructed dose in an individual rectangular field for leaf
positioning errors up to 5 mm was correct within 5% in 50% of the locations.
At the remaining locations, the reconstruction of leaf position errors larger than
3 mm can show inaccuracies, even though these errors were detectable in the
dose reconstruction. Errors larger than 9 mm created inaccuracies up to 17% in
a small area close to the penumbra. The QA capability of the system was tested
through gamma evaluation. Our results indicate that themean gamma provided
by the system is slightly increased and that the number of points above gamma 1
ensures error detection for QA purposes. Overall, the correction kernel method
used by the COMPASS system is adequate to perform QA of IMRT treatment
plans with a regular MLC, despite local inaccuracies in the dose reconstruction.
Original languageEnglish
Pages (from-to)5029-5043
JournalPhysics in Medicine & Biology
Publication statusPublished - 2011


  • radiotherapy


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