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. 2010 Jan 28;11(1):3057.
doi: 10.1120/jacmp.v11i1.3057.

Angular dose dependence of Matrixx TM and its calibration

Luciant D Wolfsberger  1 , Matthew Wagar, Paige Nitsch, Mandar S Bhagwat, Piotr Zygmanski
Affiliations

Angular dose dependence of Matrixx TM and its calibration

Luciant D Wolfsberger et al. J Appl Clin Med Phys. .

Abstract

One of the applications of MatriXX (IBA Dosimetry) is experimental verification of dose for IMRT, VMAT, and tomotherapy. For cumulative plan verification, dose is delivered for all the treatment gantry angles to a stationary detector. Experimental calibration of MatriXX detector recommended by the manufacturer involves only AP calibration fields and does not address angular dependency of MatriXX. Angular dependency may introduce dose bias in cumulative plan verification if not corrected. For this reason, we characterized angular dependency of MatriXX and developed a method for its calibration. We found relatively large discrepancies in responses to posterior vs. anterior fields for four MatriXX (Evolution series) detectors (up to 11%), and relatively large variability of responses as a function of gantry angle in the gantry angle ranges of 91 degrees-110 degrees and 269 degrees-260 degrees. With our calibration method, the bias due to angular dependency is effectively removed in experimental verification of IMRT and VMAT plans.

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Figures

Figure 1
Figure 1
CT of MatriXX and reference phantoms. MatriXX phantom (left): MatriXX detector sandwiched between 8.2 cm (ant) and 5 cm (post) solid water slabs; reference phantom (right): A12 ionization chamber placed at the center of solid water phantom of total thickness of 17 cm.
Figure 2
Figure 2
Doses as a function of gantry angle in MatriXX and reference phantoms. The attenuation due to phantom shape is naturally included in these profiles.
Figure 3
Figure 3
Angular dependency of one of the MatriXX detectors. Data points are the measured doses relative to A12. Solid line is a cubic interpolation of mirrored data CF (Θ) → (CF(Θ) + CF(360° − Θ) / 2.
Figure 4
Figure 4
Typical dose profiles for cumulative IMRT and VMAT plans measured with MatriXX (corrected for angular dependency with CF approx./equal to 0.97) and compared to TPS. Before correction there was a dose bias of about 3%, and the isodose lines did not match well.
Figure 5
Figure 5
Relative water equivalent path length (left) and relative monitor units (right) as functions of gantry angle calculated in planning system. Relative is defined for MatriXX phantom with respect to the reference phantom. Monitor units are for the same amount of dose at the isocenter for 10×10cm open beam.
Figure 6
Figure 6
Dose profiles measured with MatriXX and reference ion chamber in water for 10×10cm at gantry angle 60°.
See this image and copyright information in PMC

References

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