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Two-dimensional ionization chamber arrays for IMRT plan verification
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View: Figures


Image of FIG. 1.
FIG. 1.

Schematic drawings of the 2D-ARRAYs commercially available. Left: version 1 (PTW type 10017), ionization chambers. Right: version 2 (PTW type 10024), chambers.

Image of FIG. 2.
FIG. 2.

(a) Scheme of the collimation of a slit beam that had a full half-width of in the ISO plane. A: Upper collimator, slit width corresponding to in the ISO plane. B: Lower collimator (MLC), one leaf pair opening with a slit length of in the ISO plane. C: Tertiary collimator (cast metal), positioned in the accessory holder, slit width . (b) Profile of the slit beam in the ISO plane, measured by film dosimetry. Squares: film placed on top of a PMMA phantom, no preabsorber. Circles: film at depth in PMMA.

Image of FIG. 3.
FIG. 3.

A comparison of a measured and planned Virtual Wedge profile (, ). Points: 2D-ARRAY (version 2); line: Dose profile measured with ionization chamber.

Image of FIG. 4.
FIG. 4.

Field-size-dependent output factors for photon radiation, measured with different detectors at a depth of in a RW3 phantom (source-to-surface distance , backscatter material). At the small field sizes, the volume effect of the ionization chambers is clearly detectable.

Image of FIG. 5.
FIG. 5.

Lateral response functions of types 1 and 2 of 2D-ARRAYs measured by slit scanning with a slit of half-width. The gray areas denote the positions of the ridges between two adjacent chambers.

Image of FIG. 6.
FIG. 6.

Dose profile at the edge of a photon field at in the in-plane direction. A comparison of the true profile (diode values), the true profile convolved with the response function of the single chamber of the 2D-ARRAY, and the measured values of a single chamber of the 2D-ARRAY obtained by stepwise scanning across the dose distribution. (a) 2D-ARRAY version 1; (b) 2D-ARRAY version 2.

Image of FIG. 7.
FIG. 7.

Work flow chart of the comparison between expected and measured readings of the 2D-ARRAY. The procedure is shown for version 1 (type 10017).

Image of FIG. 8.
FIG. 8.

Dose profile along the dashed line marked in the insert. Solid line: dose values from planning system. Dotted line: profile from film measurement. Squares: expected readings of the 2D-ARRAY (version 1). Crosses: measured values . The visible shift of the expected and measured readings from the profile is due to dose gradients in the direction vertical to the scanning line.

Image of FIG. 9.
FIG. 9.

Chamber readings failing the gamma-index criterion for the 2D-ARRAY’s versions 1 and 2. Black squares: chambers failing the criterion with , . Lines: isodoses from the planning system.

Image of FIG. 10.
FIG. 10.

Left: MLC shape of a segment of the IMRT field with introduced error. Solid line: segment without error. Dashed line: artificial MLC error of . The relative weight of this segment to the overall field is 30%. Right: chambers failing to meet the gamma-index criterion after irradiation of the sequence (black square), with and . Lines: isodoses from a planning system (2D-ARRAY, version 2).


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Two-dimensional ionization chamber arrays for IMRT plan verification