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Quality assurance for IMRT

The complex beam intensity modulation in each IMRT field has required a new system for patient set-up, treatment delivery and inverse treatment planning. QA for IMRT plan verification is more complicated as 3D-conformal therapy. Individual fields are composed of many small segments of varying intensity located on and off the central axis of the beam. The main approaches used for verification of IMRT plans are dosimetric measurement for small fields (dose profiles and PDD (present depth dose) and monitor unit calculation.

Individual quality control for IMRT treatment plan

For the quality control of IMRT treatment delivery is not enough to make a measurement only in one point, these plans are too complicated. With 2D dosimetric equipment (film, 2D array) we can measure the dose in the different planes. For this purpose we need special type of phantoms, which allows to determinate the dose distribution for one field or the resultant dose distribution from all fields. In the treatment planning system we can transfer the treatment plan on the QC phantom together with field configuration and dose constraints, and we can make a recalculation for the phantom. The calculation values can be exported in appropriate matrix points which comply with arrangement of detectors. We can make a quantitative evaluation of measured and calculated values after the irradiation of phantoms.
The accuracy of MU calculation can be measured in one dose point with ionisation chamber in homogeneous water equivalent phantom. The measuring point has to be placed in region where the dose distribution is homogeneous. With this method we cannot check those segments which are distant from the selected point. Therefore it is recommended to use the 2D array, which is suitable for checking all segments.

Field 1.: 2D detector.
Figure 2.: A 2D detector in phantom.
Figure 3.: Dosimetric controls for dose distribution receive from IMRT treatment.

Comment: számolt=calculated, mért=measured, különbség=difference.

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