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Investigating the Importance of Phantom Geometry in Intensity-Modulated Radiation Therapy Quality Assurance


B Hamilton

B Hamilton*, U Langner, University of Kentucky, Lexington, Kentucky

SU-E-T-192 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
To determine the effect of beam weighting through the phantom geometry on the error detection for phantoms used in intensity modulated radiation therapy quality assurance (IMRT QA) programs.

Methods:
PTW Octavius II and MapCHECK2 with MapPHAN were evaluated due to their differing phantom geometries. PTW is an octagonal phantom with its central diode at ~15cm equivalent depth from all directions in its axial plane, whereas MapPHAN is a rectangular phantom. Beams with a field size of 10x10 cm, were delivered onto each phantom at gantry angles of 0, 20, 45, and 70 degrees for varying monitor units. The central axis reading was recorded and normalized to the reading for gantry angle at 0 and 100 monitor units. Errors of different magnitude were introduced in a beam from each gantry angle and compared to the beam with no errors.

Results:
This work showed that while the response at large angles for MapCHECK2 is still linear (constant response vs. monitor units delivered), the reading per monitor unit falls off by 26% from that of the enface beam. This is caused by increased attenuation because of more material in the beams path.This results in an underestimation of the error in this beam. A test on a plan involving beams from all four angles with a passing criteria of 5%/3mm, passed for an error of 25 percent introduced in the 70 degree beam for MapCheck2, while it failed for PTW.

Conclusion:
Phantom geometry intrinsically introduces beam weighting for IMRT QA, which might cause some errors to be undetected. A phantom with geometry similar to that of the patient should therefore be chosen when IMRT QA plans are delivered in order to minimize this effect. This will ensure that beams are weighted similarly than for the intended plan.

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