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A Method of Improving the Effective Spatial Resolution of IMRT QA for 2D Array Devices


J Zhang

J Zhang*, W Laub, Oregon Health & Science Univ, Portland, OR

SU-E-T-367 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose:
To improve the effective spatial resolution of small field IMRT QA with 2D-array device.

Methods:
When small field IMRT QA is measured with 2D-array device, the spatial resolution (5-10mm) may be considered too sparse to accurately verify dose over high gradient area. Reducing the incident angle can improve spatial resolution along the transverse direction of the beam eye view (BEV). Rotating collimator can improve resolution along the other direction. However, detector angular dependency and electron scattering may potentially degrade measurement accuracy when the incident angle becomes too small. In this study, a Scandidos Delta4 device was used for IMRT QA measurements. It has two orthogonal dose planes with 5mm spatial resolution in central 6x6 cm. We measured IMRT plans with two settings: one with 40 and 50 incident angles and the other with 15 and 75 incident angles. The measured 2D dose planes were compared with the ones calculated by TPS. 3D doses were estimated by interpolating two measurement points along beamlets based on the PDD curve and compared with the one from TPS.

Results:
Calculation showed that for single detector plane, the spatial resolution along the transverse direction of the BEV is improved from 3.21 mm (for the 40 degree) to 1.29 mm (for the 15 degree). When both orthogonal planes were considered, the spatial resolution was improve from 1.75 mm to 1.02 mm. The QA measurements showed that the average passing rate with 2% dose deviation criterion was 74 % for 15-degree incident angle, only slightly lower than the passing rate of 80% for the 40-degree incident angle. On the other side, the 15/75 setting reconstructed 3D dose in high gradient region more accurately than the 40/50 setting due to higher spatial resolution.

Conclusions: A moderately small incident angle can improve effective spatial resolution without significant accuracy loss.

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