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Experimental Measurements of 3D Dose Distribution for a Moving Target Treated with IMRT and VMAT


H Yan

H Yan1*, A Thomas1, M Oldham1, F Yin1, (1) Duke University Medical Center, Durham, NC

SU-E-T-381 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: Torgan motion during radiotherapy may cause under-dose of target and over-dose on surrounding normal tissues. To investigate its effect, a phantom study was performed to assess the dosimetric uncertainty caused by organ motion during IMRT and VMAT.

Methods: A motion phantom equipped with high-precision step motor was employed to mimic organ motion in anterior-posterior (AP) direction. A cylindrical Presage dosimeter was attached to the moving organ, in which a planning target volume (PTV=GTV+0.5cm GTV along longitudinal axis) was contoured. Multiple sets of computerize tomography (CT) data were acquired: stationary CT for the phantom without organ motion and 4-dimensional CT (4DCT) with organ motion using two different breathing patterns. A 9-field conventional intensity modulated radiotherapy (IMRT) plan and a partial-arc volumetric modulated arc therapy (VMAT) plan were generated based on the stationary CT set. It is designed to deliver prescription doses into the PTV in the Presage dosimeter. After the phantom was setup at treatment couch, on-board CBCT images and fluoroscopic images were taken to verify the positioning accuracy of each Presage dosimeter. The same IMRT and VMAT plans were delivered the phantom three times each with a new Presage dosimeter: one for stationary, two for different motion patterns. The irradiated Presage dosimeters were scanned using an optical scanning system and the 3D dose distributions were reconstructed from the scanned projections. The reconstructed 3D doses were compared with the calculated doses from a planning system to evaluate the delivery accuracy of each treatment.

Results: DVHs measured and 2D dose distributions from Presage dosimeters showed comparable dose coverage for GTV between stationary and motion targets. Considerable dose degradation was observed for PTV of moving targets.

Conclusions: Breathing-induced motion will cause considerable loss of dose coverage of PTV for both IMRT and VMAT treatments.

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