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Quantifying Intrafractional Prostate Rotation From Cone-Beam Computed Tomography with Radiopaque Markers

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C Huang

CY Huang*, J Tehrani, J Ng, P Keall, The University of Sydney, Camperdown, New South Wales

SU-E-J-118 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Rotation greater than one degree can severely compromise the accuracy of radiotherapy treatments. Prostate intrafractional rotation is not negligible because peristaltic motion has a time scale that is shorter than the delivery time of a single fraction. The objectives of this study were (a) to investigate a clinically feasible method to calculate tumor rotation with radiopaque markers; (b) to evaluate the accuracy of the method and (c) to quantify prostate intrafractional rotation in intensity-modulated arc radiotherapy (IMAT).

Methods: 2D kilovoltage (kV) cone-beam computed tomography (CBCT) images were acquired for prostate cancer patients with three implanted gold fiducial markers as surrogates for the tumor position during fractionated IMAT. Post-treatment, 3D marker trajectories were determined from the 2D projection images by maximum likelihood estimation of a 3D probability density function. Rotations about each axis (roll (right/left), pitch (superior/inferior) and yaw (anterior/posterior)) were then calculated based on the iterative closest point algorithm (ICP) method, with the origin set at the centroid of the three markers.

Results: Intrafractional rotation was studied for10 patients, 268 fractions. Average roll, pitch and yaw were calculated as 0.85±1.16, 0.41±0.51 and 0.37±0.4 degrees, respectively. Roll was the predominant rotation among the three axes which is consistent with literature. Rotations showed a similar pattern among different fractions for the same patient. The pattern acquired at the first few fractions could be used as a prediction for the rest of the fractions.

Conclusion: This study demonstrated a novel method to quantify intrafractional tumor rotation from 2D kV projections with radiopaque markers. Intrafractional rotation is small but significant for certain cases. Real-time radiopaque marker-based adaptive radiotherapy with rotation correction could make a positive impact on reducing treatment margins.

Funding Support, Disclosures, and Conflict of Interest: This project is supported by an NHMRC Australia Fellowship and NHMRC project grant 1034060.

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