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Real-Time Intrafractional Motion Tracking During VMAT Delivery Using a Conventional Elekta CBCT System

Y Park

Yang-Kyun Park1*, Gregory C. Sharp1 , David P. Gierga1 , Sung-Joon Ye2 , Brian A. Winey1 , (1) Massachusetts General Hospital, Boston, MA, (2) Seoul National University, Seoul, Korea


SU-D-207-5 (Sunday, July 12, 2015) 2:05 PM - 3:00 PM Room: 207

Purpose:Real-time kV projection streaming capability has become recently available for Elekta XVI version 5.0. This study aims to investigate the feasibility and accuracy of real-time fiducial marker tracking during CBCT acquisition with or without simultaneous VMAT delivery using a conventional Elekta linear accelerator.

Methods:A client computer was connected to an on-board kV imaging system computer, and receives and processes projection images immediately after image acquisition. In-house marker tracking software based on FFT normalized cross-correlation was developed and installed in the client computer. Three gold fiducial markers with 3 mm length were implanted in a pelvis-shaped phantom with 36 cm width. The phantom was placed on a programmable motion platform oscillating in anterior-posterior and superior-inferior directions simultaneously. The marker motion was tracked in real-time for (1) a kV-only CBCT scan with treatment beam off and (2) a kV CBCT scan during a 6-MV VMAT delivery. The exposure parameters per projection were 120 kVp and 1.6 mAs. Tracking accuracy was assessed by comparing superior-inferior positions between the programmed and tracked trajectories.

Results:The projection images were successfully transferred to the client computer at a frequency of about 5 Hz. In the kV-only scan, highly accurate marker tracking was achieved over the entire range of cone-beam projection angles (detection rate / tracking error were 100.0% / 0.6±0.5 mm). In the kV-VMAT scan, MV-scatter degraded image quality, particularly for lateral projections passing through the thickest part of the phantom (kV source angle ranging 70°-110° and 250°-290°), resulting in a reduced detection rate (90.5%). If the lateral projections are excluded, tracking performance was comparable to the kV-only case (detection rate / tracking error were 100.0% / 0.8±0.5 mm).

Conclusion:Our phantom study demonstrated a promising result for real-time motion tracking using a conventional Elekta linear accelerator. MV-scatter suppression is needed to improve tracking accuracy during MV delivery.

Funding Support, Disclosures, and Conflict of Interest: This research is funded by Motion Management Research Grant from Elekta.

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