TH-E-M100F-06: Estimation of the Error in Internal Target Volume (ITV) of Lung Tumor Obtained From Free-Breathing Cine-Mode 4DCT: A Simulation and Comparison Study Based On Dynamic MRI

Purpose: To quantitate the error of tumor internal target volume (ITV) as determined from simulated free-breathing cine-mode 4DCT using dynamic magnetic resonance imaging (dMRI). Method and Materials: 8 healthy volunteers and 6 lung tumor patients underwent a 5-minute MRI scan in the sagittal plane to acquire dynamic images of lung motion. A MATLAB program was written to simulate the cine-mode 4DCT acquisition by segmenting and resorting the MR images. Maximum intensity projection (MIP) images were generated from both simulated 4DCT (sCT) and dMRI, and the errors in MIP-based ITV from sCT (ε), comparing to those from dMRI, were determined and correlate to the subjects' respiratory variability (ν). Results: MIP-based ITVs from sCT were comparatively smaller than those from dMRI in both digital-phantom studies (ε=−21.64±8.23%) and lung tumor patient studies (ε=−20.31±11.36%). The errors in MIP-based ITV from sCT linearly correlated (, r²=0.76) with the subjects' respiratory variability.

Conclusions: Because of the low temporal resolution and retrospective resorting, 4DCT may not accurately depict the excursion of a moving tumor. Using 4DCT MIP image to define ITV may therefore cause under-dosing and increased risk of subsequent treatment failure. Patient-specific respiratory variability may also be a useful predictor of the 4DCT-induced error in MIP-based ITV determination.