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 ([formula omitted], 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. © 2007, American Association of Physicists in Medicine. All rights reserved.
