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Purpose: In treatment planning of the lung tumor with 4D-CT, maximum-intensity-projection (MIP) images have been used for delineation of the gross tumor volume plus motion or iGTV, which can then be revised with the multiple phases of the 4D-CT images. Although majority of contouring can be performed with MIP, the MIP images are not recommended for delineation of iGTV if the tumor is near or connected to the diaphragm or other structures of a similar density due to insufficient contrast between the tumor and the surrounding tissues in the MIP images. To remedy this shortcoming, the authors developed a new weighted MIP (wMIP) from cine CT without respiratory gating for contouring the iGTV. Methods: The wMIP images are obtained by keeping one phase of the cine CT images with the largest tumor in the overlap region of the tumor and the diaphragm. Outside the overlap region, the wMIP images are identical to the MIP images. Both MIP and wMIP images are obtained without respiratory gating from cine CT. Results: The authors demonstrated in a study of seven patients that wMIP can achieve 92% of the iGTV from 4D-CT. The maximum surface separation of the two iGTVs between wMIP and 4D-CT was 1.7 mm and six out of the seven studies had less than 1 mm in surface separation between the iGTVs of wMIP and 4D-CT. Conclusions: This development has the potential of enabling many CT scanners capable of cine CT to assess the respiratory motion of a lung tumor without 4D-CT. [ABSTRACT FROM AUTHOR]