Impact of 4D-CT MIP reconstruction method on iGTV definition: A clinical case report

To present a clinical case demonstrating the impact of 4D-CT maximum intensity projection (MIP) reconstruction methodology on internal gross tumor volume (iGTV) definition and to highlight implications for motion management quality assurance. During routine thoracic radiation therapy treatment planning, a discrepancy in tumor extent was identified during physician contouring. The inferior extent of the tumor appeared artificially truncated on the MIP dataset used for target delineation. Further review revealed that the default MIP had been generated from phase-sorted 4D-CT images. A comparison was performed between the phase-sorted MIP and a MIP reconstructed from the original cine images to evaluate differences in motion representation. The phase-sorted MIP under-represented the full tumor motion envelope, most notably in the inferior direction. In contrast, the cine-based MIP demonstrated a more complete representation of tumor extent throughout respiration. This discrepancy was not readily apparent during the routine clinical workflow, and verification of the MIP reconstruction method was not part of the standard QA review process at the time. Differences in MIP reconstruction methodology can impact iGTV definition. Although these differences may be subtle in most cases, clinically meaningful discrepancies may occur, particularly in patients with irregular breathing patterns. This case highlights the importance of awareness and verification of 4D-CT MIP reconstruction methods and supports comprehensive QA across CT simulation, treatment planning, and motion management workflows.