Evaluating the utility of simulated respiratory-gated four-dimensional computed tomography in internal-target-volume-based treatment planning for irregular breathing patterns.

In internal target volume (ITV)-based treatment planning, respiratory-gated four-dimensional computed tomography (4D-CT) is used to visualize the respiratory motions of lung tumors. However, in cases of irregular breathing, conventional 4D-CT cannot be synchronized with patient-specific respiratory waveforms, which poses challenges to obtaining images that accurately represent the ITV of the tumor. To address this challenge, we propose "simulated respiratory-gated 4D-CT" synchronized with the simulated waveform. This study aimed to assess the precision of ITV delineation using simulated respiratory-gated 4D-CT in patients with irregular breathing. A respiratory motion phantom was employed by inserting a simulated tumor and assessing the method under sinusoidal and irregular breathing conditions. Irregular respiratory waveforms of 20 patients were used to simulate irregular breathing. The phantom was imaged using conventional 4D-CT, slow-scan CT, and our method. For conventional 4D-CT and our method, the average and maximum intensity projections (MIP) were obtained via postprocessing. ITVs corresponding to each respiratory motion were manually contoured. Furthermore, the Dice similarity coefficient (DSC) was computed based on the ITV of conventional 4D-CT. No significant disparity was found between the ITVs obtained using our method and those obtained using conventional 4D-CT under sinusoidal motion. The DSC values of the ITVs derived from our method using MIP were the highest. In the assessment of the irregular respiratory waveforms, the ITVs obtained via MIP using our method closely approximated the ideal ITV in 20 patients. These findings demonstrate that our method can enhance the accuracy of ITV delineation in lung tumors during irregular breathing episodes.