Objective To give target outline guidance for lung tumor radiation therapy by respiratory gating(RG) four-dimensional PET/CT for lung cancer. Eventually reasonable radiation target regions in treatment planning are received by patients.

Methods Twenty malignant nodules were studied by RG PET/CT and conventional PET/CT. The differences of gross tumor volume defined by average four-dimensional PET and three-dimensional PET were compared in different lung locations. Differences of gross tumor volume defined by average four-dimensional CT and three-dimensional CT were also investigated. Differences between volume defined by average four-dimensional volumes and three-dimensional volumes were defined as relative difference of gross tumor volume, and influences for volume defined by four-dimensional PET and CT and three-dimensional PET and CT based on the nodal position and respiratory motion amplitude were also investigated in this study.

Results Both volumes defined by average four-dimensional PET measured with two techniques were 17.2% greater than the volume defined by three-dimensional PET on average. The relative difference of the volume defined by average four-dimensional PET and three-dimensional PET was related to the nodal position and respiratory motion amplitude. The mean difference was 26.5% for the lower lobe and hilus of the lung, which was much greater than that of the upper lobe and pleura(2.7%). When the respiratory motion amplitude of nodules was > 3 mm, the difference between the volumes defined by average four-dimensional PET and three-dimensional PET was 24.3%(as < 3 mm, difference was 1.8%). The volume defined by average four-dimensional CT was 3.9% larger than the volume defined by three-dimensional CT, with difference range of nodule volume at 0.2 cm³ to 5.9 cm³ and ratios of 1.10±0.32. Only the nodules in the lower lobe showed significantly larger volumes defined by average four-dimensional CT than those defined by three-dimensional CT with average difference of 11.3%.

Conclusion For lung nodules close to the liver and spleen, the volume defined by average four-dimensional PET showed more accuracy for sketching tumor target. For nodules around the hilus of the lung, the volume defined by average four-dimensional PET can be considered as tumor target. For the upper lobe and pleural nodules, the volume defined by average four-dimensional CT was selected for the target outline, which adopted low-dose RG scan and considered breathing movement.