Impact of FDG PET/CT on delineation of the gross tumor volume for radiation planning in non-small-cell lung cancer.

Purpose: Computed tomography (CT) remains the gold standard for delineation of tumor volumes for radiotherapy (RT) planning. However, positron emission tomography (PET) overlay on CT has shown to impact the gross target volume (GTV), decrease intraobserver variability, and change the treatment planning in a significant number of patients. The objective of this study was to evaluate the influence and accuracy of FDG PET in GTV definition as a complementary modality to CT for patients with non-small-cell lung carcinoma at Vanderbilt University Medical Center. Methods: Data from 11 consecutive patients with non-small-cell lung carcinoma, which were referred to FDG PET/CT for initial staging and RT planning were analyzed retrospectively. All patients had undergone routine staging using a RT noncontrasted CT. Both the RT CT and PET/CT images were acquired using standard protocols but with the patients positioned in the same RT immobilization devices. Both the CT and PET/CT images were transferred to the RT planning workstation for contouring. GTV, pathologic nodal and metastases volumes were first defined in the conventional manner based on RT CT. The FDG PET and RT CT planning image datasets were coregistered with the help of the transmission CT from PET/CT. FDG PET GTVs were determined by a team of radiation oncologists and nuclear physician with expertise in PET/CT, and displayed simultaneously with the CT contours. The RT CT and PET GTV were measured and the percent difference was calculated for the primary tumor, pathologic lymph nodes, and distant metastases. A difference of 15% was considered significant. Results: The primary tumor GTV was decreased in 36% (n = 4) of patients by differentiating atelectasis and postobstructive pneumonia from tumor, and increased GTV in 27% (n = 3) of patients by detecting additional tumor burden. Increased nodal disease burden was detected in 18% (n = 2) of patients. The use of PET/CT changed treatment from curative to palliative by detecting distant metastasis in 27% (n = 3) of patients. Conclusions: Our results are consistent with the published data of PET/CT altering GTV in a significant number of patients, detecting tumor spread to additional lymph nodes and distant metastases. While these advantages support the use of PET/CT in RT planning, it remains unknown what impact this will have on patient outcomes.