[en] Purpose: We discuss the effect of ¹⁸F-fluoro-deoxy-glucose (FDG) positron emission tomography (PET)/computed tomography (CT) data on target volume definition for radiotherapy planning. We compared the effect of various thresholding methods on the PET-based target volume vs. the standard CT-based tumor volume. Methods and Materials: Different thresholding methods were reviewed and compared to our PET-based gross tumor volume data obtained from a cohort of 31 non-small-cell lung carcinoma patients who had undergone preoperative PET/CT scans for staging. The feasibility and limitations of FDG-based PET/CT data on target volume delineation in radiotherapy planning have been demonstrated with frequently used approaches for target outlining such as the qualitative visual method and the fixed 15% or 40% of the maximal iso-uptake value threshold methods. Results: The relationship between PET-based and CT-based volumes generally suffers from poor correlation between the two image data sets, expressed in terms of a large statistical variation in gross tumor volume ratios, irrespective of the threshold method used. However, we found that the maximal signal/background ratios in non-small-cell lung carcinoma patients correlated well with the pathologic results, with an average ratio for adenocarcinoma, large cell carcinoma, and squamous cell carcinoma of 10.5 ± 3.5, 12.6 ± 2.8, and 14.1 ± 5.9, respectively. Conclusion: The fluctuations in tumor volume using different quantitative PET thresholding approaches did not depend on the thresholding method used. They originated from the nature of functional imaging in general and PET imaging in particular. Functional imaging will eventually be used for biologically tailored target radiotherapy volume definition not as a replacement of CT- or magnetic resonance imaging-based anatomic gross tumor volumes but with the methods complementing each other in a complex mosaic of distinct biologic target volumes.

[en] Purpose: Fluorodeoxyglucose-positron emission tomography (FDG-PET)/computed tomography (CT) is more accurate than CT in determining the extent of non-small-cell lung cancer. We performed a study to evaluate the impact of FDG-PET/CT on the radiotherapy volume delineation compared with CT without using any mathematical algorithm and to correlate the findings with the pathologic examination findings. Methods and Materials: A total of 32 patients with proven non-small-cell lung cancer, pathologic specimens from the mediastinum and lung primary, and pretreatment chest CT and FDG-PET/CT scans were studied. For each patient, two data sets of theoretical gross tumor volumes were contoured. One set was determined using the chest CT only, and the second, done separately, was based on the co-registered FDG-PET/CT data. The disease stage of each patient was determined using the TNM staging system for three data sets: the CT scan only, FDG-PET/CT scan, and pathologic findings. Results: Pathologic examination altered the CT-determined stage in 22 (69%) of 32 patients and the PET-determined stage in 16 (50%) of 32 patients. The most significant alterations were related to the N stage. PET altered the TNM stage in 15 (44%) of 32 patients compared with CT alone, but only 7 of these 15 alterations were confirmed by the pathologic findings. With respect to contouring the tumor volume for radiotherapy, PET altered the contour in 18 (56%) of 32 cases compared with CT alone. Conclusion: The contour of the tumor volume of non-small-cell lung cancer patients with co-registered FDG-PET/CT resulted in >50% alterations compared with CT targeting, findings similar to those of other publications. However, the significance of this change is unknown. Furthermore, pathologic examination showed that PET is not always accurate and histologic examination should be obtained to confirm the findings of PET whenever possible