Defining Radiotherapy Target Volumes Using 18F-Fluoro-Deoxy-Glucose Positron Emission Tomography/Computed Tomography: Still a Pandora's Box?

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. 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. 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. 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.