A prospective evaluation of staging and target volume definition of lymph nodes by ¹⁸FDG PET/CT in patients with squamous cell carcinoma of thoracic esophagus

Purpose: To determine an optimal standardized uptake value (SUV) threshold for detecting lymph node (LN) metastases in esophageal cancer using ¹⁸F-Fluorodeoxyglucose positron emission tomography/computer tomography ( ¹⁸FDG PET/CT) and to define the resulting nodal target volume, using histopathology as a "gold standard." Methods: Sixteen patients with esophageal squamous cell carcinoma who underwent radical esophagectomy and three-field LN dissection after ¹⁸FDG PET/CT and CT scans were enrolled into this study. Locations of LN groups were recorded according to a uniform LN map. Diagnostic performance of different SUV thresholds was assessed by receiver operating characteristic analysis. The optimal cutoff SUV was determined by plotting the false-negative rate (FNR) and false-positive rate (FPR), the sum of both error rates (FNR+FPR), and accuracy against a hypothetical SUV threshold. For each patient, nodal gross tumor volumes (GTVNs) were generated with CT alone (GTVNCT), PET/CT (GTVNPET), and pathologic data (GTVNpath). GTVNCT or GTVNPET was compared with GTVNpath by means of a conformity index (CI), which is the intersection of the two GTVNs divided by the sum of them minus the intersection, e.g., CI _CT&path = GTVN _CT&path/(GTVN _CT+ GTVN _path - GTVN _CT&path). Results: LN metastases occurred in 21 LN groups among the 144 specimens taken from the 16 patients. The area under the receiver operating characteristic curve was 0.9017 ± 0.0410. The plot of error rates showed a minimum of FNR+FPR for an SUV of 2.36, at which the sensitivity, specificity, and accuracy were 76.19%, 95.93%, and 93.06%, respectively, whereas those of CT were 33.33%, 94.31%, and 85.42% (p values: 0.0117, 0.7539, and 0.0266). Mean GTVN _CT, GTVN _PET, and GTVN _path were 1.52 ± 2.38, 2.82 ± 4.51, and 2.68 ± 4.16cm ³, respectively. Mean CI _CT&path and CI _PET&path were 0.31 and 0.65 (p value = 0.0352). Conclusions: Diagnostic superiority of PET/CT at an SUV threshold of 2.36 over CT has potential value in nodal target volume definition, but whether this can contribute to better treatment outcomes needs prospective analyses of recurrences in a larger cohort of patients.