Correlation Between SUVmax and CT Radiomic Analysis Using Lymph Node Density in PET/CT-Based Lymph Node Staging

In patients with lung cancer (LC), malignant melanoma (MM), gastroenteropancreatic neuroendocrine tumors (GEP NETs), and prostate cancer (PCA), lymph node (LN) staging is often performed by ¹⁸F-FDG PET/CT (LC and MM), ⁶⁸Ga-DOTATOC PET/CT (GEP NET), and ⁶⁸Ga-labeled prostate-specific membrane antigen PET/CT (PCA) but is sometimes not accurate because of indeterminate PET findings. To better evaluate malignant LN infiltration, additional surrogate parameters, especially in cases with indeterminate PET findings, would be helpful. The purpose of this study was to evaluate whether SUV_max in the PET examination might correlate with semiautomated density measurements of LNs in the CT component of the PET/CT examination.

After approval by the institutional review board, 1,022 LNs in the PET/CT examinations of 148 patients were retrospectively analyzed (LC: 327 LNs of 40 patients; MM: 224 LNs of 33 patients; GEP NET: 217 LNs of 35 patients; and PCA: 254 LNs of 40 patients). PET/CT was performed before surgery, biopsy, chemotherapy, or internal or external radiation therapy, according to the clinical schedule; patients with prior chemotherapy or radiation therapy were excluded. SUV_max analyses were based on uptake 60 min after tracer injection, and volumetric CT histogram analyses were based on the unenhanced CT images of the PET/CT scan.

PET findings were considered positive or negative on the basis of SUV_max in the LN compared with that in the blood pool; histologic confirmation was not available. Of the 1,022 LNs, 331 were PET-positive (3 times the SUV_max of the blood pool), 86 were PET-indeterminate (1-3 times the SUV_max of the blood pool), and 605 were PET-negative (less than the SUV_max of the blood pool). PET-positive LNs had significantly higher CT densities than PET-negative LNs, irrespective of the type of cancer.

CT density measurements of LNs in patients with LC, MM, GEP NET, and PCA correlated with¹⁸F-FDG uptake, ⁶⁸Ga-DOTATOC uptake, and ⁶⁸Ga-PSMA uptake, respectively, and might therefore serve as an additional surrogate parameter for differentiating between malignant and benign LNs. The use of a 7.5-Hounsfield unit CT density threshold to differentiate between malignant and benign LN infiltration and 20 Hounsfield units to exclude benign LN processes might be possible in clinical routine and would be especially helpful for PET-indeterminate LNs.