To compare histological findings of FDG-PET false-positive and true-negative hilar and mediastinal lymph nodes.

Sixty-seven lymphnode areas in 11 patients who were diagnosed to have N3 lymph nodes by FDG-PET and underwent surgery were histologically examined, and the histopathological findings in false-positive and true-negative lymph nodes were compared. Lymph nodes with higher accumulation of FDG than the surrounding mediastinum level were judged as positive. On histological sections, proportions of macrophages and lymphocytes, amount of coal dust deposit, presence of silicotic nodules, long- and short-axes of the largest node, and volume of macrophages and lymphocytes were evaluated. Correlations between the above-mentioned factors and FDG accumulation were evaluated.

FDG uptake was not correlated with the proportion of macrophages and lymphocytes, coal dust amounts, or the presence of silicotic nodules. The long- and short-axes of the largest node in the false-positive areas were significantly longer than those in the true-negative areas (p = 0.01, and 0.001, respectively). Volumes of lymph nodes (mean ± SD: 150 ± 190 mm³) and macrophages (78 ± 71 mm³) in false-positive areas were markedly larger than those in true-negative areas (68 ± 87 mm³, p = 0.0009 and 34 ± 54 mm³, p = 0.0001, respectively). The volume of lymphocytes was also larger in false-positive areas but less markedly.

Our study suggested that false-positive results of FDG-PET in hilar and mediastinal lymph nodes were closely related to the size of lymph node and the volume of macrophages.