Increased 18F-FDG uptake within the reticuloendothelial system in patients with active lung cancer on PET imaging may indicate activation of the systemic immune response.

The reticuloendothelial system (RES) cells are in the defense against certain pathogens, and in the re- moval of dying cells, cell debris, microorganisms, and malignant cells. Liver, spleen, and bone marrow represent the major organs with high RES activity. We hypothesized that in subjects with active lung cancer, the metabolic activity of these organs would be greater than that of the subjects with no ac- tive tumor. We have studied two groups of subjects who had undergone 18 F-FDG-PET imaging for clinical purposes. The first group consisted of 39 subjects (20 women, 19 men, mean age 64.8±10.2 years) with benign lung nodules as demonstrated by 18 F-FDG-PET imaging. The second group con- sisted of 30 subjects (18 women, 12 men; mean age 65.1±11 years) who were known to have active lung cancer with or without distant metastases as seen on 18 F-FDG-PET imaging. The subjects in the second group did not have any evidence of liver, spleen, bone marrow, or heart involvement on 18 F- FDG-PET images. We measured the mean SUV of the liver, spleen, bone marrow, heart, and of the contralateral unaffected lung, and compared the average SUV for these organs between the two groups. We found that the mean SUV of the liver, bone marrow, and spleen were significantly greater in subjects with evidence of active primary or metastatic lung cancer compared with those of sub- jects who had benign lung nodules and no evidence of active malignant disease. There was a statisti- cally significant difference between mean SUV for organs noted above between the two groups (P<0.05). In contrast, mean SUV for the heart and contralateral normal lung did not show any signifi- cant difference between the two groups. In conclusion, the mean SUV for the major organs of RES, liv- er, spleen, and bone marrow were higher in subjects with active lung cancer with or without metas- tases than in those without active malignancy. We believe these differences in SUV may indicate a differential activation of the systemic immune response, related to the presence or absence of active lung cancer, which can be detected and quantified non-invasively through 18 F-FDG-PET imaging.