FDG brain uptake on PET-CT scans in patients with large tumors

Objectives There is a growing body of clinical reports suggesting that cancer patients have symptoms of fatigue and lethargy at the time of diagnosis, even without evidence of brain metastases. In clinical practice, we often observe decreased FDG uptake in the brain of patients with large tumors on routine whole-body staging PET-CT scans. The aim of this study was to characterize FDG uptake in the brain of patients with large tumors on PET/CT to determine the effect of tumor size on brain metabolism.

Methods We included 15 patients with large tumors (mean age 62) and 15 patients with small tumors (mean age 66) on staging base skull-thigh PET-CT scans. Exclusion criteria included glucose>140 mg/dL, children and pregnant women, patients with known brain metastases, or known mental disease. Two investigators evaluated each scan using MIM fusion software. We measured total tumor volume, maximum standard uptake value (max SUV), metabolic tumor volume (MTV), and total glycolytic activity (TGA). Max SUV in the bilateral inferior frontal lobes, lateral temporal lobes, and cerebellum was also measured.

Results The large tumor group had average tumor volume 833 mL, max SUV 18.5 and TGA 5309, while the small tumor group had average tumor volume 6 mL, max SUV 4.8 and TGA 24. Statistical analysis using student T-test found a significant difference between the two groups in FDG uptake within the brain, with max SUV significantly lower in the group with large primary tumors across all 6 studied brain regions (p<0.001). The max total brain SUV was 7.49 (SD ± 1.99) in the large tumor group and 11.76 (SD ± 2.44) in the small tumor group.

Conclusions This preliminary retrospective analysis suggests that patients with large tumor volume have decreased brain metabolism, which may possibly explain the cognitive dysfunction reported in these patients, even in the absence of brain metastases.