Evaluation of the early effect of local irradiation on normal rodent bone marrow metabolism using FDG: preclinical PET studies.

Our aim was to evaluate the early effect of local irradiation on normal bone marrow glucose metabolism in rodents, assessed by FDG biodistribution measured by tissue excision and gamma counting. METHODS: Sixty-one rats were divided into nine groups (n = 4-11 per group). Eight groups of rats received either local irradiation (10 Gy) or sham irradiation to the right femur on day 0. Irradiation was performed using a 60Co gamma-ray unit under anesthesia. Each group of rats was fasted overnight and then injected with 5.5-7.4 MBq FDG on day 1, 9, 18, or 30 after the local or sham irradiation. A control group of rats that received neither local nor sham irradiation was studied with FDG on day 0. 18F activity in tissue 1 h after injection was measured using a gamma counter. Smear specimens of bone marrow from bilateral femurs were examined by light microscopy. RESULTS: Tracer uptake was relatively stable in marrow from the sham-irradiated rats. By contrast, FDG uptake of the irradiated marrow on day 1 was significantly higher (mean +/- SD, 0.257 +/- 0.036 percentage injected dose [ID] per gram of tissue per kilogram of rat weight [%ID/g/kg]) than that of the sham group on day 1 (0.187 +/- 0.028 %ID/g/kg) and the control group (0.184 +/- 0.009 %ID/g/kg) (P < 0.05). Tracer uptake in the irradiated marrow on day 9 was significantly lower (0.148 +/- 0.023 %ID/g/kg) than that of the sham group on day 9 (0.193 +/- 0.021 %ID/g/kg) and the control group (P < 0.01). In contrast, the nonirradiated contralateral marrow from irradiated rats showed increased FDG uptake on day 18 (0.274 +/- 0.063 %ID/g/kg) that was significantly higher than that of the sham group on day 18 (0.208 +/- 0.030 %ID/g/kg) and the control group (0.183 +/- 0.018 %ID/g/kg) (P < 0.05). The irradiated marrow smear specimens initially revealed increased percentages of neutrophils on day 1 (45% of 500 nucleoid cells examined per slide) compared with that of the sham group (20%), followed by severely decreased overall cellularity on day 9. CONCLUSION: In this experimental system, normal marrow uptake of FDG transiently rose, then fell, and ultimately returned to baseline after external beam irradiation. Knowledge of this biphasic early irradiation effect on normal bone marrow may be important when the efficacy of radiation therapy on bone metastasis is evaluated using FDG PET after irradiation.