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 bon e marrow glucose metabolism in rodents, assessed by FDG biodistribution mea sured by tissue excision and gamma counting. Methods: Sixty-one rats were d ivided 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 Co-60 gamma -ray unit under anest hesia. Each group of rats was tasted 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 co ntrol group of rats that received neither local nor sham irradiation was st udied with FDG on day 0.(18F) activity in tissue 1 h after injection was me asured using a gamma counter. Smear specimens of bone marrow from bilateral femurs were examined by light microscopy. Results: Tracer uptake was relat ively stable in marrow from the sham-irradiated rats. By contrast, FDG upta ke of the irradiated marrow on day 1 was significantly higher (mean +/- SD, 0.257 +/- 0.036 percentage injected dose IID] per gram of tissue per kilog ram 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 %/D/g/kg) (P < 0. 05). Tracer uptake in the irradiated marrow on day 9 was significantly lowe r (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 nonirr adiated contralateral marrow from irradiated rats showed increased FDG upta ke 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 %[D/g/kg) (P < 0.05), The irradiated marrow smear s pecimens initially revealed increased percentages of neutrophils on day 1 ( 45% of 500 nucleoid cells examined per slide) compared with that of the sha m group (20%), followed by severely decreased overall cellularity on day 9. Conclusion: in this experimental system, normal marrow uptake of FDG trans iently rose, then Tell, and ultimately returned to baseline after external beam irradiation. Knowledge of this biphasic early irradiation effect on no rmal bone marrow may be important when the efficacy of radiation therapy on bone metastasis is evaluated using FDG PET after irradiation.