Historically, it has been difficult to monitor the acute impact of anticancer therapies on hematopoietic organs on a whole-body scale. Deeper understanding of the effect of treatments on bone marrow would be of great potential value in the rational design of intensive treatment regimens. 3 -deoxy-3 -18F-fluorothymidine (18F-FLT) is a functional radiotracer used to study cellular proliferation. It is trapped in cells in proportion to thymidine-kinase 1 enzyme expression, which is upregulated during DNA synthesis. This study investigates the potential of 18F-FLT to monitor acute effects of chemotherapy on cellular proliferation and its recovery in bone marrow, spleen, and liver during treatment with 2 different chemotherapy regimens. METHODS: Sixty patients with non-small cell lung cancer underwent concurrent radical chemoradiotherapy to 60 Gy in 6 wk with either cisplatin/etoposide (C/E, n = 28) weeks 1 and 5 or weekly carboplatin/paclitaxel (C/P, n = 32) regimens. 18F-FLT and 18F-FDG PET with CT were performed at baseline, week 2 (day 9 for 18F-FLT and day 10 for 18F-FDG PET), and week 4 (day 23 for 18F-FLT and day 24 for 18F-FDG PET). Visual and semiquantitative standardized uptake value (SUV) measurements were performed in bone marrow outside the radiotherapy field, liver, spleen, and small bowel. These were correlated to blood counts and smears in a subset of patients. RESULTS: The C/E group exhibited a drop in bone marrow 18F-FLT uptake at week 2 (median SUVmax [maximum SUV] decrease to 31 , 8.7-6.0, P <0.001), with recovery at week 4, reflecting the absence of chemotherapy between these times. By contrast, the weekly C/P group showed gradually declining bone marrow uptake (P > 0.05). Spleen uptake in both cohorts decreased at week 2, with intense rebound activity at week 4 (SUVmax week 4 at 58 above baseline: 2.4-3.8, for C/E, respectively, 30 for C/P: 2.7-3.5, P <0.001). Liver uptake changed little. 18F-FLT changes preceded neutrophil count reductions. 18F-FDG uptake in marrow liver and spleen changed much less than 18F-FLT. CONCLUSION: 18F-FLT imaging may be used to quantify impairment and recovery of bone marrow by specific chemotherapy regimens and may also enable imaging of organ-specific processes such as spleen activation. 18F-FLT is superior to 18F-FDG for this purpose. This technology may support novel treatment planning and monitoring approaches in oncology patients.