[en] Objective: To evaluate whether ¹⁸F-fluorothymidine (FLT) can be used to monitor early response to irradiation in colorectal cancer (CRC). Methods: SW480 cells were cultured and irradiated with 0, 10, and 20 Gy. Twenty-four hours later, morphological changes, apoptosis, necrosis, proliferation, and cell cycle phases were observed. Uptake of ¹⁸F-FLT was measured in these tumors in vitro from 24 h to 72 h after irradiation. The one-way analysis of variance was used to analyze the data. Results: Apoptotic and necrotic cells were detected 24 h after radiotherapy. SW480 cells proliferation was significantly delayed after irradiation in 3- (4, 5-dimethylthiazol-2-yl) -2, 5-diphenylte-trazolium bromide (MTT) assay. Cell cycle analysis showed that SW480 cells had a decreased fraction of cells in S phase (from 33.23% to 9.24%, then to 5.43%) and an arrested fraction in G₀-G₁. After SW480 cells were cultured for 60 min, the uptake of ¹⁸F-FLT was (5.21±1.60)%; and 24 h after irradiation of 10 Gy, the uptake decreased significantly to (4.27±0.48)% (F=8.253, P=0.009). And 72 h after irradiation, the uptake further decreased significantly to (3.39±0.59)% (F=36.715, P<0.001). In tumor tissue, the uptake of ¹⁸F-FLT reduced significantly 72 h after radiotherapy (10 Gy: F=12.388, P=0.007; 20 Gy: F=16.744, P=0.004) and the attenuation degree increased with the radiation dose. Conclusion: The uptake of ¹⁸F-FLT in SW480 cells or in CRC could reflect the changes of SW480 cells in proliferation, cell cycle re-distribution, cell apoptosis and necrosis. The results suggest that ¹⁸F-FLT may used for monitoring early response to irradiation of CRC. (authors)