Abstract: Objective To evaluate the role of ¹⁸F-fluorothymidine（FLT） in monitoring early responses of colorectal cancer（CRC） to radiotherapy. Methods Changes in growth and cycle of SW480 and SW620 tumor cells were detected by MTT and flow cytometry after the cells were irradiated with 20 Gy.After 24h and 72h of irradiation,a tumor-bearing rat model of SW480 and SW620 cells was established,and PET/CT scanning was performed after injection of FLT（20±1.84MBq/0.25ml） through the tail veins of rats.Ratio of SW480 and SW620 tumor cells in ROI of tumor and normal tissues was calculated. Results The growth of SW480 tumor cells was much more significantly inhibited than that of SW620 tumor cells after irradiation.The distribution of SW480 tumor cells was decreased 83.66% and 45.23%,respectively,in S phase 72h after irradiation.PET/CT scanning showed that the ratio of SW480 and SW620 tumor cells in tumor and normal tissues was 3.65±0.51 and 2.22±0.42,respectively,and decreased to 45.25% and 29.28% after 24h and 72h of irradiation,respectively.Western blot revealed that Ki67 was expressed in SW480 and SW620 tumor cells and its expression was decreased at a different extent after irradiation. Conclusion Changes of FLT intake in CRC after irradiation can reflect the proliferation of CRC cells,and FLT-PET/CT can be used to judge the early responses of CRC to radiotherapy.