18F-FDG PET-CT in early assessing the therapeutic effects of nasopharyngeal carcinoma xenograft in nude mice after radiotherapy and it's relativity to the expression of Ki67

Objective: To investigate the value of 18F-FDG PET-CT scans on early assessing the therapeutic effects on human nasopharyngeal carcinoma (NPC) xenograft in nude mice after radiotherapy, and preliminarily analyze the relation of 18F-FDG PET-CT scans and the positive expression rate of Ki67, and provide a basis for further clinical studies. Methods: Fifteen xenograft nude mice of NPC were randomly divided into three groups (5 nude mice per group). Control group: all mice immediately executed after the 18F-FDG PET-CT scans and taken pathology; 6 Gy radiotherapy group: all mice took 18F-FDG PET-CT scans before and 1 d after receiving 6 Gy irradiation, and were killed immediately to take pathology; 12 Gy radiotherapy group: all mice received 12 Gy irradiation (6 Gy each time, totally 2 times). They took 18F-FDG PET-CT scans before radiotherapy, after receiving the first 6 Gy irradiation, 2 d and 6 d after receiving 12 Gy irradiation respectively, and then were executed to take pathology after the last scan. The expression of Ki67 in xenografts of different groups was detected and the relation between their changes and T/NT were analyzed. Results: 1) For 12 Gy radiotherapy group, the average T/NT ratios of the NPC xenograft of pre-radiotherapy, after the first 6 Gy irradiation, 2 d and 6 d after 12 Gy radiotherapy were 2.17 ± 0.31, 1.68 ± 0.42, 1.41 ± 0.40, and 0.70 ± 0.12, respectively. The average T/NT ratios were not statistically difference before and after the first 6 Gy irradiation. The average T/NT ratio of 2 d after radiotherapy decreased significantly than pre-radiotherapy (t=2.80, P<0.05) and after the first 6 Gy irradiation (t=3.14, P<0.05); The average T/NT ratio of 6d after radiotherapy was decreased seriously than 2 d after radiotherapy (t=3.49, P<0.05, compared with pre-radiotherapy t= 8.01, P<0.01). 2) The positive expression rates of Ki67 of NPC xenograft markedly decreased after receiving irradiation. There were significant differences in the positive expressions of Ki67 between different group s (F=21.95, P<0.01). The positive expression of Ki67 in the 12 Gy radiotherapy group decreased significantly compared with the control group (t=7.145, P<0.01) and the 6 Gy radiotherapy group (t=2.384, P<0.05). The positive expression of Ki67 in the 6 Gy radiotherapy group was less than the control group, also have statistical significance (t=4.320, P<0.01). 3) There was no correlation between the T/NT ratio of NPC xenograft and the positive expression of Ki67. 4) Positive correlation was found between the variance of T/NT ratio and the variance of the positive expression of Ki67 of NPC xenograft (r=0.532, P<0.05). Conclusions: 18F-FDG PET-CT imaging has an important value on early assessing the therapeutic effects of NPC xenograft in nude mice after radiotherapy, and the 6th day after radiotherapy is an appropriate time point. 18F-FDG PET-CT imaging can reflect the variance of the positive expression of Ki67 of NPC, and yet indirectly reflect the sensitivity of NPC to radiotheraphy. (authors)