Intriguingly, it would be a novel strategy to help better application of 18F-FDG PET in the diagnosis and treatment of HCC patients based on the Ki-67 index. In this research, we found that (1) visual grouping and semi-quantitative analysis by lesion-to-liver SUVmax ratio were highly consistent (AUC=0.987, 95% CI [0.967–1.000], p<0.001), and cut-off value was 1.59 [13], lesions with the ratio greater than or equal to 1.59 were considered as FDG-avid (easy to be detected by 18F-FDG PET). (2) Tumor differentiation and HePpar-1 status were negatively correlated with the FDG-avidity of tumor lesion, while the Ki-67 index was positively correlated with it, but after correction, only the relationship between Ki-67 index and the FDG-avidity was significant. Compared to the tumor differentiation, Ki-67 index might be a more relevant and precise correlative factor for the 18F-FDG uptake of HCC tumors. (3) When the Ki-67 index of HCC lesion was more than 17.5%, this lesion might have higher FDG-avidity, the sensitivity of 18F-FDG PET/CT examination also increased significantly.

Many previous studies suggested that 18F-FDG PET/CT has low sensitivity in the diagnosis of HCC, the sensitivity was based on tumor cells uptake more 18F-FDG than surrounding [6, 14]. There were many reasons found recent years why the sensitivity was low in the diagnosis of HCC by 18F-FDG PET/CT, the most popular theory was that the low hexokinase activity and high glucose-6-phosphatase activity in well-differentiated HCC tumors, and resulting in the uptake of 18F-FDG by HCC lesion similar to that of normal liver parenchyma [7], however, this theory could not take into account the moderately differentiated HCC (accounts for a considerable proportion of HCC).

Ki-67 is well known as a kind of proliferative nuclear marker, which was first identified by Gerdes, etc. [15]. Ki-67 is a nuclear DNA binding protein, it is widely expressed in proliferating cells, many previous studies about cell cycle had shown that Ki-67 was present in the G1, S, and G2 phases of the cell cycle, but not in the G0 phase [10, 16], therefore, its high expression often represents that the proliferation of the cells is relatively vigorous. Many studies have shown that Ki-67 plays an important role in tumor grading and prognosis [17, 18], including HCC, studies had shown that high expression of Ki-67 predicted poorer prognosis [19], but its application in the interpretation of 18F-FDG PET/CT has not been studied. In this research, we found that Ki-67 index is positively correlated with the FDG-avidity of HCC lesion, and when an FDG-avid HCC lesion was found, the Ki-67 index might be more than 17.5% by immunohistochemistry. The reason might be the expression of glucose transporters (GLUTs). GLUT1 and GLUT3 are highly expressed in many tumors and both are thought to be highly correlated with the FDG-avidity of tumor [20, 21]. The expression of GLUTs protein is regulated by different signaling pathways and transcription factors in different tumors, however, in HCC tumors, the relationship between Ki-67 expression and GLUT1, GLUT3 protein expression is not clear until now [22].

In this study, we found that the FDG-avidity of HCC tumors have a correlation with the tumor differentiation, thus, poorly differentiated tumor with higher FDG-avidity and well-differentiated tumor with lower, which was also consistent with the classical views [22,23,24,25,26]. However, when the differentiation and the Ki-67 index entered the logistic regression equation together, only the correlation between Ki-67 index and FDG-avidity was significant. The reason might be that there is a correlation between Ki-67 index and differentiation of HCC tumors [19, 27], we also reached similar conclusions in this study, but Ki-67 index might be a more independent, relevant, and precise correlative factor for the 18F-FDG uptake of HCC tumors [28]. Another major reason might be that the FDG-avidity can be high or low for moderately differentiated HCC [7, 14, 29], as shown in Figs. 5 and 6.

When the Ki-67 index of HCC lesion was above 17.5%, this lesion might have high FDG-avidity. This might not only guide clinicians in the selection of 18F-FDG PET/CT examinations (because PET/CT is a radiological and high-cost examination [30]), but also play a role in image-guided treatment. For example, some targeted therapies targeting Ki-67 protein mainly guided by pathological biopsy results [10], however, the results of pathological biopsy might be affected by many conditions, such as biopsy site, section, and staining [31, 32], analyzing the FDG-avidity of the lesions might also help to guide the biopsy. When the Ki-67 index of HCC lesion was below 17.5%, other tumor imaging agents such as choline might need to be given more consideration [33]. The postoperative follow-up of HCC was often complicated by inflammation and bile leakage [34], and active foci of FDG metabolism in the hepatic region might be challenging to identify [35]. This study's findings might facilitate their detection.

Our study has some limitations. In the validation cohort group 1 (Ki-67 index above 17.5%), there were still 30% of patients with non-FDG-avid tumors, which might need to be combined with additional indicators for a more precise determination. Additionally, the attainment of a more precise Ki-67 index cut-off value requires a substantial sample size. These requirements necessitate further research efforts, and we are in the process of establishing prospective multi-center clinical trials to acquire more accurate data. At last, the intrinsic relationship between Ki-67 expression and FDG-avidity in HCC tumor remains unclear, these may require more laboratory research work by more people.

In conclusion, this study found a positive relationship between Ki-67 index and FDG-avidity in HCC tumors by 18F-FDG PET/CT that Ki-67 index might be a more effective predictor of 18F-FDG PET/CT sensitivity than tumor differentiation in HCC diagnosis and treatment. 18F-FDG PET/CT might be a good hint for HCC when Ki-67 index is higher than 17.5%, the sensitivity of 18F-FDG PET/CT examination increased significantly at this time. This might not only guide clinicians in the selection of 18F-FDG PET/CT examination, but also might provide a new way in image-guided treatment, and this discovery has the potential to assist nuclear medicine physicians in effectively utilizing FDG PET/CT for differential diagnosis of HCC concerning treatment effectiveness and the possibility of recurrence.