Relationship Between Tumor Microenvironment And The Dose Of Single Brachytherapy

AimBrachytherapy(BT)delivers high-dose irradiation to local tumor lesions while reducing dose to surrounding organs,making a relatively good therapeutic effect possible due to its unique advantages.However,the application of BT is relatively limited and some tumors are radiation resistant to BT.Therefore,it is necessary to explore the changes of tumor microenvironment(TME)in BT,combined with its dose distribution characteristics,in order to reveal the relationship between different BT doses and TME.Materials and MethodsIn this study,Balb/c female mice were subcutaneously injected with CT26 cancer cells at the age of 6 weeks to establish a tumor-bearing mouse model.When the tumor diameter reached 8-10mm,BT was conducted with Ir-192 radiation source,and the prescription doses was 5 Gy,10Gy,15 Gy and the control group(0Gy).On the second day after irradiation,tumor tissues were extracted for pathological biopsy,including Hematoxylineosin(HE)staining,Ki-67 and terminal-deoxynucleotidyl transferase mediated nick end labeling(TUNEL)immunohistochemical staining.On the 7th day after irradiation,tumor tissues and spleen were extracted,and flow cytometry was performed to detect the level of immune cells in tumor microenvironment,including CD8+T cells,CD4+T cells,natural killer(NK)cells,regulatory T(Treg)cells.In order to test the expression of inflammatory cytokines in the serum of mice,the peripheral blood of the mice was drawn for enzymelinked immunosorbent assay(ELISA)experiment,and the interferon-y(IFN-y),interleukin(IL)6,tumor necrosis factor(TNF)α expression were tested.Research resultsCompared with control group,the tumor growth of the irradiation experimental groups with different dose was significantly inhibited,and the 15Gy group was the most obvious.The mean Tumor growth inhibition rate(TGRI)of the 15Gy,10Gy and 5Gy groups were 83.6%,77.2%and 66.4%,respectively.After brachytherapy,the levels of CD8+T cells,NK cells,and M1 macrophages in the TME were significantly higher than the control group.In terms of pathological sections of tumor tissue,HE staining showed that compared with the control group,the tumor tissue in the experimental group was damaged to varying degrees,and there were obvious cell necrosis and liquefaction areas,which were most obvious in the 10Gy and 15Gy groups;as for Ki-67 Immmunohistochemistry,the area and proportion of Ki-67 positive cells in the control group was the highest,but decreased in varying degrees in the experimental group;compared with the control group,the apoptosis area was significantly larger in the 10Gy group and 15Gy group.The apoptotic area with high fluorescence intensity,and the damage and apoptosis of tumor cells are also related to the distance from the radiation source.ConclusionsSingle BT can promote the activity of TME and help kill tumor cells by increasing the infiltration of anti-tumor immune cells such as CD8+T cells,NK cells,and M1 macrophages.Among the different dose gradients explored in this experiment,prescription dose of 10Gy is with the best effect on TME activation.In addition,the damaging effect of BT on tumors in the pathological biopsy is related to the distance from the radiation source,showing the unique spatial heterogeneity of brachytherapy.