Quantitative evaluation of abscopal effect based on biological effective dose in breast cancer tumors in mice

Due to abscopal effect, cell damage may occur outside of the radiation field and the quantification of this effect is one of the most challenging debates in radiation therapy. The aim of this study was to estimate the abscopal effect induced in non-irradiated tumors quantitatively by means of biological effective dose (BED).
Breast tumors using 4T1 and MC4-L2 cells, were induced into the flank region of Balb/c mice. When palpable, the tumor on one side of the body was irradiated with dose of 28Gy in 14 fractions and 2 Gy per fraction, 5 fractions per week. The tumor on the other side of the body was shielded with a lead plate. BED was estimated based on tumor volume. H&E staining and TUNEL assay were performed to assess histological changes and apoptosis in irradiated and non-irradiated tumors.
The effect of radiation on non-irradiated tumors was more than that on irradiated ones. The BED was 4.49 and 6.74 in 4T1 and MC4-L2 tumors, respectively. The ratio of the tumor volume in the last fraction to that in the first fraction for irradiated 4T1 tumors was 2.32 and in non-irradiated was 1.50. This ratio in irradiated and non- irradiated MC4-L2 tumors was 2.64 and 1.98, respectively. The number of apoptotic cells was higher in non-irradiated tissues.
Results indicate that the occurrence of abscopal effect is highly depends on the type of tumor. By means of the abscopal effect, more radiation dose can be delivered to the tumor and metastatic sites.