Radiotherapy is one of the most important options for treating breast cancer in humans. The development of biomarkers to monitor radiosensitivity is scarce. The aim of this study is to investigate the γH2AX levels in the human blood samples 0.5 h after radiotherapy compared to the levels before radiotherapy in breast cancer patients in relation to their respective body mass index (BMI). Blood plasma samples were collected from a total of 20 breast cancer patients before and after radiotherapy to measure γH2AX levels with an antibody against γH2AX based on enzyme-linked immunosorbent assay technique. The median BMI of the patients was 30 kg/m². γH2AX was differentially expressed in breast cancer patients before radiotherapy. γH2AX levels significantly increased in 14 patients after radiotherapy (P = 0.006), whereas γH2AX levels decreased in three patients after radiotherapy, and three patients were excluded. There was no correlation between γH2AX values after radiotherapy and BMI (P = 0.5, r = 0.1). Our results suggest that γH2AX can be used by ELISA technique to measure γH2AX in the blood plasma of breast cancer patients undergoing radiotherapy and can be considered a biomarker of radiosensitivity.