Radiotherapy is a well-established cancer treatment; it is estimated that approximately 52% of oncology patients will require this treatment modality at least once. However, some tumors, such as triple-negative breast cancer (TNBC), may present as radioresistant and thus require high doses of ionizing radiation and a prolonged period of treatment, which may result in more severe side effects. Moreover, such tumors show a high incidence of metastases and decreased survival expectancy of the patient. Thus, new strategies for radiosensitizing TNBC are urgently needed. Red light therapy, photobiomodulation, has been used in clinical practice to mitigate the adverse side effects usually associated with radiotherapy. However, no studies have explored its use as a radiosensitizer of TNBC. Here, we used TNBC-bearing mice as a radioresistant cancer model. Red light treatment was applied in three different protocols before a high dose of radiation (60 Gy split in 4 fractions) was administered. We evaluated tumor growth, mouse clinical signs, total blood cell counts, lung metastasis, survival, and levels of glutathione in the blood. Our data showed that the highest laser dose in combination with radiation arrested tumor progression, likely due to inhibition of GSH synthesis. In addition, red light treatment before each fraction of radiation, regardless of the light dose, improved the health status of the animals, prevented anemia, reduced metastases, and improved survival. Collectively, these results indicate that red light treatment in combination with radiation could prove useful in the treatment of TNBC.

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