Radiotherapy has been found to be valuable for the control and eradication of local foci in various malignant tumors. The abscopal effect is determined as a systemic antitumor response at a distance from the irradiation site invoked by local irradiation. We herein present an extremely rare case of breast cancer in a 64-year-old woman, in whom the abscopal effect was observed after radiotherapy induced an antitumor response in all metastatic lesions, without any combination therapy. The patient was admitted to our hospital complaining of a breast mass and pain at the left hip, and was diagnosed with breast cancer with multiple bone, lung and lymph node metastases. She received treatment with local radiotherapy delivered to the breast tumor and some of the bone metastases but did not receive chemotherapy due to her poor performance status. However, 10 months after radiotherapy, spontaneous regression was observed, not only within the irradiated field, but also in the non-irradiated areas. All signs of cancer throughout the body disappeared, and the patient's performance status drastically improved. To the best of our knowledge, there have been no reports of advanced breast cancer cases in which the abscopal effect was observed after radiation monotherapy; therefore, this case report is extremely rare and highly valuable.
Keywords: abscopal effect; anti-tumor response; breast cancer; radiotherapy; spontaneous regression.