Successful treatment with proton beam therapy for a solitary sternal metastasis of breast cancer: a case report

BACKGROUND: Breast cancer infrequently metastasizes to the sternum as solitary metastasis. We experienced successful treatment with proton beam therapy for a case of sternal metastasis of breast cancer. This case demonstrates for the first time the role of proton therapy in the treatment of oligometastatic sternal metastasis with limited tolerance of normal tissue due to previous photon irradiation. CASE PRESENTATION: A 40-year-old Japanese female presented with lumpiness in her left breast. The patient was diagnosed with breast cancer (cT1N0M0, cStage IA) and underwent partial mastectomy with axillary lymph node dissection. After the mastectomy, the patient received radiation therapy with 50 Gy in 25 fractions for initial irradiation of the left breast. After the initial irradiation of 50 Gy, the patient received 10 Gy in five fractions of a sequential boost for the tumor bed to a total dose of 60 Gy. Although the patient was administered tamoxifen after radiation therapy, solitary sternal metastasis occurred 6 months after radiation therapy. She refused chemotherapy and requested proton beam therapy for her sternal metastasis. The daily proton beam therapy fractions were 2.5 relative biological effectiveness, receiving a total dose of 70 Gy relative biological effectiveness. An acute side effect of grade 2 dermatitis according to the National Cancer Institute Common Terminology Criteria for Adverse Events version 4.0. occurred during proton beam therapy, but there was no acute or late complication of more than grade 3. At 3 years after proton beam therapy, the patient remains in complete remission without surgery or chemotherapy. DISCUSSION AND CONCLUSION: Proton beam therapy for solitary sternal metastasis of breast cancer is considered to be a therapeutic option.