Progressive Liver Metastasis of Breast Cancer on Endocrine Therapy Showing Favorable Response to Additive Alternate-Day S-1 Therapy: A Case Report.

Breast cancer with multiple organ metastases sometimes shows a discordant response to treatment depending on the site of the lesion. In addition, oncologists generally convert the treatment upon progression of a small part of metastatic lesions in a specific organ, even if most metastatic lesions continue to respond well to the treatment. A 49-year-old postmenopausal woman had undergone mastectomy and sentinel node biopsy followed by breast reconstruction using a silicone implant for her left breast cancer 11 years before and was referred to our hospital due to the muscle weakness of her left upper arm. The patient had presumed multiple bone metastases, supraclavicular lymph node metastasis, and liver metastasis on images. Based on the judgment of estrogen receptor (ER)-positive and human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer, the patient received fulvestrant, palbociclib, and denosumab, leading to the alleviation of upper arm muscle weakness. Positron emission tomography (PET) showed a decrease in fluorodeoxyglucose (FDG) uptake in the extrahepatic lesions and an increase in FDG uptake in the liver lesion at six months, and continued decrease of FDG uptake in the extrahepatic lesions and retained increase of FDG in the liver metastasis at 14 months. The patient, therefore, received additive alternate-day S-1 therapy to the first-line endocrine-based therapy, leading to the normalization of the elevated CA15-3 level, regression of the liver metastasis with decreased FDG uptake, and no additive adverse events. It was noteworthy that the addition of alternate-day S-1 therapy to the first-line endocrine-based therapy, clinically effective for extrahepatic lesions, led to the shrinkage of liver metastases without notable side effects.