Abstract: Background: The efficacy of third-generation aromatase inhibitors for hormone receptor-positive postmenopausal metastat­ic breast cancer is well established. Although several clinical trials have reported incomplete cross-resistance between different aromatase inhibitors, few cases of complete responses of recurrent metastatic breast can­cer occurring after substituting a second aromatase inhibitor have been reported. We here present a rare case of non-steroidal aromatase inhibitor-tolerant metastatic breast cancer with long-term complete remission fol­lowing substitution of a steroidal aromatase inhibitor.
Case Report: We present the case of a 56-year-old Japanese woman who underwent right breast-conserving surgery for breast cancer, TNM staging T1, N0, M0, Stage I. She received adjuvant chemotherapy with 6 cycles of FEC100 and radiation therapy, and then began hormonal therapy with anastrozole. Twelve months postoperatively, computed tomography (CT) revealed multiple lung metastases. Exemestane was substituted for anastrozole. After 3 months of exemestane, CT showed that all lung metastases had completely resolved. Her complete re­sponse was maintained for 5 years: she died during a tsunami 6 years after the initial surgery.
Conclusions: Substitution of a steroidal for a non-steroidal aromatase inhibitor produced a sustained complete remission in a patient with hormonal receptor-positive postmenopausal recurrent breast cancer.
Achieving complete response after switching from a non-steroidal to a steroidal aromatase inhibitor in a hor­monal receptor-positive postmenopausal recurrent breast cancer contributed to a higher quality of life for the patient. Further investigation is needed to identify the predictors of long-term remission following such a switch.

Keywords: Neoplasm Metastasis, Breast Neoplasms - therapy, Aromatase Inhibitors