Appendicitis caused by the metastasis of HER2-positive breast cancer.

The appropriate therapy for metastatic breast cancer must be selected based on the immunohistochemical phenotype of the cancer. However, biopsy for metastatic lesions is difficult. We herein report a patient with incidental appendicitis caused by a metastatic breast cancer which was successfully treated with effective therapy chosen based on the pathological diagnosis obtained on resection. The patient was a 56-year-old female with right breast cancer and an immunohistochemical status of estrogen receptor (ER) (+), progesterone receptor (PgR) (+), human epidermal growth factor receptor 2 (HER2) (3+), and Ki67 40 %. She received epirubicin and cyclophosphamide therapy followed by docetaxel and trastuzumab, and total mastectomy with axillary dissection was performed. Thereafter, she received postmastectomy radiation, adjuvant trastuzumab, and adjuvant hormone therapy with letrozole. One year and 8 months after the operation, she developed right hydronephrosis and swollen para-aortic lymph nodes and her hormone therapy was changed to fulvestrant therapy. However, she additionally developed left hydronephrosis and multiple bone metastases, and pertuzumab, trastuzumab, and docetaxel therapy was started. After six cycles, her disease became well-controlled, and maintenance with pertuzumab and trastuzumab was introduced. However, after another 7 months, she developed new vertebral metastasis and acute appendicitis and laparoscopic appendectomy was performed. A pathological investigation of the resected appendix revealed some clusters of atypical cells in the subserosa and muscle layer, which showed an immunohistochemical status of ER (+), PgR (-), HER2 (3+), and E-cadherin (-). These findings led to the diagnosis as appendiceal metastasis of invasive lobular carcinoma (ILC) from the breast. Thereafter, she received trastuzumab-DM1 and her disease was well-controlled again. Appendicitis caused by breast cancer is very rare. However, ILC sometimes develops metastases in the abdominal cavity; an appendiceal tumor should therefore be included in the differential diagnosis. A pathological diagnosis of metastatic tumor could be very useful for selecting the effective therapy.