[A Case of Pregnancy-Associated Breast Cancer Which Underwent Surgical Treatment during Pregnancy and Chemotherapy after Delivery].

Case: A 37-year-old pregnant woman arrived at our hospital with an abnormal mammogram.

Medical History: Mammography performed in June 2018 revealed an abnormal shadow on the left breast. Cytology from the 6-mm tumor in the left upper-outer quadrant revealed a malignancy. At the same time, a transvaginal echo revealed cysts, and the patient was diagnosed at 5 weeks gestation. Needle biopsy revealed a luminal A-like cStage Ⅰ, cT1bN0M0 invasive ductal carcinoma (IDC). Tumor resection and sentinel lymph node biopsy were performed under local anesthesia at 12 weeks gestation, and post-delivery adjuvant therapy was planned. Histologic examination of the resected tumor revealed that it was HER2-positive( immunohistochemistry score 3+); therefore, we had to reconsider the use of trastuzumab and decided to administer it to the patient after childbirth. The patient gave birth by cesarean section, and weekly paclitaxel plus trastuzumab was initiated 7 months after surgery. The patient is currently alive without recurrence.

Discussion: We faced several difficulties during the treatment of this patient. Postoperative adjuvant therapy is recommended to be administered 8 weeks after the surgical resection of the tumor. However, in our case, given that the tumor was HER2-positive, we could administer adjuvant therapy with trastuzumab only after delivery. Although the prevalence of breast cancer in women below the age of 40 years in Japan is currently as low as 4-6%, the incidence of pregnancy-associated breast cancer is predicted to increase as the number of elderly primigravida increases due to later marriage.