Stage 4 hormone receptor-positive breast cancer: remission with PARP inhibitor

What happened, in summary

This 41-year-old woman had several breast cancer events and a prior bone marrow transplant for aplastic anemia. She was first diagnosed with left breast cancer at age 29. That tumor was hormone receptor-positive and HER2-negative, and treatment included mastectomy, sentinel lymph node biopsy, and postoperative endocrine therapy. At age 31, she developed contralateral right breast cancer and underwent partial resection, axillary dissection, endocrine therapy, and radiotherapy. That tumor was ER 90%, PR 5%, HER2 1+, and Ki-67 20%. At age 33, she underwent bone marrow transplant for aplastic anemia. Two years later, at age 35, a left pleural effusion was diagnosed as malignant pleural effusion, consistent with metastatic breast cancer recurrence. Weekly paclitaxel achieved a clinical complete response, maintained for 3 years with tumor-marker monitoring and imaging. At age 37, she developed right breast recurrence with right-sided pleural effusion and had a right mastectomy. Pathology showed ER >10%, PR >10%, HER2 1+, grade 2 disease with Ki-67 30%. Anastrozole was selected first because of safety considerations after transplant; abemaciclib was tried but stopped for toxicity. She then received 4 cycles each of FEC chemotherapy and docetaxel, achieving clinical complete response. Tumor-based gene panel testing was used instead of circulating tumor DNA testing because her prior bone marrow transplant could complicate blood-based analysis. The panel found a pathogenic BRCA2 R2318* mutation with VAF 71.52%, supporting PARP inhibitor therapy. She started olaparib and continued it for 12 months, with ongoing monitoring by tumor markers and imaging. As of January 2025, she had maintained complete response for 36 months.