HER2-low lobular breast cancer with bone marrow involvement responding to T-DXd

What happened, in summary

This patient, a woman over 72, had a history of lobular breast cancer treated with total mastectomy. Her surgical specimen was ER-positive, PR-negative, and HER2-negative by IHC 0. After surgery, she received adjuvant chemotherapy and was monitored for years while taking the aromatase inhibitor letrozole. During 8 to 9 years of follow-up, imaging and laboratory testing did not suggest metastatic breast cancer. The relapse became apparent through blood-count abnormalities rather than a typical mass. She developed thrombocytopenia, with platelets below 60,000, along with mild anemia. Because that pattern could suggest a separate blood disorder, she was evaluated for a possible secondary hematologic malignancy. Bone marrow aspiration instead showed infiltration by breast cancer, and the bone marrow disease showed low HER2 staining, different from the original HER2-negative surgical specimen. With limited established options for this revised HER2-low setting, she was treated with T-DXd at 5.6 mg/kg intravenously every 21 days. The response was rapid in the blood counts: after only 2 infusions, both platelet and hemoglobin levels recovered. The improvement in thrombocytopenia and anemia supported a strong response in the bone marrow, the site that had revealed the recurrence. Two years after starting T-DXd, she remained free of bone marrow disease and continued the same treatment regimen. Her course illustrates how lobular breast cancer can recur in the marrow after years of quiet surveillance and how low HER2 staining can open a targeted treatment path. No other metastatic site was described in this marrow-focused relapse, and ongoing therapy continued to maintain the response.