Diagnostic challenge of occult ER-positive breast carcinoma within bone marrow indolent B-cell lymphoma and response to CDK4/6 inhibitor: a case report

BACKGROUND: Bone marrow metastasis from hormone receptor-positive breast cancer can be challenging to detect, especially in patients with concurrent hematologic malignancies, where isolated epithelial cells may be hidden by a dominant lymphoid infiltrate. This situation requires careful histopathologic and immunophenotypic evaluation to prevent misdiagnosis. CASE PRESENTATION: We report a 74-year-old White woman with Waldenström's macroglobulinemia carrying the MYD88 L265P mutation and bone marrow infiltration by indolent B-cell non-Hodgkin lymphoma. She was evaluated for suspected skeletal disease progression due to rising tumor markers and inconclusive PET findings. Bone marrow biopsy revealed small clusters of epithelial cells positive for estrogen receptor (ER), cytokeratin AE1/AE3, and GATA3, consistent with metastatic breast carcinoma within a lymphoid background. Notably, ER positivity was the only marker indicating epithelial origin. The patient had previously received empirical aromatase inhibitor therapy after the initial detection of scattered ER-positive epithelial cells in 2018. Following disease progression in December 2024, she was treated with palbociclib and fulvestrant, while the indolent lymphoma was managed with active surveillance. At 6-month follow-up, the patient showed clinical stability, a biochemical response, and radiologic evidence of stable disease. CONCLUSIONS: This case emphasizes the importance of maintaining a high level of clinical and pathological suspicion when assessing bone marrow infiltration, especially in patients with a history of malignancy. The presence of isolated ER-positive epithelial cells within a lymphoid marrow environment should raise suspicion of occult breast carcinoma metastasis. Due to the rarity of this coexistence, the underlying mechanisms and optimal management strategies are still unclear. To our knowledge, this is one of the few reported cases of occult ER-positive breast carcinoma presenting solely as bone marrow involvement in a patient with indolent B-cell lymphoma, with documented response to CDK4/6 inhibitor-based therapy. This case report was prepared in accordance with the CAse REport (CARE) guidelines.