Paraneoplastic Immune Thrombocytopenia in Estrogen Receptor (ER)-Positive/Human Epidermal Growth Factor Receptor 2 (HER2)-Positive Advanced Breast Cancer: Clinical Implications and Therapeutic Strategies.

This case report describes the clinical course, diagnosis, and management of a premenopausal woman with a history of estrogen receptor (ER)-positive and human epidermal growth factor receptor 2 (HER2)-positive early breast cancer (EBC) who developed severe thrombocytopenia. After six years of the diagnosis of EBC (treated with curative intent), she presented with back pain, significant bruising, and menorrhagia. During the work-up, laboratory tests revealed severe thrombocytopenia, and imaging studies, including computed tomography scan and bone scintigraphy, demonstrated extensive bone lesions and adenopathies. A multidisciplinary approach was crucial in addressing her complex condition, considering potential diagnoses such as immune thrombocytopenia secondary to paraneoplastic syndrome versus hematologic malignancies. Given her history and clinical presentation, the most likely diagnosis was a recurrence of breast cancer with extensive bone metastasis. Despite challenges, including limited biopsy options due to thrombocytopenia, targeted anti-HER2 therapy and endocrine therapy (ET) led to significant clinical improvement.