Breast cancer metastasis mimicking pacemaker pocket infection: a diagnostic pitfall and long-term preservation of the cardiac implantable electronic device-a case report.

Background: Cardiac implantable electronic device (CIED) pocket infection is the most common infectious complication following pacemaker implantation or generator replacement and generally requires complete device and lead extraction according to current international guidelines. Nevertheless, non-infectious disorders, including malignant disease, may closely mimic pocket infection, creating a challenging differential diagnosis and potentially leading to delayed diagnosis or unnecessary CIED extraction. Neoplastic involvement of the CIED pocket is exceptionally rare. Case summary: An 81-year-old woman with a permanent VDD pacemaker implanted for symptomatic complete atrioventricular block underwent elective generator replacement in January 2020. Several months later, she developed progressive pain, erythema, and swelling over the left prepectoral generator pocket, raising suspicion of CIED pocket infection. Despite empirical antibiotic therapy and surgical pocket revision, repeated microbiological cultures remained sterile and no intraoperative evidence of infection was identified. Progressive enlargement of the lesion prompted further diagnostic investigation. Ultrasonography demonstrated a highly vascularized solid mass, while contrast-enhanced computed tomography (CT) revealed a lobulated soft-tissue lesion adjacent to the pacemaker generator together with bilateral pulmonary nodules, bilateral breast nodules, and multiple osteolytic skeletal lesions. Complete surgical excision of the pocket mass was performed while preserving the pacing system. Histopathological examination demonstrated metastatic well-differentiated invasive ductal carcinoma of breast origin (oestrogen receptor 95%, progesterone receptor 95%, human epidermal growth factor receptor 2 score 0, Ki-67 7%). The unusually large pocket lesion represented the first clinical manifestation leading to the diagnosis of an otherwise clinically occult disseminated breast carcinoma. Following oncological staging, endocrine therapy consisting of letrozole followed by the addition of a CDK4/6 inhibitor resulted in prolonged disease control. At the most recent follow-up in 2026, contrast-enhanced CT demonstrated partial regression of metastatic disease, without local recurrence, while the CIED continued to function normally. Discussion: Persistent or atypical CIED pocket lesions associated with negative microbiological investigations should prompt consideration of alternative diagnoses, including foreign-body reaction. Integration of clinical assessment, microbiology, multimodality imaging, and histopathological examination may prevent unnecessary device extraction, facilitate timely oncological diagnosis, and allow successful long-term preservation of a normally functioning CIED.