Postoperative Pericardial Cyst Enlargement After Recurrent Breast Cancer: Distinguishing Benign vs Malignant Etiologies.

BACKGROUND: Pericardial cysts (PCs) are benign congenital anomalies typically managed conservatively. Interval enlargement in patients with active malignancy may mimic metastatic pericardial involvement. CASE SUMMARY: A 52-year-old woman with previously treated right-sided ductal carcinoma in situ presented with new ipsilateral breast lobular carcinoma. A right-sided pericardial lesion was incidentally identified during workup, and cardiac-gated computed tomography characterized this as a PC. Seven months after bilateral mastectomies and adjuvant chemoradiation, the patient underwent bilateral breast reconstruction. A postoperative computed tomography showed interval enlargement of the pericardial lesion, raising concern for possible residual malignant disease. Clinically, the patient remained asymptomatic, and multimodal imaging over the next month showed interval size regression, supporting a diagnosis of benign PC with inflammation-related size change. DISCUSSION: PC enlargement during recurrent malignancy may mimic metastatic pericardial disease; however, prevalent benign etiologies should be ruled out. Multimodal imaging and clinical evaluation are key in determining benign vs malignant pathology.