Supplementary Material for: Anastrozole-Induced Tumor Lysis Syndrome in Breast Cancer: A Case Report of a Rare Critical Care Emergency

Introduction: Tumor lysis syndrome (TLS) is an oncologic emergency most commonly encountered in hematologic malignancies following cytotoxic therapy. Its occurrence in solid tumors is rare and is exceedingly uncommon after endocrine therapy; aromatase inhibitor–associated TLS is sparsely documented. Case Presentation: We report a 67-year-old woman with advanced estrogen receptor–positive (ER+) breast cancer who developed fatal TLS within 48 hours of initiating anastrozole. She presented with progressive dyspnea and a large left breast mass with extensive thoracic lymphadenopathy and pleural effusion. Her course was complicated by atrial fibrillation with rapid ventricular response, respiratory failure requiring intubation, and an acute ischemic stroke. Anastrozole (1 mg daily) was started as temporizing endocrine therapy. Within 48 hours, laboratory testing showed hyperkalemia, hyperphosphatemia, hyperuricemia, and acute kidney injury, meeting Cairo–Bishop criteria for TLS. Despite aggressive critical care management, she experienced cardiac arrest and died. Conclusion: This case may represent the first reported fatal occurrence of anastrozole-associated TLS in advanced breast cancer, underscoring the need for vigilance for TLS even with hormonal agents traditionally considered low-toxicity.