C51-37 Spontaneous Tumor Lysis Syndrome in Metastatic Breast Carcinoma: A Rare and Fatal Presentation

Abstract Introduction Tumor lysis syndrome (TLS) is a metabolic emergency most often observed after the initiation of cytotoxic therapy in hematologic malignancies. A rare variant, spontaneous tumor lysis syndrome (STLS), occurs without exposure to cytotoxic therapy. Although STLS has been documented in solid tumors such as hepatic, colorectal, and pulmonary carcinomas, it remains exceedingly rare in breast carcinoma. We present a case of fatal STLS in a patient with newly diagnosed metastatic breast cancer, highlighting diagnostic challenges and the importance of early recognition. Case Presentation A 67-year-old woman with a history of hypertension, hyperlipidemia, type 2 diabetes mellitus, and tobacco use disorder presented with worsening abdominal and left breast pain. Her laboratory findings were consistent with leukocytosis (21.2 × 109/L) and severe metabolic abnormalities: hyperkalemia (7.2 mmol/L), hyponatremia (132 mmol/L), hypermagnesemia (5.4 mg/dL), hyperphosphatemia (13.2 mg/dL), hyperuricemia (21.9 mg/dL), and metabolic acidosis (bicarbonate 10 mmol/L, anion gap 25 mmol/L, creatinine 7.92 mg/dL). Lactic acid was 10 mmol/L, and lactate dehydrogenase (LDH) was 707 U/L. Imaging revealed left breast thickening with heterogeneous, hypoechoic poorly defined lesion in the left breast, multiple bilateral pulmonary nodules and a 2.2 cm left thyroid lobe lesion, consistent with metastatic disease. She was admitted to the intensive care unit for progressive hemodynamic instability and acute hypoxic respiratory failure requiring intubation and vasopressors. Continuous intravenous sodium bicarbonate infusion was started for acidosis. She received rasburicase for hyperuricemia, thiamine for lactic acidosis, and empiric vancomycin plus cefepime. Serum tumor markers were markedly elevated: CA 15-3 = 969 U/mL and CA 27.29 = 1010.4 U/mL. Continuous renal replacement therapy (CRRT) was initiated. Over the next 24 hours, leukocytosis worsened, lactic acid rose to 20.3 mmol/L, and the anion gap increased to 30 mmol/L. Despite partial biochemical improvement; potassium decreased to 5.1 mmol/L, creatinine improved to 3.3 mg/dL, and uric acid dropped to 3.0 mg/dL, while phosphorus and magnesium declined to 7.9 mg/dL and 3.1 mg/dL, respectively, she continued to deteriorate. Despite maximal intervention, she succumbed to multi-organ failure, refractory acidosis, and circulatory collapse within 48 hours of admission. Conclusion Spontaneous tumor lysis syndrome is a rare but deadly metabolic complication that may reveal an undiagnosed malignancy or complicate advanced solid tumors such as breast cancer. Clinicians should suspect STLS in patients presenting with severe hyperuricemia, hyperphosphatemia, or unexplained AKI even prior to initiation of oncologic therapy. Early identification, rasburicase initiation, and renal support remain essential to improving outcomes. This abstract is funded by: None