RESUMO
Introduction: Tumor lysis syndrome (TLS) is an oncologic emergency characterized by several metabolic derangements, such as hyperuricemia, hyperkalemia, hyperphosphatemia, and hypocalcemia. TLS is typically observed in hematologic malignancies, especially after starting the first administration of antineoplastic therapies. TLS in a solid malignancy is very unusual, and exceedingly rare when occurring spontaneously, in the absence of chemotherapy. Case Presentation: We report a case of a 76-year-old man with lung adenocarcinoma, which started as a cancer with indolent behavior and small tumor burden but relapsed in 5 months with rapidly proliferating metastatic disease. Spontaneous TLS was the presenting clinical manifestation of the tumor relapse, and it led to the patient's death. Conclusion: To our knowledge, this is the first case of spontaneous TLS in a relapsed adenocarcinoma of the lung reported in the medical literature. The development of the metabolic derangements of TLS should prompt the consideration of tumor relapse during the follow-up of patients with solid malignancies.