Supplementary Material for: Rectal Gastrointestinal Stromal Tumor Complicated by Tumor Lysis Syndrome After Imatinib Initiation: A Rare and Fatal Case

Introduction: Gastrointestinal stromal tumors (GISTs) are rare mesenchymal tumors, typically located in the stomach or small intestine. Rectal GISTs are exceptional, accounting for only 5% of GISTs and 0.1% of all rectal tumors. Although imatinib is the standard treatment for advanced or unresectable GISTs, severe complications such as tumor lysis syndrome (TLS) are exceedingly rare. Case presentation: We report the case of a 73-year-old woman with multiple cardiovascular comorbidities, who was diagnosed with a high-risk, spindle-cell rectal GIST with hepatic metastases. Genetic testing revealed a KIT exon 11 mutation. Treatment with imatinib (400 mg/day) was initiated. Within days, the patient developed tumor lysis syndrome, with biologically confirmed hyperkalemia and hypocalcemia, necessitating transfer to the intensive care unit. Despite supportive treatment and discontinuation of imatinib, the clinical course was complicated by massive rectal bleeding of intratumoral origin, leading to the patient’s death. Conclusion: TLS is a rare but potentially fatal complication of imatinib therapy in GISTs, particularly in high-risk or metastatic forms. This case underscores the importance of close biochemical and clinical monitoring when initiating tyrosine kinase inhibitors, even in solid tumors where TLS is considered uncommon.