HER2-Positive Metastatic Stomach Cancer Presenting With Tumor Lysis Syndrome

What happened, in summary

A man in his 70s came to the emergency department with rectal pain, worsening constipation, back pain, poor appetite, shortness of breath, dark urine, and a 24-pound weight loss over 3 weeks. Blood tests showed anemia, kidney injury, high potassium, phosphorus, and uric acid, while CT showed a heavy tumor burden with liver involvement. He had not received chemotherapy, yet the laboratory pattern met criteria for spontaneous tumor lysis syndrome, a rare emergency caused by rapid breakdown of cancer cells.

An upper endoscopy found a large stomach mass. Biopsy confirmed invasive, moderately differentiated gastric adenocarcinoma that was HER2 positive. The tumor lysis syndrome was treated with insulin, dextrose, kayexalate, intravenous rasburicase 1.5 mg, allopurinol 300 mg, aggressive fluid replacement, and furosemide when fluid overload developed. His kidney function and electrolyte abnormalities improved enough for discharge on daily allopurinol.

At oncology follow-up, he began FOLFOX6 plus trastuzumab for HER2-positive metastatic gastric cancer. The regimen included 5-fluorouracil, leucovorin, oxaliplatin, and trastuzumab. After 4 cycles over 8 weeks, creatinine, potassium, uric acid, and lactate dehydrogenase had improved. The cancer nevertheless continued to progress despite treatment. Approximately 1 year later, he chose hospice care and died a few months afterward. His course combined an uncommon metabolic emergency at diagnosis with progressive metastatic disease despite HER2-directed therapy.