RESUMEN
Introduction The use of trastuzumab with a fluoropyrimidine and platinum compound is currently the standard first-line treatment of patients with metastatic HER2-positive gastric cancer, but it appears that serum levels of trastuzumab determine the clinical effectiveness of this treatment, affecting progression-free survival and overall survival. Case report We report the case of a patient with metastatic HER2-positivegastric cancer, receiving XELOX (fluoropyrimidine and oxaliplatin) plus trastuzumab at standard doses, who presented sub-therapeutic serum levels during the first two treatment cycles and rapid disease progression (progression-free survival = 5.6 months). Discussion This case reveals a possible cause of poor effectiveness of trastuzumab treatment for metastatic gastric cancer in some patients, namely low circulating levels of the drug. It highlights the importance of monitoring as a possible tool for individual dose adjustment to optimize this therapy.