HER2 expression status should be determined in this patient with metastatic gastric cancer. This patient continues to have adequate caloric intake and good performance status. As such, he is an appropriate candidate for systemic chemotherapy. Approximately 20% of gastric cancers and 30% of gastroesophageal junction tumors overexpress HER2 growth factor receptor. The anti-HER2 monoclonal antibody trastuzumab, when added to a systemic chemotherapy regimen, is beneficial in treating patients with these tumor types. For example, an early trial of patients with gastric and gastroesophageal junction adenocarcinomas expressing HER2 found that the median survival was statistically significantly improved by adding trastuzumab to cisplatin plus 5-fluorouracil or capecitabine (13.5 months versus 11.1 months). Use of trastuzumab is limited to those patients whose tumors overexpress HER2.

BRAF mutations are present in 40% of melanomas, and patients with these tumors are treated with selective BRAF inhibitors. However, these agents are not part of gastric cancer therapy, and knowledge of the BRAF mutation status of a gastric tumor would not alter treatment.

Determining the estrogen and progesterone receptor status of a tumor is important when planning treatment of patients with breast cancer but is not relevant when selecting therapy for patients with gastric cancer.

K-ras genotyping is needed for all patients with newly diagnosed metastatic colorectal cancer, as K-ras mutations inhibit activity of the anti–epidermal growth factor receptor agents cetuximab and panitumumab that are used to treat many of these tumors. However, because these agents are not active in treating upper gastrointestinal malignancies, determining K-ras mutation status is not indicated for this patient.