Abstract
BACKGROUND: Adenocarcinoma originating from heterotopic pancreas tissue is a rare disease. Furthermore, to our knowledge, no HER2-positive cases in the duodenum have been reported in the scientific literature nor has the efficacy of trastuzumab treatment for the disease been reported. CASE SUMMARY: A 65-year-old woman whose clinical diagnosis was unresectable advanced duodenal cancer with HER2 overexpression responded well to trastuzumab chemotherapy. The main tumor in the duodenum reduced drastically. The patient underwent pancreaticoduodenectomy and lymph node dissection. A small number of cancer cells remained in the submucosal layer of the duodenum and pancreas head. After histological and immunohistochemical examination, the patient was diagnosed with duodenal adenocarcinoma originating from heterotopic pancreas tissue. CONCLUSION: Trastuzumab treatment is effective in HER2-positive adenocarcinoma originating from heterotopic pancreas tissue in the duodenum.