Gastric wall thickening and gastric outlet obstruction due to peritoneal metastasis in breast cancer: A case report

Peritoneal metastasis from breast cancer is rare and diagnostically challenging. This challenge is particularly compounded in patients receiving CDK4/6 inhibitors, as their characteristic gastrointestinal adverse effects can mimic the symptoms of peritoneal carcinomatosis. A woman with hormone receptor‑positive/HER2‑negative metastatic breast cancer developed symptoms of functional gastric outlet obstruction (manifesting as delayed gastric emptying) 2 months after initiating palbociclib treatment. Imaging revealed diffuse and marked gastric wall thickening. Initial gastroscopic biopsies were non‑diagnostic, but laparotomy confirmed peritoneal carcinomatosis, with histology and immunohistochemistry confirming metastatic breast cancer. The present case underscores the diagnostic difficulty in differentiating drug toxicity from disease progression in patients on CDK4/6 inhibitors. The key clinical messages are: i) Persistent gastrointestinal symptoms despite CDK4/6 inhibitor withdrawal warrant investigation for peritoneal metastasis; ii) serosal‑based metastases have the potential to cause notable gastric wall thickening and functional gastric outlet obstruction while sparing the mucosa, potentially evading initial endoscopic diagnosis; and iii) a multifactorial pathogenesis involving structural infiltration and neuro‑myogenic dysfunction is proposed to underlie this clinical presentation.