Hereditary Cancer Syndromes in Familial Breast Cancer: Insights Into Proactive Gastric Cancer Surveillance.

Hereditary diffuse gastric cancer (HDGC) is a rare and aggressive cancer that accounts for 1% to 3% of gastric cancer cases, often diagnosed late with poor outcomes, and it is due to a mutation in the Cadherin 1 (CDH1) gene. We present 2 sisters, ages 62 and 70, both carrying the CDH1 mutation, which is associated with significantly increased risks of HDGC and breast cancer. Both were diagnosed with invasive lobular carcinoma of the breast, a known manifestation of CDH1 mutations, emphasizing the need for genetic screening in patients with strong family cancer histories. Despite their elevated HDGC risk, the sisters declined prophylactic surgery. Instead, they pursued annual endoscopic gastro-duodenoscopies with ultrasound surveillance tailored to their needs. This case highlights the importance of recognizing rare genetic mutations and integrating family history into personalized clinical management.