Frequent Gastrointestinal Cancer Complications in Japanese Patients with Acute or Chronic Coronary Syndrome Undergoing Percutaneous Coronary Intervention

<b>Background/Objective:</b> Gastrointestinal bleeding is a major complication of dual antiplatelet therapy (DAPT) in patients undergoing percutaneous coronary intervention (PCI). Malignancy may be detected due to gastrointestinal bleeding, necessitating critical decisions regarding treatment selection and influencing patient prognosis. <b>Methods:</b> This single-center, retrospective, observational study included 501 Japanese patients who underwent initial PCI between January 2019 and January 2023. Of these patients, 393 who underwent perioperative upper and lower gastrointestinal endoscopy were evaluated for the presence of gastrointestinal malignancy. <b>Results:</b> Of the total patients, 36% presented with acute coronary syndrome (ACS). Gastrointestinal malignancies were identified in 30 patients (8%), including 18 cases of colorectal cancer and eight cases of gastric cancer. No difference in the frequency of malignancies was observed between patients with ACS and chronic coronary syndrome (CCS) (<i>p</i> = 0.7398). Malignancies were significantly more common in patients with positive fecal immunochemical testing (FIT) (<i>p</i> < 0.0001); however, FIT did not detect all malignancies. The 1500-day survival rate for patients with gastrointestinal malignancies was 64%, with no difference in overall survival between treatment modalities. <b>Conclusions:</b> A considerable proportion of Japanese patients undergoing PCI had gastrointestinal malignancies, regardless of whether they had ACS or CCS, and their prognosis was poor. Gastrointestinal endoscopic evaluation in the perioperative period of PCI could detect malignancy without complications and might lead to appropriate cancer treatment.