This study is testing a special type of scan called GEH300079 (68Ga) PET/CT to find a condition called Peritoneal Carcinomatosis (PC) in people with certain cancers like colorectal, gastric, ovarian, or pancreatic. PC is when cancer spreads to the lining of the abdomen. In this study, participants will receive this scan before or after their usual medical imaging. The study is divided into two parts: Phase 2 and Phase 3. Phase 2 will test the best dose and timing for the scan, while Phase 3 will check how safe and effective it is.

• The study requires a commitment to multiple visits for the scan and follow-up.
• Participants must be 18 or older with confirmed cancer and suspected PC.
• Pregnant women or those with certain health issues are not eligible to participate.

Participants will need to have surgery planned and cannot have certain health issues like severe kidney or liver problems. You must be able to follow all study procedures as outlined by the researchers.