The hospital was shut down for three hours when a cylinder of gas used to sterilise medical equipment was reported to have ruptured.
A hospital spokeswoman says nine surgery cases were cancelled and radiotherapy and chemotherapy treatments delayed, but there were no reports of injuries.
Specialist fire brigade crews wearing protective clothing were called in to check the area, before staff and patients were allowed to go back inside.
Commander Martin Braid says it was a complicated job.
"Firefighters were dressed in splash suits and they used detection analysis equipment to determine whether or not the product was still present in the room," he said.
"The site has been assessed as safe to re-enter."
No patients were evacuated. Only staff who worked in the immediate area were made to leave.
Radiation and chemotherapy treatments have resumed.