My mum(53)got a hepatic cyst (6cmx5cm now)found through ultrasound scan 15 years ago. is any treatment for this or can leave it if it grows slowly?

Hepatic cyst: There's certainly nothing wrong with leaving a hepatic cyst alone in certain circumstances. It sounds as if your mum is getting periodic ultrasound scans since you mention that it was found 15 years ago and a current measurement. It would be smart for your mum to take the ultrasound results to a doctor who can do a physical exam and additional lab work to determine the cause of the cyst.