Ovarian tumour. Light micrograph of a section through a mucinous cystic tumour in an ovary. Both benign (non-cancerous) and malignant (cancerous) forms of this tumour develop as large masses, virtually replacing the ovary. This is a benign mucinous cystadenoma which has a smooth outer surface (top) since the tumour exhibits no tendency to invade through the capsules outer layer (pink, top).