Ovarian tumour. Light micrograph of a section through a benign (non-cancerous) teratoma in an ovary. Teratomas are tumours derived from germ cells (sperm and eggs) and most commonly arise in the testes of ovaries. Such tumours range from benign to highly malignant (cancerous). Those arising in the ovary are usually benign, whereas those in the testes are usually malignant.