Ovarian fibroma. Coloured transmission electron micrograph (TEM) of a section through an ovarian fibroma. An ovarian fibroma is a rare, benign tumor of the ovary. Most commonly found in women in their 50s during perimenopause (transition to menopause) or postmenopause, ovarian fibromas account for approximately four percent of all ovarian tumors. It is composed of spindle cells forming variable amounts of collagen. Magnification: x3000 when printed at 10 centimetres wide.