Dysgerminoma of an ovary, light micrograph. Dysgerminoma is biologically similar to testicular seminoma and other extragonadal germinomas and makes up about 50% of all malignant germ cell tumours of the ovary and about 1-2% of all ovarian cancers in adults. In children and young adults, it is second most common malignant germ cell tumour of the ovary (after immature teratoma), accounting for about 33% of cases. Most dysgerminomas are diagnosed in second to third decade of life (median age 22 years). They are rare in premenarchal or postmenopausal women. Microscopically, dysgerminomas resemble seminomas. They are composed of monomorphic tumour cells arranged in diffuse sheets, trabeculae, cords, or nests separated by delicate fibrous septae containing lymphocytes and plasma cells. Haematoxylin and eosin stained section. Magnification: x200 when printed at 10 centimetres wide.