Testicular cancer. Light micrograph of a section through a differentiated teratoma in a testis (testicle). Teratomas are encapsulated malignant (cancerous) tumours that contain tissue or organ components, such as hair or teeth. Differentiated teratomas are the least aggressive form and have the best prognosis. They contain a wide variety of differentiated tissues that closely resemble those of the mature adult body. Testicular teratomas tend to occur most often in men between the age of 20 to 35 years, however this form is most common in children.