Testicular cancer. Gross specimen of a testicle (sliced open) in a case of a seminoma, a malignant tumour (cancer) of the testis (testicle). An abnormal black area is seen at centre right. This type of cancer usually appears as a firm, painless swelling on a testicle. Although this is a rare cancer, it is the most common cancer in 15- to 35- year-old men. It arises from abnormal germ cells (precursors to sperm cells) in the seminiferous tubules. Surgical removal of the affected testis (orchidectomy) is followed by radiotherapy and chemotherapy if required.