Testicular cancer. Gross specimen of a testicle in a case of a seminoma, a malignant tumour (cancer) of the testis (testicle). The testicle is covered in numerous abnormal swellings. The swellings are usually painless and are most often detected by self-examination. Although this is a rare cancer, it is the most common cancer in 15- to 35-year-old men. The cancer 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.