Testicular cancer, a malignant spermatocytic seminoma, computer illustration. A seminoma is a malignant tumour (cancer) of the testis (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.