Most (95%) testicular cancer originates in undeveloped cells (germ cells) that produce sperm. These tumors, called germ cell tumors (GCTs), are most common in men between the ages of 20 and 40 and are curable in more than 95% of cases. There are two main types: seminomas and nonseminomas. A third type, called stromal tumors, develops in the supporting tissues of the testicle.
Approximately 40% of GCTs are seminomas, which are classified as either typical or spermatocytic. Typical seminomas account for 90% of this type. They often cause unilateral (i.e., on one side) testicle enlargement or more often a painless lump in the testicle. Spermatocytic seminomas grow slowly, usually do not spread to other parts of the body (metastasize), and are most common around age 65.
Nonseminomas account for 60% of GCTs and develop in younger men (usually between 15 and 35). Most nonseminomas contain cells from at least two subtypes, including the following:
- Choriocarcinoma (rare; aggressive; likely to metastasize)
- Embryonal carcinoma (accounts for 20% of cases; likely to metastasize)
- Teratoma (usually benign in children; rarely metastasize)
- Yolk sac carcinoma (most common in young boys; rare in men)
Testicular cancer may also develop in the supportive, hormone-producing tissue of the testicles (stroma). This type accounts for 4% of testicular cancer in men and 20% in boys. Types of stromal tumors include Leydig cell tumors and Sertoli cell tumors.
Seventy-five percent of Leydig cell tumors develop in men and 25% develop in boys. Most tumors of this type are benign and are treated successfully with surgery. If the tumor metastasizes, it often does not respond well to radiation or chemotherapy and the prognosis is poor.
Sertoli cell tumors develop in Sertoli cells that nourish the sperm-producing germ cells. These tumors are usually benign; metastatic tumors of this type are rare, yet resistant to treatment.
Secondary tumors in the testicles usually migrate from the lymph or lymph nodes. Testicular lymphoma is more common than primary testicular cancer in men over 50. Other cancers (e.g., prostate, lung, skin) may also spread to the testicles.