Most testicular tumours are testicular germ cell tumours, which includes seminoma and a number of non-seminomatous germ cell tumours. The most common non-germ cell tumour are lymphomas. Sex cord stromal tumours are a rare but important differential. There are three distinct phases of germ cell tumours:
- Infancy (yolk sac tumour, teratoma)
- Young adults (seminoma, non-seminomatous germ cell tumour)
- Elderly (spermatocytic seminoma)
The great majority occur in young adults and have been well studied. The other two phases are less well understood.