Most testicular cancer starts in the germ cells, which are the cells that make sperm. The 2 main types of germ cell cancer are seminomas and non-seminomas. Other types include carcinoma in situ of the testicle and stromal tumours, but these are not as common as germ cell tumours. ^[1]

Germ cell cancers

Testicular cancer often includes a mix of both seminoma and non-seminoma cancer cells. These will often be treated like non-seminoma cancers, because they grow and spread like other non-seminomas. Testicular cancer can also be of a ‘pure’ type – for example, 100% embryonal carcinomas or choriocarcinomas. 

Seminomas

Seminomas grow and spread slower than non-seminomas. There are 2 subtypes: 

• classical (typical) seminoma: most seminomas are classical seminomas 
• spermatocytic seminoma: a rare type of seminoma that tends to affect older men. These are slow-growing and not likely to spread. 

Non-seminomas

Non-seminomas usually affect younger men in their late teens to early 30s. There are 4 subtypes: 

• embryonal carcinoma: these are often found as a part of other testicular cancer cells, but don’t often occur on their own. They tend to grow quickly and spread outside of the testicle 
• yolk sac tumour: these are most often found in children, especially infants, and are highly treatable in the younger age group. Yolk sac tumours in older men are more difficult to treat. They may also be called yolk sac carcinoma, endodermal sinus tumour, infantile embryonal carcinoma or orchidoblastoma 
• choriocarcinoma: these are very rare in adults, but they are fast-growing and spread quickly, especially to the lungs, bones or brain 
• teratoma: these are usually seen as a part of mixed germ cell tumours. They are often treatable, but some do come back (recur). 

Stromal tumours

Stromal tumours start in the cells that make up the supporting structure of the testicles and in the cells that produce hormones. These are usually benign (not cancer) and straightforward to treat. 

Carcinoma in situ of the testicle

Carcinomas in situ of the testicle (CISs) are abnormal looking cells under a microscope, but are non-invasive (do not spread). These pre-cancerous cells may or may not develop into another type of invasive cancer, so your doctor may or may not decide to treat it. 

Testicular CISs do not cause symptoms, so they are usually detected if you have had a biopsy done for another reason, such as infertility.