Lethal if untreated. Testicular cancers can spread to other body parts (back, lungs, brain, etc.) if not treated or if not detected early. Thankfully, even very advanced testicular cancers can be very effectively treated and cured in most patients (example lance armstrong). Treatments include chemotherapy, surgery, radiation and bone marrow transplants in select cases.
Www. Cancer. Net. Generally depends on several factors e.g. The histology, stage, etc. Here is an excellent resource guide for patients on www. Cancer. Net on testicular cancer: http://www. Cancer. Net/cancer-types/testicular-cancer.