Testicular germ cell tumor (TGCT) is the most common malignancy in males aging 15–35. Known risk factors for TGCT include: cryptorchidism, testicular dysgenesis, infertility, testicular microlithiasis, previously diagnosed TGCT and a family history of the disease .
Familial aggregations of TGCT have been well described, suggesting the existence of a hereditary TGCT subset. Approximately 1.4% of newly diagnosed TGCT patients report a positive family history of TGCT . Epidemiological studies have shown that there is an eight to ten fold increase in relative risk of TGCT for brothers of patients and a fourfold increased risk for fathers and sons [3, 4]. Moreover, testicular cancer risk was raised in twins of men with testicular cancer (relative risk = 37.5 [12.3-115.6]); risk was greater but not significantly in monozygotic than in dizygotic twins (p = 0.45). The cumulative risk of testicular cancer by the age of 40 years in men whose monozygotic twins had testicular cancer was 14% (4–46) . A previous study using segregation analysis of testicular cancer favored an autosomal recessive model of inheritance  and, although no high-penetrance cancer susceptibility gene has been mapped yet, linkage analyses have identified several genomic regions of modest interest . Y chromosome gr/gr deletion and PDE11A gene mutations have been suggested to modify the risk of familial testicular germ cell tumor (FTGCT) [8, 9]. Phenotypic aspects of FTGCT include 2 cases of TGCT per family, mean age at diagnosis about 2.5 years younger than that observed for sporadic TGCT and greater prevalence of bilateral cancer .
The establishment of a universal screening program using testicular palpation (clinical or patient self-examination) or serum biomarker for healthy young patients lacks clinical evidence . And, although some risk factors for TGCT are well known, effectiveness of high-risk patients screening is controversial. Recent systematic review did not detect published randomized clinical trials comparing screening versus no screening for testicular cancer . On the basis of uncertain benefits and some likelihood of harms, the US Preventive Services Task Force and the American Academy of Family Physicians discourage screening. But others, such as the American Cancer Society and the European Association of Urology, recommend different approaches: testicular examination to be part of a periodic cancer-related checkup  and self-testicular examination for individuals with clinical risk factors , respectively.
Extragonadal germ cell tumors account for fewer than 10% of all germ cell malignancies. All reports of family inheritance focused exclusively in isolated germ cell tumors of testis [4, 6, 7] and the estimation of the contribution of extragonadal germ cell tumor in the familial setting is not known. Therefore, the omission of extragonadal tumors in the published reports may have contributed to the underestimation of the importance of the hereditary factor of germ cell tumors, especially when there is a presence of an affected twin. Here we present a report of simultaneous occurrence of seminomas in dizygotic twins: one in the testicle and the other in the mediastinum.