Impact of classification of mixed germ-cell tumours on incidence trends of non-seminoma.

Seminomas and non-seminomas [embryonal carcinomas, yolk sac tumours, teratomas, choriocarcinomas, mixed germ-cell tumours (MGCT)] are the major histological types of testicular germ-cell tumours (TGCT). TGCTs composed of both seminomatous and non-seminomatous elements have been coded as their non-seminoma component in the World Health Organization classification. In the late 1980s, a provisional International Classification of Diseases for Oncology (ICD-O) morphology code for MGCT was introduced. Using data from the Surveillance, Epidemiology and End Results Program and two population-based German cancer registries, we examined the impact of MGCT classification on TGCT trends. Cases were identified using ICD-O topography (ICD-9: 186; ICD-10: C62) and morphology codes (seminoma=9060-9062, 9064; embryonal carcinoma=9070; yolk sack tumour=9071; teratoma=9080-9084, 9102; choriocarcinoma=9100, 9101; MGCT=9085; all non-seminoma=9065-9102). As MGCTs and teratoma are often grouped as a single histological group, we analysed teratoma both including and excluding MGCTs. Between 1988 and 2007, incidence rates of MGCT in the US increased 407%. Rates of teratoma including MGCT increased 80%, whereas rates of teratoma excluding MGCT decreased 71%. Rates of embryonal carcinoma [-40%] and choriocarcinoma [-22%] also declined, suggesting that the code for MGCT is now being used for any mixed histology. Similar declines in incidence were observed in the German comparison populations. The declines in incidence of teratoma (excluding MGCT), embryonal carcinoma and choriocarcinoma in the US data since 1988 are likely in part because of increases in classifying any TGCT with mixed histology as MGCT. These results suggest that analysis of trends in specific histological types of non-seminoma should be interpreted cautiously.