Table 2.

Summary of the included possible risk factors and definitions

Variable	Type	Definition
Demographical Age	Continuous variable	Age at diagnosis, years
Biochemical AFP	Continuous variable	Preorchiectomy level, IU/L
hCG	Continuous variable	Preorchiectomy level, IU/L
LDH	Continuous variable	Preorchiectomy level, U/L
Pathological Tumour size	Continuous variable	Largest tumour diameter, mm In case of multifocality: (a) maximal diameter of the largest focus and (b) ‘total tumour diameter’, calculated as the sum of the maximal diameter in each lesion. Multifocality is defined by coexistence of independent tumoural foci; largely depending of the macroscopic description in the original pathology report. However, if section described as taken from normal parenchyma macroscopically, but consists of tumour microscopically, this will be counted as a tumoural focus. In order to be calculated as a tumour focus, the lesion must at least measure 1 mm in diameter. A focus of regression will be calculated as a tumoural focus as well, if independent and measures 1 mm or more in largest diameter. Areas of necrosis and fibrosis (regression) will be calculated in the maximal tumour diameter, if it is measured as part of the tumour in the gross description.
Pagetoid involvement of the rete testis	Binary variable (present/absent)	Extension into the rete testis epithelium of individual or groups of GCNIS cells.
Rete testis invasion	Binary variable (present/absent)	Tumour cells in the stroma between rete tubular channels, or clear destruction of the testicular hilum.
Hilar soft tissue invasion	Binary variable (present/absent)	Tumour extension into the soft tissue beyond the rete testis at the same plane of section as the testis parenchyma.
Epididymis invasion	Binary variable (present/absent)	If epididymis is described without tumour involvement on the gross description, but (exceptionally) is not visualised in the section taken from the area, we will rely on the macroscopic description and state no involvement (unless we expect it to be involved from other sections taken).
Spermatic cord invasion	Binary variable (present/absent)	Tumour extending grossly beyond the hilum, with the base of the cord defined as a section just superior to the head of caput epididymis, or tumour is adjacent to or surrounds the vas deferens. Discontinuous involvement of the spermatic cord by vascular-lymphatic soft tissue invasion will be registered separately as a local metastasis.
Tunica albuginea invasion	Binary variable (present/absent)	Invasion of tumour into the fibrous layer immediately surrounding the testicular parenchyma.
Tunica vaginalis invasion	Binary variable (present/absent)	Penetration of the mesothelium of the visceral layer of tunica vaginalis.
Lymphovascular invasion (LVI)	Binary variable (present/absent)	Cohesive cells often adherent to the wall of the vessel, located preferably in tunica albuginea or peritumoural location. Associated fibrin material further supports the presence of true LVI. Lack of obvious background artifactual deposition of tumour. The location of LVI in testis or/and spermatic cord will be registered separately, as will any soft tissue invasion in the spermatic cord through lymphovascular spaces. In cases where LVI is indeterminate from artefact/contamination, the LVI status will be regarded as negative.
Tumour necrosis	Continuous variable	The amount of tumour necrosis in percentage.
Spermatic cord margin involvement	Binary variable (present/absent)	Tumour in section taken from the margin (excluding implantation artefact and tumour cells confined to the vascular spaces at the margin).
Tumour subtype, non-seminoma	Continuous variables	The amount of histologic tumour types in percentages: embryonal carcinoma seminoma yolk sac tumour choriocarcinoma teratoma. If teratoma with somatic type malignancy: in addition, the type and diameter of the largest focus.
Scrotum invasion	Binary variable (present/absent)	Tumour invades beyond the tunica vaginalis and spermatic fascia into soft tissue or skin of the scrotum.

AFP, α-fetoprotein; GCNIS, germ cell neoplasia in situ; hCG, β-human choriogonadotropin; LDH, lactate dehydrogenase; LVI, lymphovascular invasion.