Primary mediastinal
seminomas are unusual
tumors that can present in a pure form or as part of a mixed
germ cell tumor. Contrary to testicular
seminomas, little is known about the expression of novel immunohistochemical markers in mediastinal
seminomas. This study investigates the immunohistochemical features of these
tumors with a focus on novel markers. Thirty-two cases of primary mediastinal
seminomas were reviewed; and representative whole-tissue sections were selected for immunohistochemical studies using
antibodies directed against high molecular weight
cytokeratin 5/6 (CK5/6), low molecular weight
cytokeratin (
CAM5.2), octamer-binding
transcription factor 3/4 (OCT3/4), spalt-like
transcription factor 4 (SALL4), GATA
binding protein 3 (GATA-3), sry-related HMG box 2 (SOX2), SOX17,
human T cell leukemia/lymphoma 1 (TCL1),
glypican 3,
melanoma associated
antigen C2 (MAGEC2), and paired box gene 8 (Pax8). The percentage of positive
tumor cells as well as the intensity of staining was evaluated and scored. Thirty-one cases (97%) expressed SOX17, whereas 29 cases (91%) were positive for OCT3/4 and SALL4, respectively. Twenty-eight cases (88%) expressed MAGEC2 and
CAM5.2, respectively. Two cases (6%) were positive for Pax8, and a single case (3%) was positive for TCL1. None of the cases stained with CK5/6, GATA-3, SOX2, or
glypican 3. Similar to testicular
seminomas, mediastinal
seminomas show consistent expression of OCT3/4, SALL4, SOX17, and MAGEC2 and are negative for SOX2,
glypican 3, GATA-3, and CK5/6. Pax8 positivity is only inconsistently identified in mediastinal
seminomas. Contrary to their testicular counterparts, mediastinal
tumors show diffuse expression of low-molecular-weight
cytokeratin in up to 90% of cases and are commonly negative for TCL1. Although there is some immunohistochemical overlap between testicular and mediastinal
seminomas, considerable differences also exist and should be acknowledged when dealing with these
tumors.