Metastatic treated malignant germ cell tumors: Is SALL4 a better marker than placental alkaline phosphatase?

Studies have shown that in the metastatic setting and after treatment, expression of immunohistochemical markers may be diminished or lost. Transcription factor SALL4 (sal-like protein 4) has been recognized as a sensitive marker for both primary and metastatic malignant germ cell tumors (MGCTs), but has not been tested in the posttreatment setting. We sought to determine the level of SALL4 expression in treatment-resistant metastatic MGCT in comparison with pan-GCT marker placental alkaline phosphatase (PLAP). Thirty-six previously treated MGCTs, 16 untreated primary testicular MGCTs, and 4 cytology specimens were immunostained for SALL4 and PLAP, and staining characteristics were evaluated. In the treated MGCT group, there was diffuse SALL4 nuclear immunoreactivity in the majority of cases (27/36, 75%), labeling seminoma, yolk-sac tumor, embryonal carcinoma, and primitive neuroectodermal components. No treated metastatic MGCT lacked SALL4 immunoreactivity. In contrast, PLAP was diffusely expressed in only 14/36 (39%) cases of treated MGCTs, showed scattered focal weak to moderate positivity in 13/36 (36%), and was virtually absent in 9/36 (25%) cases. Both markers had scattered expression limited to the epithelial components of teratomatous regions. SALL4 also outperformed PLAP on a small sample of cytology blocks. Although SALL4 is not entirely specific, it is a highly sensitive marker with strong diffuse nuclear reactivity in the majority of MGCTs in the posttreatment setting, at significantly higher levels than PLAP (P<0.001). Persistent expression of SALL4 in metastatic MGCTs resistant to chemoradiation also raises the possibility for targeted systemic therapy as the anti-SALL4 peptide continues to be developed.