Loss of expression of STK11/LKB1 in intratubular large cell hyalinizing Sertoli cell neoplasm

Aims: Intratubular large cell hyalinizing Sertoli cell neoplasia (ILCHSCN) is a rare testicular sex cord stromal tumour that occurs in paediatric patients in the context of Peutz-Jeghers syndrome, a condition caused by germline STK11 alterations. The diagnosis of ILCHSCN and its distinction from large cell calcifying Sertoli cell tumour (LCCSCT), a neoplasm associated with loss of function PRKAR1A mutations, can be challenging. We therefore aimed to assess the immunohistochemical expression of STK11 in a cohort of ILCHSCN and LCCSCT to investigate its diagnostic utility. Methods and results: Immunohistochemistry (IHC) for STK11 was assessed in 7 ILCHSCN and 9 LCCSCT. Six ILCHSCN were also stained for PRKAR1A, a biomarker known to be lost in LCCSCT. The majority of ILCHSCN (4/7; 57%), including one case with invasion, showed complete loss of expression of STK11. All LCCSCT (9/9) showed retained STK11 expression. All ILCHSCN assessed with PRKAR1A (6/6) showed retained expression. Conclusions: The majority of ILCHSCN demonstrate loss of expression of STK11, while LCCSCT shows retained expression. Our findings support that STK11 IHC is highly specific and moderately sensitive in distinguishing between these two tumours. The combination of STK11 and PRKAR1A IHC has even greater utility for distinguishing ILCHSCN and LCCSCT.