High interobserver variability of PTEN immunohistochemistry defining PTEN status in low- to intermediate-risk prostate cancer: results of the first German ring trial.

The prognostication of individual disease trajectory and selection of optimal therapy in patients with localized, low-grade prostate cancer often presents significant difficulty. The phosphatase and tensin homolog on chromosome 10 (PTEN) has emerged as a potential novel biomarker in this clinical context, based on its demonstrated prognostic significance in multiple retrospective studies. Incorporation into standard clinical practice necessitates exceptional diagnostic accuracy, and PTEN's binary readout-retention or loss-suggests its suitability as a biomarker. This multi-institutional ring trial aimed to validate the diagnostic precision of PTEN immunohistochemistry in localized, low- to intermediate-risk prostate cancer, across ten university pathology institutes in Germany. The trial incorporated 90 cases of patients diagnosed with acinar adenocarcinoma of the prostate of grade groups 1 (n = 8, 8.9%) and 2 (n = 82, 91.1%) post-radical prostatectomy. Remarkably, the interpretation of PTEN immunohistochemistry displayed substantial variation (12.5-51.2% PTEN loss rates) within an identical cohort of prostate cancer. Fluorescence in situ hybridization analysis demonstrated PTEN hemizygous deletions in 5.5% (5/90) of cases. All cases with hemizygous deletions presented a distinct loss of PTEN expression by immunohistochemistry and were unanimously identified as PTEN loss by all participants (sensitivity 100%). However, negative (loss) immunohistochemistry was relatively non-specific for an underlying genomic deletion. Improved inter-observer agreement was observed in a subsequent ring trial. Finally, we identify S473-pAKT immunohistochemistry as a useful marker in equivocal cases. In summary, this multi-institutional ring trial illustrates surprisingly heterogeneous outcomes in defining PTEN status by immunohistochemistry.