Immunohistochemistry-Based Biomarkers and Impact on Outcomes Following Salvage Radiotherapy for Prostate Cancer.

Abstract

Salvage radiation therapy (SRT) is a mainstay of treatment for prostate cancer patients who experience biochemical relapse following radical prostatectomy. We sought to characterize the prognostic impact of previously identified deleterious molecular phenotypes-loss of PTEN, ERG expression, and p53 mutation- upon survival in patients undergoing SRT. We analyzed an institutional database of 320 patients treated with SRT for whom prostatectomy tissue was available; tissue microarrays (TMAs) were constructed and used for genetically validated immunohistochemistry assays for PTEN, ERG, and p53. Chi-squared and Mann-Whitney's tests were used to assess association between molecular expression and clinical risk factors for adverse survival. Uni- and multivariate Cox-proportional hazards models were constructed to determine effects on biochemical (bRFS) and metastasis-free survival (MFS), calculated from end of SRT to event. Loss of PTEN (n = 123, 50%) and ERG expression (n = 118, 39.1%) were common, while p53 overexpression (signifying TP53 missense mutation) was less common (n = 21, 6.6%). On univariate Cox regression, loss of PTEN portended worse bRFS [HR 1.86; 95% CI 1.36-2.57] and MFS [HR 1.89; 1.21-2.94], with homogeneous loss associated with particularly worse MFS [HR 2.47; 1.54-3.95]. Similarly, p53 overexpression predicted worse bRFS [HR 1.95; 1.14-3.32] and MFS [HR 2.79; 1.50-5.19]. ERG expression was associated with only worse MFS [HR 1.6; 1.03-2.48]. On multivariate analysis adjusting for known prognostic features (grade, nodal involvement, pathologic stage, androgen deprivation therapy (ADT), and PSA velocity), homogeneous PTEN loss remained predictive of adverse bRFS [HR 1.73; 1.06-2.82] and MFS [HR 2.35; 1.04-5.34]. PTEN loss identified by immunohistochemistry is an independent adverse prognostic factor for both biochemical and metastasis-free survival in prostate cancer patients treated with SRT. Although clinically validated RNA expression assays also exist which are prognostic for outcomes following SRT, stromal contamination in bulk RNA could mask "loss" signals specific to tumor cells, such as PTEN. Further work is necessary to determine the optimal approach to treating patients with poor molecular prognostic features in a salvage setting.