Utility of NKX3.1 as a Diagnostic Immunohistochemical Marker in Prostatic Adenocarcinoma

Abstract Prostatic adenocarcinoma is a common malignancy with variable histologic patterns and clinical behavior. Although NKX3.1 is an established marker of prostatic origin, region-specific validation data, particularly in Indian populations and in routine diagnostic settings, remain limited. This study aimed to evaluate the diagnostic utility of NKX3.1 in prostatic adenocarcinoma in an Indian cohort, assess its expression across different Gleason grade groups including high-grade tumors, and determine its specificity in differentiating prostatic adenocarcinoma from histologic mimics and nonprostatic carcinomas encountered in routine practice. This 2-year prospective observational study included 100 formalin-fixed, paraffin-embedded tissue blocks comprising 50 prostatic adenocarcinomas, 10 histologic mimics, and 40 carcinomas from various nonprostatic primary sites. NKX3.1 immunohistochemistry was performed and evaluated semiquantitatively for percentage positivity and staining intensity. Sensitivity, specificity, positive predictive value, and negative predictive value were calculated. All 50 prostatic adenocarcinoma cases demonstrated nuclear NKX3.1 positivity (100% sensitivity), including tumors across higher Gleason grade groups. Most cases showed diffuse (> 76%) staining with predominantly moderate intensity. Histologic variants encountered included foamy gland morphology and prostatic intraepithelial neoplasia-like features, and one high-grade tumor with neuroendocrine-like morphology with retained NKX3.1 expression while being negative for neuroendocrine markers. NKX3.1 was negative in all histologic mimics and in 39 of 40 nonprostatic carcinomas, yielding a specificity of 98%. A single salivary gland intraductal papillary mucinous neoplasm showed focal nuclear positivity, highlighting a potential diagnostic pitfall. This study provides regional validation of NKX3.1 as a highly sensitive marker for prostatic adenocarcinoma in an Indian population, including high-grade tumors. NKX3.1 is a highly sensitive marker of prostatic epithelial lineage and is particularly useful for confirming prostatic origin in high-grade, poorly differentiated, and metastatic tumors. While it has a limited role in distinguishing benign from malignant glands in routine primary prostate biopsies, its strong and consistent nuclear expression supports its use in diagnostically challenging and metastatic settings.