Precision Oncology and the Role of Pathology in Targeted Therapy Selection

This chapter reviews the indispensable and expanding role of diagnostic pathology in the era of targeted cancer therapy, addressing how immunohistochemical, fluorescence in situ hybridization–based, and next-generation sequencing–based companion diagnostic assays have become integral to treatment decision-making across major cancer types. It critically evaluates the evidence base and regulatory landscape for predictive biomarker testing, including PD-L1 expression, HER2 amplification, KRAS/NRAS mutational status, and tumor mutational burden, and examines challenges in standardization, inter-laboratory concordance, and evolving biomarker thresholds. The chapter also explores the pathological framework for assessing treatment response, including pathological complete response and residual disease scoring systems.