Tumour Markers in Clinical Oncology: Biochemistry, Classification, Diagnostic Applications, and Emerging Frontiers — A Narrative Review

Background: Cancer exacts a disproportionate burden on global health. Tumour markers — biochemical substances elaborated by malignant cells or induced in the host by tumour — have emerged as indispensable adjuncts in oncological practice. Their detection in blood, urine, or tissue specimens offers a minimally invasive window into tumour biology that complements histopathology and imaging. Objective: This narrative review synthesises the current evidence on the biochemical basis, clinical classification, diagnostic performance characteristics, and practical clinical applications of established tumour markers, while addressing their documented limitations and the trajectory of next-generation biomarker development. Methods: A structured narrative review of the English-language literature was conducted using PubMed, Scopus, and MEDLINE databases. Search terms included 'tumour markers', 'cancer biomarkers', 'oncofetal antigens', 'liquid biopsy', 'circulating tumour DNA', and their respective combinations. Guidelines from ASCO, ESMO, and NACB were incorporated to contextualise clinical recommendations. Results: Among established markers, beta-human chorionic gonadotropin (β-hCG) and alpha-fetoprotein (AFP) exhibit the highest sensitivity and specificity for gestational trophoblastic disease and germ-cell tumours, respectively (>95% for both). Prostate-specific antigen (PSA) retains utility for post-treatment surveillance, though its specificity for population screening remains problematic at 25–40%. Emerging liquid-biopsy platforms, including circulating tumour DNA (ctDNA) and cell-free RNA, demonstrate superior lead-time over conventional imaging for recurrence detection in multiple solid tumours. Conclusion: Tumour markers remain clinically indispensable when interpreted within the framework of histopathological confirmation, cross-sectional imaging, and patient-specific clinical context. The integration of multi-omic liquid biopsy platforms into routine oncological surveillance holds the potential to transform early detection and personalise therapeutic decision-making.