Cancer of Unknown Primary — Metastatic Cancer Without Origin

Cancer of unknown primary, also termed occult primary malignancy, refers to a heterogeneous group of metastatic cancers for which a primary anatomical site cannot be established after standardised diagnostic workups. It accounts for a small percentage of all cancer presentations and predominantly affects older adults.

Pathologically, the metastatic deposits are most frequently classified as adenocarcinomas or poorly differentiated carcinomas. Even with modern diagnostic imaging, endoscopic procedures, and detailed histopathology, the original tumour often remains undetected because it may be exceptionally small, dormant, or eliminated by the immune system.

Finding the primary tumour is typically the foundation of cancer treatment planning, as standard regimens are designed around the organ of origin. In CUP, the lack of a known origin means your multidisciplinary team must rely on cellular patterns, genomic markers, and clinical presentation. This approach helps personalise systemic therapies, avoid unnecessary invasive investigations, and focus resources on controlling the disease and preserving quality of life.

A comprehensive diagnostic workup includes contrast-enhanced computed tomography (CT), positron emission tomography (PET-CT), and tissue biopsy. Immunohistochemistry (IHC) testing on biopsy specimens uses antibody staining (such as CK7, CK20, TTF-1, and CDX2) to predict the tissue of origin. Next-generation sequencing (NGS) and gene expression profiling may also identify actionable molecular alterations.

By definition, CUP does not have an identified primary cancer type. However, the secondary cells usually resemble common malignancies such as colorectal, pancreatic, lung, breast, or ovarian cancers. It commonly presents as metastases in the lymph nodes, liver, lungs, bones, or peritoneum. Less frequently, neuroendocrine tumours or squamous cell carcinomas are identified as CUP presentations.

Treatment depends on whether the CUP falls into a favourable or unfavourable prognostic subset. Favourable presentations (such as isolated axillary or cervical lymph node involvement) are treated with curative intent using surgery, radiotherapy, or specific systemic therapy. Unfavourable presentations typically receive broad-spectrum empiric chemotherapy, or targeted therapy and immunotherapy if genomic alterations are detected.