I identify two cohorts of cancer patients with known and unknown primary tumours. Cancer of unknown primary (CUP) is defined by the presence of pathologically identified metastatic disease without clinical or radiological evidence of a primary tumour. Using the Ontario Cancer Registry, Same Day Surgery/Discharge Abstract Database and Ontario Health Insurance Plan, a novel linkage strategy was developed to cross-validate diagnoses. I found CUP patients represent a significant portion of all metastatic cancers. CUP patients with histological confirmed disease, squamous cell histology, or metastases localized to nodal regions had significantly better survival than other CUP patients. Knowledge of the primary site was associated with significantly improved overall survival. Known primary patients were more likely to receive treatment than CUP patients. Treatment was associated with prolonged survival in CUP patients. Adoption of gene profiling, emphasis on targeted therapeutics, and robust clinical guidelines are likely to improve CUP patient outcomes
