Derivation of malignancy status from ICD-9 codes.

To assess the severity of illness of oncology patients, it is necessary to distinguish patients with a single primary tumor from patients with metastatic disease occurring at a secondary location remote from the primary site. We developed a ranked list of cancer groupings and an algorithm that could distinguish patients with primary and metastatic cancer even if no specific code for secondary cancer was recorded. In patients with metastatic disease, the algorithm should also distinguish the primary site from the secondary site.