[Comment] Molecular diagnosis and site-specific therapy in cancer of unknown primary: an important milestone

Cancer of unknown primary (CUP)—ie, metastatic cancer for which the anatomical primary site is not clinically detectable despite an adequate workup—has posed a difficult challenge for patients and physicians for decades.1,2 Treatment for metastatic cancers has traditionally been based on the primary site. Patients with CUP presenting with a constellation of clinicopathological features suggestive of a primary tumour have favourable outcomes when treated accordingly.1 Patients with unfavourable CUP, who account for about 80% of cases, without recognised presumptive primaries, have been treated with empirical chemotherapy for the last four decades with very poor outcomes.