Unknown primary tumour (UPT) is the term applied to metastatic cancer, the origin of which remains unidentified. Since cancer treatment is primarily based on the tumour site of origin, treatment of UPT patients is challenging. The number of reports on incidence, treatment and survival of UPT is limited. We hereby report data on patients (2000-2012) with UPT in the Netherlands.

The age-standardised rate (ASR) of 'other and unspecified' malignancies in the Netherlands was compared with other European countries. Patients diagnosed with UPT between 2000 and 2012 were selected from the Netherlands Cancer Registry (NCR) to calculate incidence rates. Patient characteristics, treatment and survival rates were assessed.

The ASR of 'other and unspecified' malignancies in the Netherlands did not differ from the European average ASRs (2008-2012). A total of 29,784 patients with an unknown primary tumour were selected from the NCR (2000-2012). The incidence decreased from 14 per 100,000 person years (European standardised rate) in 2000 to 7.0 in 2012. The most common metastatic sites were liver, lymph nodes, bone and lung (42%, 22%, 16% and 14%, respectively), and approximately two-thirds of patients were diagnosed with metastases at a single site. One-third of the patients were treated; these were mainly younger patients. The overall median survival for all patients was 1.7 months. The median survival of untreated patients was 1.0 month and of treated patients 6.3 months.

The incidence of UPT between 2000 and 2012 is decreasing in the Netherlands, and one-third of these patients received treatment. Survival after diagnosis is limited to months rather than years.