OBJECTIVE: To investigate the occurrence of synchronous colorectal cancer metastases (SCCM) confined to the lungs, risk factors for these metastases and their impact on survival.

METHODS: In a nationwide cohort study of 26,200 patients data were prospectively entered into the Danish Colorectal Cancer Group's (DCCG's) database between May 2001 and December 2011. The recorded data were merged with data from the Danish Pathology Registry and the National Patient Registry. Multivariable logistic- and extended Cox regression analyses were used to adjust for confounding variables.

RESULTS: In total, 1970 patients (7.5%) had pulmonary SCCM of whom 736 (37%) had metastases exclusively in the lungs. Advanced age, recent years of diagnosis and a rectal index cancer were significantly associated with pulmonary SCCM. Adjustment for excess use of thoracic CT scans in rectal cancer patients did not alter this association (adjusted OR=1.81 (95% CI: 1.46-2.25, P<0.001)). Patients subjected to pulmonary metastasectomy, resection of primary tumour and chemotherapy had a superior overall survival compared with non-treated patients, especially when these therapeutic modalities were combined.

CONCLUSIONS: The occurrence of pulmonary SCCM was higher than previously reported and had a severe impact on survival. Our analyses suggest that pulmonary metastasectomy, resection of the primary tumour and chemotherapy may be a sound strategy in patients with confined pulmonary SCCM, but the risk of selection bias and consequent exaggeration of the treatment effect should be kept in mind. This study may serve as a reliable un-biased reference for future evaluation on detection strategies and potential therapeutic interventions.