Background
Venous thromboembolism (VTE) is a potentially preventable serious complication in patients with lung cancer undergoing thoracic operation. We examined the risk and timing of VTE after surgery for primary non-small cell lung cancer (NSCLC).

Methods
All patients undergoing operation for NSCLC in Denmark between 2003 and 2021 were identified in the Danish Lung Cancer Registry. VTE events in the year after operation were assessed by stage, patient characteristics, and surgical procedure.

Results
We identified 13,197 patients who underwent operation for NSCLC in 2003 to 2021 (mean age, 67.6 years; 50% female); 10,524 (79.7%) had stage I-II NSCLC, and 2673 (20.3%) had stage III-IV. During 1-year follow-up, there were 335 VTE events, yielding a rate of 2.87 events/100 person-years and an absolute risk of 3.3% (95% CI, 2.3-4.0). VTE risk increased with advancing cancer stage (1.8% for stage I vs 3.9% for stage IV) but varied little by pathologic type, sex, and comorbidity level. Bilobectomy was associated with highest VTE risk (4.8%; 95% CI, 3.2-6.9), followed by pneumonectomy (3.5%; 95% CI, 2.3-5.0). The hazard of VTE was highest during the first 3 months after operation, after which it declined. For stage IV cancer, hazards increased again after 6 months. At 1 year, all-cause death was 12.6% (95% CI, 12.0%-13.1%).

Conclusions
VTE developed in 3.3% of patients undergoing operation for NSCLC, most commonly within 3 months postoperatively. Prolonged thromboprophylaxis could be considered, particularly in those with advanced cancer stage and undergoing extended resections.