Introduction: Little is known about the natural history of malignant solitary pulmonary nodules (SPN). Experts' beliefs may help fill these knowledge gaps and explain variation in clinical practices.

Methods: Using a modified Delphi process, we surveyed a group of lung cancer experts about tumor growth, disease progression, and prognosis in patients with malignant SPN. After completing the first survey, experts were given the opportunity during a second survey to revise their responses in light of their peers' beliefs.

Results: The response rate was 100% (14 of 14) for both surveys. There was consensus that disease progression depends on the tumor growth rate, that survival for patients with untreated lung cancer is approximated by a declining exponential function, and that treatment is delayed by approximately 1 tumor volume doubling time (TVDT) in patients who undergo a period of watchful waiting. Just over half of experts (8 of 14) agreed that lung cancer progresses in three steps (from local to regional to distant disease), whereas 43% (6 of 14) preferred a 2-step model (from local to systemic disease). Likewise, 64% of experts (9 of 14) believed that malignant nodules grow exponentially, whereas 36% (5 of 14) believed that growth is slower than exponential. Experts' estimates of the risk of disease progression during a period of observation lasting 1 TVDT varied from 1 to 50%. Estimates of 5-year survival for patients in whom diagnosis and treatment were delayed by 1 TVDT varied between 40% and 80%.

Conclusions: There is substantial variability in experts' beliefs about the natural history of untreated, malignant SPN. Different beliefs may be partly responsible for variation in management practices.