Medical tradition and lore advised follow-up chest films for asymptomatic people 6 weeks after pneumonia to see if a lung cancer might be lurking there. IDSA guidelines from 2007 don't address the question, and UK guidelines suggest a follow-up chest X-ray only in those with a smoking history or over age 50.
Using databases, Tang et al found 3,398 patients in Edmonton (Canada) diagnosed with pneumonia between 2000 and 2002. (17% were smokers.) About 40% got follow-up chest X-rays; 57 cases of cancer were diagnosed at 90 days. (It's unclear how often the lung cancers were diagnosed by the chest X-rays -- the indications and results for the chest films couldn't be gleaned from the databases.) Results:
- The incidence of lung cancer at 90 days was 1.1%; at 1 year, 1.7%; and at 5 years, 2.3%.
- All but 1 of the 57 people found with lung cancer at 90 days were over 50.
- All but 3 of the 79 found with cancer at 5 years were over 50.
Authors say if chest films were restricted to those over 50, the yield of a post-pneumonia chest X-ray would have been 2.8%.
This wasn't randomized or prospective, so biases as to who got chest films are baked-in. But the population-level numbers are probably accurate and informative -- particularly the 5-year cancer incidence. Authors conclude chest films aren't justified routinely for anyone under 50. That seems reasonable, as this would imply a number needed to screen in the thousands-range to diagnose one cancer that may or may not have been curable among the under 50 set.
Tang KL et al. Incidence, correlates, and chest radiographic yield of new lung cancer diagnosis in 3398 patients with pneumonia. Arch Intern Med. 2011;171(13):1193-1198.