Pneumonia and the incidence of cancer: a Danish nationwide cohort study.

Objectives: To examine the risk of a subsequent pulmonary or extra-pulmonary cancer diagnosis following a first-time hospital-based diagnosis of pneumonia.
Design: Population-based cohort study using Danish medical registries.
Setting: All hospitals in Denmark.
Subjects: A total of 342,609 patients with a first-time hospital-based (inpatient, emergency room or outpatient clinic) diagnosis of pneumonia between 1995 and 2011.
Main Outcome Measures: We quantified the excess risk of various cancers amongst pneumonia patients compared to the expected risk in the general population, using relative [standardised incidence ratios (SIRs)] and absolute risk calculations. Follow-up started 1 month after a hospital-based diagnosis of pneumonia and ended on 31 December 2011.
Results: A total of 28,496 cancers were observed, compared with 21,625 expected, amongst 342,609 pneumonia patients followed for a median of 4.2 years. The absolute risk of a cancer diagnosis 1 to <6 months following a pneumonia diagnosis was 1.4%, with a corresponding SIR of 2.48 [95% confidence interval (CI) 2.41-2.55]. This was mainly due to an increased risk of lung cancer (eightfold) and haematological cancers (fourfold). The SIR for any cancer remained increased at 1.35 (95% CI 1.30-1.40) during 6-12 months of follow-up, and 1.20 (95% CI 1.18-1.22) during 1-5 years of follow-up. Beyond 5 years, an increased risk was maintained for lung, oesophageal, liver and bladder cancers, squamous cell carcinoma of the skin, lymphoma and multiple myeloma.
Conclusions: A hospital-based pneumonia diagnosis was associated with an increased risk of a cancer diagnosis, especially in the ensuing months, but the absolute risk was small.
(© 2014 The Association for the Publication of the Journal of Internal Medicine.)