Background: Individuals with undiagnosed lung and colorectal cancers present with non-specific symptoms in primary care more
often than matched controls. Increased access to diagnostic services for patients with symptoms generates more early-stage
diagnoses, but the mechanisms for this are only partially understood.
Methods: We re-analysed a UK-based case–control study to estimate the Symptom Lead Time (SLT) distribution for a range of
potential symptom criteria for investigation. Symptom Lead Time is the time between symptoms caused by cancer and eventual
diagnosis, and is analogous to Lead Time in a screening programme. We also estimated the proportion of symptoms in lung and
colorectal cancer cases that are actually caused by the cancer.
Results: Mean Symptom Lead Times were between 4.1 and 6.0 months, with medians between 2.0 and 3.2 months. Symptom
Lead Time did not depend on stage at diagnosis, nor which criteria for investigation are adopted. Depending on the criteria, an
estimated 27–48% of symptoms in individuals with as yet undiagnosed lung cancer, and 12–32% with undiagnosed colorectal
cancer are not caused by the cancer.
Conclusions: In most cancer cases detected by a symptom-based programme, the symptoms are caused by cancer. These cases
have a short lead time and benefit relatively little. However, in a significant minority of cases cancer detection is serendipitous. This
group experiences the benefits of a standard screening programme, a substantial mean lead time and a higher probability of
early-stage diagnosis.