title: Symptom lead times in lung and colorectal cancers: what are the benefits of symptom-based approaches to early diagnosis?
abstract: 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.
authors.affiliation: School of Social and Community Medicine, University of Bristol, Canynge Hall, 39 Whatley Road, Bristol BS8 2PS, UK.
authors.forename: M
authors.initials: M
authors.lastname: Biswas
authors.affiliation: School of Social and Community Medicine, University of Bristol, Canynge Hall, 39 Whatley Road, Bristol BS8 2PS, UK.
authors.forename: A E
authors.initials: AE
authors.lastname: Ades
authors.affiliation: Institute of Health Service Research, University of Exeter Medical School, College House, St Luke's Campus, Exeter EX2 4TE, UK.
authors.forename: W
authors.initials: W
authors.lastname: Hamilton
mesh_terms: D016022:Case-Control Studies; D015179:Colorectal Neoplasms / Q000175:diagnosis* / Q000453:epidemiology; D055088:Early Detection of Cancer; D006801:Humans; D015994:Incidence; D008175:Lung Neoplasms / Q000175:diagnosis* / Q000453:epidemiology; D011320:Primary Health Care; D012680:Sensitivity and Specificity