Objective: To determine the pathological and biological characteristics of breast cancers diagnosed by screening and examined at the Edinburgh University pathology department.
Methods: These cancers were classified by screening status: never screened (n = 111), prevalence screen detected (n = 105), and previously screened (n = 74). The last category arose in women who had been regularly screened during the trial; the cancers were diagnosed as interval cases before the first invitation to service screening (n = 33) or were incidence screen detected at that time (n = 41).
Results: Association (for operable invasive cancers, n = 250) of cancer characteristics with screening status reflects influences of biology (aggressiveness) or chronology (time of diagnosis), or both. The prognostic indicators tumour grade, histological type, and oestrogen receptor status were found in a smaller percentage of the patients with poor prognosis among the prevalence screen detected cases (9%, 77%, 18%) than among those previously screened (29%, 84%, 35%). The chronological factors size and node status were found in a smaller percentage of patients with poor prognosis among women previously screened (31%, 24%) than among those never screened (62%, 39%). Apart from these two, no other factors improved the diagnosis in the previously screened group compared with the never screened group.
Conclusions: These results suggest that favourable characteristics of screen detected cases are often due to the effects of length bias on "biological factors" and fail to show that current local screening practice has succeeded in advancing the diagnosis of breast cancers to a less aggressive phase.