Are there some cancers found by screening mammography that do not require treatment?

Probably, but it is difficult to determine this at the individual level, i.e. for a given patient. Some cancers are so indolent and slow-growing that they might not ever have been detected otherwise in a patient’s lifetime (“overdiagnosis”). While estimates of overdiagnosis vary, on average, of 11 breast cancers found with screening, 2 will be life saving, 1 will represent overdiagnosis, and 8 will be found earlier than they would have been without screening (with better prognosis) [1]. Some ductal carcinoma in situ found on the first screening examination represents overdiagnosis (estimated at 37% of such cases), but new findings on subsequent screens are uncommonly overdiagnosis (estimated at 4% of cancers on annual screens).