Objective: To investigate secular trends and correlates of incidence of breast cancer by histology type following the introduction of population-based mammography screening.
Methods: Analysis of age-standardised incidence rates for 1,423 in situ and 16,157 invasive carcinomas recorded on the South Australian population-based cancer registry for the 1985-2004 diagnostic period. Multiple logistic regression was undertaken to compare sociodemographic characteristics by histology. Progression from in situ disease was investigated using the Kaplan-Meier method.
Results: The incidence of in situ lesions increased approximately seven-fold over the 20-year period, compared with an increase of about 40% for invasive cancers. The increase for in situ lesions was due to increases for ductal carcinomas, with little change for lobular lesions. By comparison, the percentage increase in incidence for invasive cancer was greater for lobular than ductal cancers. Both for in situ and invasive cancers, percentage increases were greatest for the screening target age range of 50-69 years. One in 14 in situ cases was found to progress to invasive cancer within seven years of diagnosis, but insufficient detail was available to determine whether the invasive cancers were a progression of the in situ lesions or whether they originated separately. These invasive cancers were smaller than generally applying for other invasive cancers of the female breast.
Conclusions: The larger secular increases in incidence for in situ than invasive cancers would reflect the dominant role of mammography in the detection of ductal carcinoma in situ. The lack of an increase for lobular in situ lesions may have resulted from their poorer radiological visibility. The greater percentage increase for lobular than ductal invasive lesions may have been due to an increase in imaging sensitivity for these lesions, plus real increases in incidence. The smaller sizes of invasive cancers found in women with a prior in situ diagnosis may have resulted from more intensive medical surveillance, although the possibility of biological differences cannot be discounted.