Treatment and Outcomes from a Large, Prospective, National Longitudinal Cohort Study of Screen Detected Ductal Carcinoma in Situ (DCIS).

1570 Background: The degree of over-diagnosis and over-treatment for ductal carcinoma in situ (DCIS) remains contentious. Hypotheses from retrospective data were tested in a prospective, population-based study. Methods: A prospective longitudinal cohort of 12,838 women diagnosed through a population-based national breast screening program accrued 2003-2012 with proforma for imaging, surgery, pathology and adjuvant therapy. Follow-up was linked to national databases and case note review to analyse patterns of care, recurrence and mortality. Results: Data was complete for 9,972/12,838 (78%) women (mean age 60 years; range 46-87) with median follow up 62 months (range 6-116). Breast conservation (BCS) was definitive surgery for 7,028 (70.5%) women; use increased with older age (CHI2 380, 6 df p < 0.001). Mastectomy (Mx; n = 2,944) was associated with higher grade DCIS (CHI2 19.31, 2 df P < 0.001), larger size (CHI2 203.9, 5 df P < 0.001) and comedo necrosis (CHI2 9.1, 1 df P = 0.003). For patients undergoing BCS, use of radiotherapy was associated with high grade (CHI2 199.9, 2 df P < 0.001), comedo necrosis (CHI2 62.3, 1 df P = 0.001), larger size (CHI2 57.5, 5 df P < 0.001), microinvasion (CHI2 49.9, 1 df P = < 0.001), and use of endocrine therapy (CHI2 16.47, 3 df P < 0.001), but not close ( < 1mm) or involved margin(s). In women who underwent BCS, ipsilateral in-breast recurrence (n = 368; CHI2 27.14, 5 df P < 0.001) and ipsilateral invasive cancer (n = 182; CHI2 32.62, 5 df P < 0.001) were significantly reduced by radiotherapy, independent of margins. Endocrine therapy reduced local recurrence by 1.52%, independent of radiotherapy. Among 324 deaths (3.3% of the cohort), 47 were from invasive breast cancer. There was no difference in overall mortality by type of surgery (3.16% BCS vs 3.5% Mx; CHI2 0.017, 3 1f, P = 0.90), use of radiotherapy (2.48% or not 3.84%; CHI2 0.30, 1 df, P = 0.59) or endocrine therapy (3.88% or not 3.14%; CHI2 0.077, 1 df, P = 0.78). Conclusions: This prospective cohort of contemporary, screen-detected DCIS demonstrates ipsilateral recurrence, but not mortality, is reduced by radiotherapy and endocrine therapy. Death due to breast cancer was rare, outnumbered 5:1 by death from other causes.