Background: The role of neoadjuvant endocrine therapy (NET) for ER+ DCIS is an area of evolving study. It may allow down-sizing prior to surgery, converting DCIS requiring mastectomy to disease suitable for breast conservation surgery (BCS). Here we report the results from the first European single-institution series of its type.

Methods: Data were prospectively collected from patients diagnosed with ER+ DCIS and treated with NET prior to surgery, at a single unit between 2009 -2015. The size of the tumour on initial imaging (mammography) was compared to the size of the tumour on final imaging and pathology using RECIST criteria to determine disease progression. Blocks from initial core biopsy and final pathology are being interrogated by immunohistochemistry and DNA and RNA comparisons.

Results: 42 patients diagnosed with ER+ DCIS received NET with median age at diagnosis of 63y (range 37-94y). 7/42 premenopausal women were treated with tamoxifen, 35/42 post-menopausal women were treated with letrozole.

36/42 (85.7%) patients underwent surgery with 18/36 (50%) requiring mastectomy and 18/36 (50%) treated by BCS. 3/18 (16.7%) of the BCS patients required re-excision for positive margins. The median time to operation was 72d (range 15-308d). In total 12/42 (28.6%) had invasive disease on final pathology.

2/36 (5.6%) patients had a pathological complete response (PCR), 14/36 (38.9%) had a partial response (PR), 17/36 (47.2%) had stable disease and 3/36 (8.3%) had larger disease on pathology than imaging; this is a common feature of many lower grade DCIS lesions.

26/42 (61.9%) patients initially had DCIS >40mm (largest 240mm) and yet 9/26 (34.6%) of these patients still underwent successful BCS.

There was a significant correlation between length of endocrine therapy and reduction in size of disease. Immunohistochemical and molecular analyses are ongoing.

Conclusions:

•NET is an effective treatment for ER+ DCIS. It reduces the rate of re-excision to 16.7% in this series - substantially lower than the national (UK) figures for DCIS at 30%.

•It produces path CRs (5.6%) and high response rates that relate to the duration of treatment.

•This unique study shows that the optimal duration of NET is of the order of 6 months, achieving high rates of conversion for mastectomy to BCS.

Citation Format: Cartlidge CW, Johns N, Hackney RJ, Turnbull AK, Dixon JM. Neoadjuvant endocrine therapy for ER+ DCIS can lead to disease regression and allows BCS in up to a third of patients with disease >40mm at diagnosis [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P4-15-04.