Low-dose tamoxifen for breast cancer prevention in patients with ductal carcinoma in situ (DCIS) and atypical lesions: A real-world experience.

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e12537 Background: Patients diagnosed with non-invasive breast lesions such as DCIS and lobular carcinoma in situ (LCIS) and atypical breast lesions such as atypical ductal hyperplasia (ADH) and atypical lobular hyperplasia (ALH) are at increased risk of developing invasive breast cancer (BC). Tamoxifen 20mg daily for 5 years has shown to be effective for BC prevention in these at-risk patients but its use can be limited by its adverse effects. The recent Tam01 study found that low dose tamoxifen (5mg daily for 3 years) in patients with DCIS, ALH, and ADH resulted in an approximate 50% risk reduction in BC events compared to placebo with a slight increase in hot flashes but no increases in uterine cancer or venous thromboembolism within the tamoxifen group (PMID: 30973790). The objective of this study was to evaluate the use of low dose tamoxifen at our institution and determine how many patients preferred this option. Methods: We performed a retrospective chart review of women who were diagnosed with DCIS, LCIS, ADH, and/or ALH at our institution from January 2019 to December 2021 and subsequently started adjuvant tamoxifen. Patients with history of invasive BC were excluded. Patients were categorized as being on standard dose (20mg daily) or low dose (5mg daily or 10mg every other day) tamoxifen. Fisher’s exact test was used to compare rates of standard and low dose tamoxifen use in the entire cohort as well as in pathology-based subgroups. Results: We included 130 women diagnosed with DCIS, LCIS, ADH and/or ALH and who started adjuvant tamoxifen. The median age at diagnosis was 50 years and 68% were premenopausal. Overall, 62% (n = 80) of women started standard dose tamoxifen and 38% (n = 50) low dose tamoxifen. Among 63 patients who were offered both standard and low dose options, 76% (n = 48) initiated low dose tamoxifen (p < 0.0001). Twenty women were noted to be hesitant to start standard dose tamoxifen due to concern for side effects or preexisting menopausal symptoms but were agreeable to the low dose option. Of the 86 patients with DCIS, a preference for standard dose tamoxifen was observed (74% vs 26%, p = 0.0016). Among 44 patients with LCIS, ADH, and/or ALH, more patients were on low dose tamoxifen (64% vs 36%, p = 0.282). Nine percent (n = 7) of patients on standard dose tamoxifen switched to low dose due to adverse effects including hot flashes, mood changes, and insomnia. Conclusions: At our institution, in patients offered both low dose and standard dose tamoxifen, more than 75% started low dose tamoxifen. The low dose option was more frequently used for atypical lesions such as ADH, ALH and LCIS. In women at higher risk of invasive BC who may be reluctant to start standard dose tamoxifen or intolerant of it, low dose tamoxifen should be considered and offered.