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540 Background: Treatment of hormone receptor positive (HR+) ESBC is evolving. The use of chemotherapy (CT) is declining with use of the 21-gene RS assay. This validated tool predicts the likelihood of adjuvant CT benefit in HR+ ESBC. Results from the TAILOR-x study suggest up to 70% of HR+ node negative ESBC patients (pts) may avoid CT with RS ≤25. Our objectives were to assess the clinical and economic impact of RS testing on treatment decisions using real-world data. Methods: From October 2011 to February 2019, a retrospective, cross-sectional observational study was conducted of HR+ node negative ESBC pts who had RS testing in Ireland. A survey of Irish breast medical oncologists provided the assumption for the decision impact analysis that grade (G) 1 pts would not receive CT pre RS testing and G2/3 pts would receive CT. Using TAILOR-x results, pts were classified low risk (RS ≤ 25) and high risk (RS > 25). Data was collected via electronic patient records. Descriptive statistics were used. Cost data was obtained via the National Healthcare Pricing Regulatory Authority. The economic analysis was adjusted for changing treatment and assay costs over the study period. Results: 963pts were identified. Mean age 56 years. Mean tumour size 1.87cm. 114 (11.8%), 636(66%), 211(22%), 2(0.2%) pts had G1, G2, G3 and unknown G respectively (resp). 797pts (82.8%) had low RS, 159 (16.5%) had high RS, and 7pts(0.7%) unknown RS. 251pts(26%) were aged < 51 at diagnosis. Of these, 45(17.9%), 145(57.8%), 58(23%), 3(1.2) had G1, G2, G3 and unknown G resp. 208pts(82.9%) had RS ≤ 25, 39pts(15.5%) had RS > 25 and 4pts(1.6%) unknown RS. In the RS ≤ 25 group, 111pts(44%) had RS 0-15, 59(23.5%) had RS16-20, and 38(15.1%) had RS21-25. Post RS testing 595pts(61.8%) had a change in CT decision; 586 changed to hormone therapy (HT) alone, and 9 from HT to CT. In total, 227pts(23.5%) received CT, and 3pts(0.3%) declined. Of pts treated with CT; 9(4%) had RS 0-15, 89(39.2%) had RS16-25, 129(56.8%) had RS > 25. The most common CT regimen was docetaxel and cyclophosphamide(TC), administered to 121pts(53%). RS assay use achieved a 69% change in treatment decision among G2/3 pts and a net 61% reduction in CT use. This resulted in savings of over €4 million in treatment costs. Deducting the assay cost, net savings of over one million euro was achieved. Conclusions: Ireland was the first public healthcare system to approve reimbursement for RS testing. Over the 8 year period of the study, a net 61% reduction in CT use in Irish pts with HR+ ESBC was achieved with conservative net savings of over €1,000,000.

43 Background: Predicting the risk of tumour recurrence, and thus the need for chemotherapy, for lymph node-negative breast cancer patients is a significant problem for clinicians and patients. Methods: We have identified a ‘core proliferation signature,’ which is consistently high in proliferating primary cultures, and is downregulated during cellular senescence. Using a reverse engineering approach on a breast cancer-specific regulatory network, and confirmed by ChIP-seq analysis, we have identified a hierarchy of several highly interconnected Master Transcriptional Regulators upstream of these core proliferation genes. Results: Further analysis of the expression of these factors in breast cancer cohorts at the mRNA and protein levels reveals a remarkable ability to reliably predict recurrence risk for early-stage breast cancer. Strikingly, in our analyses, a combination of just two of these factors outperforms the currently used clinical biomarkers for breast cancer recurrence risk, as well as recently developed multi-gene prognostic assays. Moreover, the addition of the senescence regulator p16INK4A to this panel further increases its prognostic capability. Conclusions: We propose that this novel approach has succeeded in identifying ‘drivers’ of breast cancer proliferation which, when combined with a marker of senescence such as p16INK4A, successfully identify actively proliferating tumours with an impaired senescence response pathway. Furthermore, we suggest that this gene combination has the potential to become an improved prognostic assay for early-stage breast cancer.