William MacFaui - The University of Sydney, Sydney Adventist Hospital Clinical School, Sydney, NSW, Australia
T Michael D Hughesa - The University of Sydney, Sydney Adventist Hospital Clinical School, Sydney, NSW, Australia- Breast Multidisciplinary Team, Sydney Adventist Hospital, Sydney, NSW, Australia- Division of Surgery, Sydney Adventist Hospital, Sydney, NSW, Australia
Kerry Hitosd - Westmead Research Centre for Evaluation of Surgical Outcomes, Department of Surgery, WestmeadHospital, Sydney, NSW, Australia- Westmead Clinical School, The University of Sydney, NSW, Australia
Nimala Pathmanathane - Westmead Breast Cancer Institute, Westmead, NSW, Australia- Douglass Hanly Moir Pathology, Macquarie Park, NSW, Australia
Nicholas K Nguia - The University of Sydney, Sydney Adventist Hospital Clinical School, Sydney, NSW, Australia- Breast Multidisciplinary Team, Sydney Adventist Hospital, Sydney, NSW, Australia- Division of Surgery, Sydney Adventist Hospital, Sydney, NSW, Australia

Background: Pathological complete response (pCR) following neoadjuvantsystemic treatment(NAST) for breast cancer is associated with improvedprognosis; however, a large proportion of patients have residual disease.Oestrogen Receptor (ER) and HER۲ status have been shown to affect likelihood ofachieving pCR, with ER positive tumors being more treatment resistant. Ashormone receptor status is heterogeneous within tumors, we postulated that,following NAST, ER expression would change in patients with residual disease, asthe ER negative cells within the tumor are more treatment sensitive.Methods: A retrospective case series of patients treated with NAST prior tosurgery at our institution was conducted. Information collected includeddemographic data, tumor grade, hormone receptor and HER۲ status both beforeand after treatment, and pCR rates.Results: Of the ۴۴ patients included, half achieved pCR. HER۲ status(P=۰.۰۱), and subtype (P=۰.۰۰۸) were significantly associated with pCR. HER۲positive/ER negative tumors were most likely to undergo pCR. Approximately۸۰% of residual disease was ER positive. Higher levels of ER expression were alsoassociated with increasing residual cancer burden (RCB) class (P=۰.۰۳۷). Therewas no trend between change in ER or HER۲ expression following NAST. Medianchange in ER expression was ۸۰% to ۹۰% (P= ۰.۸۹), HER۲ intensity changed from۳.۰ to ۲.۲ (P=۰.۶۷) following treatment.Conclusion: Consistent with the literature, we have shown associationsbetween ER and HER۲ status and PCR, and between ER expression and residualdisease burden. Our study was not able to demonstrate a significant trend inhormone and HER۲ expression.

HER۲, hormone status, neoadjuvant, pathological complete response, residual disease