Abstract PS3-09-23: Neoadjuvant Aromatase Inhibitor Therapy Decreases Radiographic Tumor Size: Initial Results from the NAOMI Trial

Abstract Background: Nearly 70% of breast cancers are ER positive, and anti-estrogen therapy, such as tamoxifen and aromatase inhibitors (AI), are a mainstay of treatment. Neoadjuvant AI therapy is effective at reducing tumor burden, and may be used to improve surgical outcomes. Non-invasive assessment of response may be helpful in treatment planning, and radiographic treatment response has not been evaluated. Additionally, questions remain as to the social effects of neoadjuvant therapy. Preliminary data from this trial presented at Northern New England Clinical Oncology Society in 2024 showed an 85% adherence to aromatase inhibitor therapy at 1 year in patients who were treated in the neoadjuvant setting, with ongoing data collection, compared to 62% in the adjuvant setting, according to one study. Objective: Determine the ability of tumor imaging to detect the effect of neoadjuvant aromatase inhibitor (AI) therapy on tumor size. Methods: 52 post-menopausal women with Stage I-III ER+, HER2- breast cancer were treated with an aromatase inhibitor for 1-24 weeks (mean 6.7 weeks) prior to surgery and had pre-neoadjuvant imaging available for analysis. 12 patients did not have an imaging exam after neoadjuvant endocrine therapy and prior to surgery, or it was not possible to assess tumor size on pre-operative imaging, resulting in 40 patients for final analysis. Tumor measurements prior to neoadjuvant endocrine therapy were compared to tumor measurements after neo-endocrine therapy at the time of surgery using mammography or ultrasound to determine response to treatment. Post neo-endocrine treatment measurements were compared with pathologic tumor size. Results: Neoadjuvant aromatase inhibitor therapy decreased tumor size evaluated by imaging (mean pre-treatment size 19.4mm, mean post-treatment 14.8mm, p<.001), with size reduction in 68% of patients (AUC=.68). The larger the initial tumor size, the greater the reduction in tumor size (p=0.0017): on average, an additional 10mm increase in initial tumor size results in a 2mm decrease in post-treatment tumor size. The correlation coefficient between pathologic tumor size and post neoadjuvant AI therapy tumor size determined by specimen radiographs or localization imaging (with mammography or ultrasound) was 0.69. Discussion: AI therapy in the neoadjuvant setting could improve overall response to therapy and may improve long-term adherence. Response to therapy can be evaluated with mammography and ultrasound imaging obtained at the time of needle localization or specimen radiograph. Further work is needed to evaluate long-term outcomes with this treatment, and correlation with histology is ongoing. Due to the discordant results in lobular carcinomas, separate analysis of a large subset of lobular carcinomas is planned. Citation Format: L. Lawrence, R. diFlorio-Alexander, J. Marotti, E. Demidenko, M. Chamberlin. Neoadjuvant Aromatase Inhibitor Therapy Decreases Radiographic Tumor Size: Initial Results from the NAOMI Trial [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-09-23.