Abstract PO4-07-09: CONTRAST-ENHANCED MAMMOGRAPHY IN LOCAL STAGING OF SCREEN-DETECTED BREAST CANCER: ADDITIONAL LESIONS AND CHANGES TO CLINICAL MANAGEMENT

Abstract Background BreastScreen Australia, a mammographic population screening program, provides assessment of suspected breast cancer to the point of diagnosis using a combination of conventional imaging and percutaneous biopsy. Comprehensive local staging, not performed at BreastScreen, determines the extent of disease and may identify additional clinically significant breast abnormalities. Local staging options include completion of bilateral mammographic work-up, ultrasound, and/or contrast-based imaging (CBI) (magnetic resonance imaging (MRI) or contrast-enhanced mammography (CEM)), with biopsy as required. We introduced CBI for local staging after diagnosis of screen-detected cancer at our academic hospital breast service in Melbourne, Australia. We report diagnostic findings for otherwise occult disease and their impact on treatment decisions in women who underwent CEM for local staging. Material and methods Women with screen-detected breast cancer who underwent CEM for local staging between November 2018 to April 2022 were identified retrospectively. The reporting breast-specialised radiologist compared BreastScreen and CEM images to identify additional enhancing abnormalities. Additional abnormalities were further investigated with preoperative percutaneous biopsy, surgical biopsy, or problem-solving CBI. Additional CEM-detected abnormalities were documented, with invasive cancer or DCIS recorded as true positive (TP) and any other findings as false positive (FP). Impact on surgical decisions was assessed. Results 204 patients underwent CEM. 62/204 (30%) patients had 76 additional abnormalities, of which 36 (47%) were TP and 40 (53%) FP. CEM identified otherwise occult malignant lesions in 30/204 (15%) patients. TPs comprised 75% invasive cancers and 25% DCIS. 83% (30/36) of malignant abnormalities were ipsilateral to the index lesion, while 17% (6/36) were contralateral. The majority of additional invasive cancers were Grade 2 (20/27, 74.1%), followed by Grade 1 (4/27, 14.8%) then Grade 3 (3/27, 11.1%). All additional invasive cancers had the same phenotype as the patient’s index cancer (ER+/HER2-), except for one abnormality which was ER+/HER2+ where the index cancer was ER+/HER2-. The FP abnormalities consisted of normal breast tissue (20/40, 50%), benign lesions (16/40, 40%) and atypical proliferative lesions (4/40, 10%). Occult malignancies were more common for patients with higher background parenchymal enhancement (20% for moderate/marked vs 4% for minimal/mild, p=0.0023), with no statistically significant differences found by breast density (BIRADS A or B = 12%, vs BIRADS C or D = 20%, p=0.23) or by age (40-49=38%, 50-59=13%, 60-69=13%, ³70=19%, p=0.88). Additional abnormalities found on CEM resulted in surgical management change in 45/204 (22%) patients, including wider resection (24/45), conversion to mastectomy (8/45), contralateral breast surgery (6/45), additional ipsilateral excision (5/45), and bracketing (2/45). Conclusions The use of CEM for local staging of screen-detected breast cancers identified otherwise occult malignancy in 15% of patients. Age and mammographic density did not identify groups at minimal risk of additional findings. Pathology of additional malignancy suggests it is clinically significant. CEM may improve local staging and direct appropriate management of screen-detected breast cancers. Citation Format: Caroline MacCallum, Kenneth Elder, Carolyn Nickson, Kelly Ruecker, Allan Park, Bruce Mann. CONTRAST-ENHANCED MAMMOGRAPHY IN LOCAL STAGING OF SCREEN-DETECTED BREAST CANCER: ADDITIONAL LESIONS AND CHANGES TO CLINICAL MANAGEMENT abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-07-09.