Abstract PD9-01: Prospect trial MRI findings: High incidence of occult cancers in apparently low risk cases

Background and Aim The role of staging breast magnetic resonance imaging (MRI) of apparently localised breast cancer is controversial. Some recommend MRI in selected ‘high risk’ situations, while others suggest it has little or no role. Few studies demonstrate improved outcomes associated with MRI. ANZ 1002 : Post-operative Radiotherapy Omission in Selected Patients with Early breast Cancer Trial (PROSPECT) is a prospective single-arm study using preoperative MRI to identify a group of patients with early breast cancer in whom radiotherapy might be safely omitted. Inclusion criteria include nil/minimal or mild Background Parenchymal Enhancement (BPE), unifocal pT1N0 invasive cancer, not TNBC, no LVI. Since September 2011, 443 patients have undergone MRI and 201 have had radiotherapy omitted. Primary analysis of ipsilateral local recurrence is due in May 2021. Here we report imaging, subsequent biopsy findings for occult lesions and mastectomy rates. Methods All patients who underwent PROSPECT MRI in addition to mammogram (MMG) and ultrasound (US) were included. Imaging findings on MMG, US and MRI were documented. Breast Imaging-Reporting and Data System (BIRADS) 4 or higher lesions on MRI were subject to biopsy under US if possible, or MRI or surgical biopsy if not. Pathologic results of lesions identified by MRI were described and the extent of surgery was documented. Results: Over 8 years, 443 patients were identified; mean age 63 years (range: 50 to 84), median index tumour size 13mm (range: 2 to 111), grade 1 (188), grade 2 (212) or grade 3 (38). MRI showed nil/minimal or mild BPE in 336 patients and moderate or marked BPE in 107. A total of 189 occult BIRADS>3 lesions were identified in 140 (32%) patients; 135 (71%) were ipsilateral; 36 (19%) could be identified and biopsied with MRI-directed, US-guided biopsy, while 63 (33%) underwent MRI-guided biopsy, 82 (43%) surgical biopsy after MRI-guided hook needle localisation, 1 underwent a stereotactic biopsy and 7 were not biopsied due to proximity to the index lesion. A total of 51 occult malignant lesions were identified in 40 patients (9% of the total patient cohort). 36 were invasive cancer and 15 were DCIS. There were 17 invasive and 14 DCIS ipsilateral occult lesions in 26 patients (6% of total cohort) and 19 invasive and 1 DCIS contralateral lesions in 18 patients (4% of total cohort) An additional 16 lesions in 9 patients were malignant but considered part of the primary cancer. There were 3 ‘at risk’ lesions: 1 LCIS and 2 ADH. There were a total of 112 benign biopsies in 95 patients. 9 patients in the cohort (2%) underwent total mastectomy due to pathological extent of disease of the index cancer (4), multicentric cancer (3) or patient choice (2). A higher proportion of patients with moderate/marked BPE had occult lesions (29% vs. 39%, p=0.09) and occult malignant lesions (8% vs. 13%, p=0.09) compared to those with less BPE. Conclusion Breast MRI in selected, low risk patients over 50 years old with low risk apparently unifocal cancer identified an occult breast cancer in 9% of patients. Only 2% underwent total mastectomy. Primary analysis of PROSPECT in 2021 will help define the clinical utility of these findings. Citation Format: Allison Rose, Arlene Mou, John Collins, Anita Skandarajah, Janemary Hughes, Heath Badger, Helen Braggett, Nicholas Zdenkowski, Rebecca Asher, Andrew Spillane, Boon Chua, Gregory Bruce Mann. Prospect trial MRI findings: High incidence of occult cancers in apparently low risk cases [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr PD9-01.