Abstract P2-12-11: Lobular breast cancer: diagnostic complexity and delays to surgery

Abstract Introduction: Lobular breast cancers account for up to 15% of breast cancers but can be diagnostically complex and often require increased investigation burden. We investigated characteristics and outcomes of lobular breast cancer. Methods: We performed a retrospective cohort study of consecutive diagnoses of lobular breast cancer at two high-volume surgical centres in Liverpool, UK between March 2023-March 2024. Clinical, histological, imaging and treatment pathway data were collected. Outcomes in terms of time to surgery were computed and compared using Kaplan-Meier plots. Mean (SD) or median (IQR) are presented throughout and p-value <0.05 was considered statistically significant. Results: For the 12-month period, a new primary diagnosis of lobular breast cancer was diagnosed in 87 women with a median age of 65 years (36-96) and 25/87 (28.7%) were detected via the NHS screening programme. 3/87 (3.5%) first presented as metastatic disease (all 3 as gastrointestinal metastases). 71/87 (81.6%) were classical lobular cancer subtype, 11/87 (12.6%) were pleomorphic and 5/87 (5.7%) were mixed lobular/ductal. 59/87 (67.8%) underwent MRI, the mean tumour size on MRI was 43.7 mm (SD 28, range 6-140mm). Tumour size on MRI was similar to the tumour size on final histology (43.7mm vs 39.3mm, p=0.31), however was consistently larger than USS (43.7mm vs 25.1mm, p<0.001) and mammogram (43.7mm vs. 22.6mm, p<0.001). 25/60 (41.7%) undergoing MRI required second look imaging with 17 (68%) requiring further image-guided biopsy, of which 5 were positive for malignancy (PPV=20%). Time from diagnosis to surgery for patients going direct to surgery was significantly longer for the lobular breast cancer patients compared with total new cancers going direct to surgery (66 days vs. 55 days, p=0.004). In total, 32 (36.7%) patients had neoadjuvant endocrine therapy, 21 patients received neoadjuvant endocrine treatment as a bridging therapy to surgery (median 44 days, range 13-179 days) and 11 patients received neoadjuvant endocrine therapy for downstaging, for a duration of 9-12months. 10 frail and/or co-morbid patients were treated with primary endocrine therapy only, age range 77-96 years. Oncotype testing was undertaken in 30 patients, recurrence score range 3-25. 38/51(74.5%) patients underwent breast conserving surgery of which 8/38 (21.1%) required further surgery 4/8 (50%) underwent completion mastectomy. Pre-operative imaging consistently underestimated pathological tumour size in these patients. Conclusion: In lobular breast cancer, MRI demonstrates strong correlation with final pathological tumour size with greater accuracy than mammography and/or USS but generates a high volume of incidental findings requiring further investigations. We observed a clinically relevant and statistically significant delay to surgery in patients with lobular breast cancer. Further work is needed to improve efficiency in care pathways in lobular breast cancer, as this may improve patient outcomes. Citation Format: Hannah Lennon, Eaman Javed, Ruzan Reem, Hiba Fatayer, Vijay Sharma, Julia Henderson, Mohamed Lafi, Anupama Shrotri, Asha Shivaram, Lee Martin. Lobular breast cancer: diagnostic complexity and delays to surgery [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P2-12-11.