Abstract PS1-09-06: Is there misdiagnosis and/or late diagnosis of metastatic ILC? A quantitative and qualitative patient-reported analysis

Abstract Background Invasive lobular breast cancer (ILC) is the second most diagnosed type of breast cancer (BC) in women, after invasive breast cancer of no special type. The linear-file infiltration and diffuse growth hinder detection by conventional imaging. Metastatic ILC (mILC) often shows poor glucose uptake and is less sensitive to FDG-PET imaging. ILC metastasizes to unusual sites—peritoneum, GI tract, ovaries, urinary tract, leptomeninges, and orbit—which may go unrecognized after a long disease-free interval. Some metastatic sites cause nondescript symptoms, like GI pain, with published case reports of mILC misdiagnosed as gastric cancer. These atypical BC metastatic sites may lead to late and/or misdiagnosis, thereby delaying effective treatments. Objective Given the limitations in accurate and timely diagnosis of mILC, we developed a patient survey to investigate the patient-reported prevalence of delayed diagnosis or misdiagnosis of mILC and their impact on treatment outcomes. Methods A 45 question survey was developed and piloted with breast cancer researchers, clinical oncologists, and patient advocates. An IRB-approved survey was administered to patients with mILC via social media, advocacy organizations, and conferences to assess patients' history, symptoms, diagnosis, and the impact of misdiagnosis or delays on treatment. Analyses including data QC and visualization were conducted in R using descriptive statistics. Incomplete or inconsistent responses were excluded, and summary statistics were stratified by four common mILC sites to highlight subgroup differences. Results 520 patient surveys were completed, with 450 patients diagnosed with ILC, and 321 diagnosed with mILC. Those with mILC, 33.3% (n = 107) were diagnosed de novo at initial presentation. Of the patients diagnosed with mILC, 32.1% (n = 103) presented with other medical conditions at diagnosis. Misdiagnosis was reported by 26.2% (n = 84) of patients with mILC, and 31% (n = 26) had ≥2 misdiagnoses before receiving an accurate one. The top 5 misdiagnoses (52% of all misdiagnoses) were benign breast condition (18%), bone related condition (18%), another type of BC (6%), diagnostic delay (7%), and primary GI cancer (3%). 44.5% of patients waited ≥1 year for an accurate diagnosis. The most reported contributors to delayed or misdiagnosis were inconclusive imaging, lack of ILC knowledge by providers, and initial misdiagnosis. Of the 321 mILC patients, 138 (42.9%) reported experiencing symptoms prior to diagnosis, the most common being back pain (16.5%), fatigue/malaise (14.9%), GI symptoms (11.8%), bloating (8.4%), and weight loss (8.1%). 47 patients described their misdiagnoses, with a total of 72 misdiagnoses: 23 GI, 19 GU, 15 neurological, and 15 hematological. This includes patients who reported 1 misdiagnosis of the same type, e.g., 17 patients had GU misdiagnosis, but the total number of GU misdiagnoses was 19. 38% were treated for their misdiagnosis, mostly with musculoskeletal, pain, or GI therapies; 6 patients received unnecessary cancer treatments. Diagnostic work-up for most patients included CT, biopsy, MRI, labs, and US. Although 40% of patients indicated they had a mammogram at the time of their initial misdiagnosis, ILC was detected in only 20.5% of these cases, detecting only 5 of 20 de novo mILC cases. Patients reported additional diagnostic testing within 1-3 mos. of their initial mammogram, including biopsy, US, and MRI. 47.6% of patients were in active BC surveillance at the time of their mILC diagnosis; however, no difference was seen in time to diagnosis vs those not under surveillance. Conclusion Our survey results underscore the urgent need to improve diagnostic strategies for mILC. Addressing delays and diagnostic errors in mILC is critical to optimizing treatment strategies and improving patient outcomes. Citation Format: M. E. Cody, M. Balic, R. Chang, T. Cushing, C. Desmedt, C. Donnelly, J. Foldi, S. Freeney, R. C. Jankowitz, J. Levine, L. Petitti, N. Ryan, K. Spencer, G. Tseng, C. Turner, S. Oesterreich, A. V. Lee. Is there misdiagnosis and/or late diagnosis of metastatic ILC? A quantitative and qualitative patient-reported analysis 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 PS1-09-06.