Abstract Background: Invasive lobular carcinoma (ILC) accounts for 15% of breast cancer (BC) cases and is increasingly recognized for its unique biological characteristics. Metastatic lobular breast cancer (mILC) can have different organ tropism and patterns of infiltration than invasive ductal carcinoma (IDC), which are inadequately described in contemporary literature. Due to loss of e-cadherin, ILC often exhibits radiographically occult infiltrative growth patterns rather than the discrete masses as seen in IDC, limiting early detection and intervention, and putting patients at risk for rapid organ compromise. Hydronephrosis is one such dangerous manifestation of mILC caused by diffuse infiltration of the periureteral tissue and lymphatics that leads to encasement and constriction of the ureters, causing potential renal failure if not treated early. Incidence of malignant hydronephrosis (MH) in mILC is underreported. We describe the characteristics and treatment course of patients with mILC and MH who received care at our institution. Methods: We conducted a retrospective chart review of 82 patients with mILC treated at our institution. We collected data on patient demographics, disease characteristics, and interventions received. Results: Among 82 patients with mILC, 27 patients (32.9%) developed MH. Other common metastatic sites included bone (69.5%), peritoneum (34.1%), leptomeninges (28%), and liver (24.4%). On imaging, 10 of the 27 cases were classified as having mild hydronephrosis, while 8 were moderate and 9 were severe. 15 patients had bilateral hydronephrosis. 12 patients underwent ureteral stenting, and 6 underwent percutaneous nephrostomy tube placement. Of the 27 patients with MH, 11 retained normal renal function. 16 developed acute kidney injury (AKI) of any grade, which notably included 4 patients with only mild hydronephrosis. Of those 16 patients, 1 went hospice, 11 had grade 1-2 AKI with renal function recovery after urological intervention and 4 had grade 3 AKI as defined by anuria for at least 12 hours, rise in creatinine over 3x baseline, or recommendations for renal replacement therapy. Among the 4 patients with severe AKI, 2 recovered with urological intervention, and 2 transitioned to hospice. Conclusion: The observed rate of MH (32.9%) in our cohort notably exceeds previous reported estimates in both general BC populations (1.4%) and ILC-specific cohorts (11%-22%). MH can be challenging to diagnose and even mild cases can cause rapid renal impairment due to the infiltrative nature of mILC. With clinical attention, the majority of our patients had recovery and preservation of renal function. Given these high rates and the risk of renal injury often exceeding severity on imaging, clinicians should maintain a high level of suspicion and awareness as early recognition and prompt management are critical to minimize renal damage and improve outcomes. Citation Format: N. Reddy, R. Shatsky, K. Yeung, A. Beck, B. Larkin, X. Zhang. Elevated Incidence of Malignant Hydronephrosis in Metastatic Invasive Lobular Carcinoma: A Single-Center Retrospective Study 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 PS1-08-22.
Reddy et al. (Tue,) studied this question.