IMPORTANCE

Comparing the clinical and molecular features of metastatic invasive lobular carcinoma (mILC) and metastatic invasive ductal carcinoma (mIDC) is essential to enhance understanding of breast cancer biology and improve personalized treatment approaches.

OBJECTIVE

To compare mILC and mIDC in terms of survival outcomes and to investigate the association of clinicopathologic characteristics with those outcomes.

DESIGN, SETTING, AND PARTICIPANTS: This cohort study included adult patients at the University of Texas MD Anderson Cancer Center with their first metastatic diagnosis occurring between January 1997 and December 2020. Patient records were obtained from an institutional database. The study follow-up concluded in July 2023, and data were analyzed from July to December 2024.

EXPOSURE

Diagnosis of mIDC and mILC.

MAIN OUTCOMES AND MEASURES

Progression-free survival (PFS), overall survival (OS), and disease-free interval (DFI) were estimated using the Kaplan-Meier method. Survival distributions were compared using the log-rank test. Multivariable Cox proportional hazards regression was used to assess the association of metastasis onset, estrogen receptor (ER) expression level, and tumor grade with OS and PFS.

RESULTS

The analysis included a total of 9714 patients (9628 women [99%]), 8535 with mIDC and 1179 with mILC. The median age at metastasis was 53.3 years (range, 17.6-62.0 years). Generally, patients with mILC were older and had lower nuclear grade tumors and fewer metastasis sites than patients with mIDC. Patients with mILC had longer PFS (median, 0.65 years; 95% CI, 0.58-0.74) than patients with mIDC (median, 0.46 years; 95% CI, 0.45-0.48) (hazard ratio [HR], 0.78; 95% CI, 0.73-0.84; P < .001). For OS, patients with mILC had longer OS (median, 3.06 years; 95% CI, 2.87-3.29) than patients with mIDC (median, 2.60 years; 95% CI, 2.52-2.67 years) (HR, 0.91; 95% CI, 0.84-0.98; P = .01). Overall, patients with mILC had longer DFI than patients with mIDC (HR, 0.69; 95% CI, 0.64-0.75; P < .001). At initial diagnosis, patients with mILC were less likely to present with visceral metastasis (522 patients [44.3%]) than patients with mIDC (4909 patients [57.5%]). A higher proportion of patients with mILC (931 patients [79.0%]) than patients with mIDC (5431 patients [63.6%]) had bone-only metastasis.

CONCLUSIONS AND RELEVANCE

In this cohort study, patients with mILC had longer OS and PFS than those with mIDC. Metastasis onset, ER positivity, and tumor grade were associated with survival outcomes, and distinct metastatic patterns of mIDC and mILC were also associated with survival for mIDC and mILC, which may help guide more personalized treatment strategies for each subtype.