Invasive Lobular Carcinoma Has Worse Outcome Compared with Invasive Ductal Carcinoma in Stage IV Breast Cancer with Bone-Only Metastasis

BackgroundInvasive lobular carcinoma (ILC) is more likely to have bone metastasis than invasive ductal carcinoma (IDC). However, the prognosis for bone metastasis in ILC and IDC is barely known. So, the aim of this study was to investigate the difference of prognosis between ILC and IDC accompanied by bone metastasis.MethodsWe evaluated the women with bone-only metastasis of defined IDC or ILC reported to the Surveillance, Epidemiology and End Results program from 2010 to 2016. Pearson''s χ² test was used to compare the differences of clinicopathologic factors between IDC and ILC. Univariate and multivariate analyses were performed to verify the effects of histological types (IDC and ILC) and other clinicopathologic factors on the overall survival (OS) and cancer-specific survival (CSS).ResultsOverall, 3,647 patients with IDC and 945 patients with ILC met the inclusion criteria and were analyzed in our study. The patients with ILC were more likely to be older and to have lower histological grade and a higher proportion of the HR*/HER2− subtype. However, less treatment was administered to ILC than IDC, such as surgery of the breast, radiation, and chemotherapy. Compared to patients with IDC, patients with ILC showed worse OS (median OS, 36 and 42 months, respectively, p < 0.001) and CSS (median CSS, 39 and 45 months, respectively, p < 0.001), especially in subgroups with HR*/HER2− subtype (OS, hazard ratio: 1.501, 95% CI 1.270–1.773, p < 0.001; CSS, hazard ratio: 1.529, 95% CI 1.281–1.825, p < 0.001), lower histological grade (I–II) (OS, hazard ratio: 1.411, 95% CI 1.184–1.683, p < 0.001; CSS, hazard ratio: 1.488, 95% CI 1.235–1.791, p < 0.001), or tumor burden, such as T_0–2 (OS, hazard ratio: 1.693, 95% CI 1.368–2.096, p < 0.001; CSS, hazard ratio: 1.76, 95% CI 1.405–2.205, p < 0.001) and N_1–2 (OS, hazard ratio: 1.451, 95% CI 1.171–1.799, p = 0.001; CSS, hazard ratio: 1.488, 95% CI 1.187–1.865, p = 0.001). Furthermore, older age, black race, unmarried status, higher tumor burden (T_3–4 and N₃), triple-negative subtype, and higher histological grade were independent risk factors for both OS and CSS. Surgery of the breast and chemotherapy could significantly improve the prognosis for patients.ConclusionPatients with ILC have worse outcomes compared to those with IDC when associated with bone-only metastasis, especially in subgroups with lower histological grade or tumor burden. More effective treatment measures may be needed for ILC, such as cyclin-dependent kinase 4/6 inhibitors, new targeted drugs, etc.