Metastatic behavior of mixed invasive ductal lobular carcinoma (mIDC/ILC).

Abstract

1085 Background: Mixed invasive ductal lobular carcinoma (mIDC/ILC) is a poorly described subtype of invasive breast cancer, characterized by its composition of both ductal and lobular histopathology. It is unclear if individual or both components drive metastasis. Literature is sparse regarding sites of metastatic spread of this elusive subtype of invasive breast cancer. Methods: Cohorts of patients with mIDC/ILC, invasive ductal carcinoma (IDC), and invasive lobular carcinoma (ILC) were identified from the UPMC Network Cancer Registry. Among these, 46 patients with mIDC/ILC, 1,131 patients with IDC, and 145 patients with ILC seen at UPMC Magee Women’s Hospital from 1990 – 2017 were found to have developed distant metastasis during the course of their disease. The metastatic pattern of spread was compared between the cohorts. Formalin-fixed, paraffin-embedded patient samples from the metastatic sites of a portion of the mIDC/ILC cases (n = 19) was acquired and evaluated by H&E staining. Results: Patients with IDC were more likely than patients with ILC to have metastasis to the liver (p = 0.001) and lung (p < 0.001), and less likely to have metastasis to the peritoneum (p < 0.001). Patients with mIDC/ILC were more likely than patients with IDC to have peritoneal metastasis (p = 0.01), similar to patients with ILC. Compared to patients with ILC, patients with mIDC/ILC were more likely to have liver metastasis (p = 0.001), similar to patients with IDC. Evaluation of the metastatic lesions originating from mIDC/ILC displayed a spectrum of histopathology including mixed histology (n = 3), pure IDC (n = 3), pure ILC (n = 5), and indeterminate lesions with features of both IDC and ILC (n = 6). Two cases were uninterpretable due to significant crush artifact. Metastatic mIDC/ILC lesions with retained mIDC/ILC histology were found in vertebral, pleural, and skin tissues. Metastatic mIDC/ILC lesions with IDC histology were found in a cerebellar, liver, and chest wall lesion; whereas, those with ILC histology were found in bowel, omental fat, ovary, bone, and sacrum. Indeterminate histology metastatic lesions were found at liver, chest wall, cerebellar, and bone sites. Conclusions: mIDC/ILC metastasizes to a range of distant sites with a higher preference to the liver and peritoneum as compared to ILC and IDC, respectively. Metastatic lesions arising from mIDC/ILC tumors showed a spectrum of histologies, including mIDC/ILC, IDC, ILC and indeterminate lesions with features of both IDC and ILC. Ongoing genomics studies will provide further insight into development of metastases from mIDC/ILC tumors.