A 46-year-old woman with erythema of the right breast presented to our hospital and was diagnosed with stage IV breast cancer (HER2-positive invasive ductal carcinoma). She received 4 courses of anthracycline-based regimens and 4 courses of trastuzumab + pertuzumab + docetaxel (Tmab + Pmab + DTX). Since she responded well to these therapies, only Tmab + Pmab was continued thereafter. Twenty-three months after starting treatment, she developed a headache. A tumor was identified in the right temporal lobe. Craniotomy was performed for definitive diagnosis. Intraoperative pathological assessment suggested the tumor to be brain metastasis of breast cancer. However, the final pathological diagnosis was diffuse large B-cell lymphoma of central nervous system (DLBCL-CNS) based on re-assessment with immunohistochemical examinations. Therefore, the Tmab + Pmab was discontinued, and 6 courses of high-dose methotrexate therapy were administered. This case highlights the importance of considering rare entities, such as DLBCL, when diagnosing a solitary brain tumor in a patient with a primary cancer, based on imaging and pathological findings.

Background Metastatic breast tumors from extramammary malignancies are quite rare. Characteristics of such tumors are unclear due to small number of reported cases. During 2012–2019, approximately 3,500 malignant breast tumors were diagnosed with needle biopsy at our hospital and we experienced three cases (0.09%) of metastatic extramammary malignancies. We herein report these cases focused on imaging and pathological findings. Case presentation The first case was a 59-year-old woman who underwent curative surgery for thyroid cancer. After developing lung and ovarian metastases, she visited our department with a mass in her right breast. Ultrasound revealed a 7 mm-sized oval mass. With high depth–width ratio and abundant blood flow, primary breast cancer was suspected. Core needle biopsy revealed atypical cells with nuclear grooves proliferating in papillary formation. With immunohistochemical examination, her final diagnosis was metastatic thyroid cancer. The second case was a 74-year-old woman with metastatic spinal tumors and referred to our department for searching primary tumor. She was diagnosed with gastric cancer at the age of 41. Ultrasound revealed a hypoechoic area including cysts and the internal echo level was uneven. Contrast-enhanced magnetic resonance imaging showed a non-mass lesion with heterogeneous internal enhancement pattern, suggesting ductal carcinoma in situ. Core needle biopsy showed alveolar lesion with predominant signet cell-like morphology. We histologically diagnosed her disease as metastatic gastric cancer. The last case was 33-year woman with Stage IV clear cell sarcoma of the left foot. She came to our department after she felt a lump on her right breast. Ultrasound revealed a 45 mm-sized mass. Her disease was confirmed as metastatic clear cell sarcoma by needle biopsy. Conclusions Imaging suggested malignancies, but it was difficult to distinguish them from primary breast cancer. Our cases indicate that metastatic tumors to the breast might have imaging patterns specific to primary organs, although more cases should be accumulated to establish such patterns on imaging. The first two cases shared some similar pathological findings with breast cancer, but also had some histological characteristics of the primary tumors. Hence, it was possible to diagnose these cases as metastatic tumors with careful observation.

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Adjuvant chemotherapy has played a major role in the treatment of hormone receptor-positive breast cancer for many years. To better determine which patient subsets need adjuvant chemotherapy, various gene expression analyses have been developed, but cost-effective tools to identify such patients remain elusive. In the present report, we retrospectively investigated immunohistochemical expression and subcellular localization of MUC1 in primary tumors and examined their relationship to tumor malignancy, chemotherapy effect and patient outcomes. We retrospectively examined three patient cohorts with hormone receptor-positive/human epidermal growth factor receptor 2-negative invasive breast cancer: 51 patients who underwent 21-gene expression analysis (multi-gene assay-cohort), 96 patients who received neoadjuvant chemotherapy (neoadjuvant chemotherapy-cohort), and 609 patients whose tumor tissue was used in tissue-microarrays (tissue-microarray-cohort). The immunohistochemical staining pattern of the anti-MUC1 monoclonal antibody, Ma695, was examined in cancer tissues, and subcellular localization was determined as apical, cytoplasmic or negative. In the multi-gene assay-cohort, tumors with apical patterns had the lowest recurrence scores, reflecting lower tumor malignancy, and were significantly lower than MUC1-negative tumors (P = 0.038). In the neoadjuvant chemotherapy-cohort, there was no correlation between MUC1 staining patterns and effects of chemotherapy. Finally, in the tissue-microarray-cohort, we found that patients with apical MUC1 staining patterns had significantly longer disease-free-survival and overall survival than other patterns (P = 0.020 and 0.039, respectively). Our data suggest that an apical MUC1 staining pattern indicates luminal A-likeness. Assessment of the subcellular localization of MUC1 glycoprotein may be useful for identifying patients who can avoid adjuvant chemotherapy.