Rare sites of breast cancer metastasis: a review

Breast cancer (BC) metastasis accounts for the majority of deaths from BC. The rate of metastasis to uncommon sites is on the rise due to the more effective therapy prolonging survival and to the early detection on imaging. The evaluation of patient-reported symptoms is essential in detecting a recurrence as early as possible, which may impact survival. Hence, the knowledge of even the rare sites of BC metastasis is of paramount importance for the clinical interpretation of new symptoms in BC survivors. The term "unusual metastasis" defines a systemic failure with a frequency of ≤1% at each site and according to this unusual metastasis involve the central nervous system, secretory/endocrine organs and glands, internal organs and structures, and gynecological organs. The literature search was performed using the electronic database PubMed up to December 2018, with the following key words: {[rare(Title/Abstract)] OR [unusual(Title/Abstract)] OR [unconventional(Title/Abstract)]} AND {[metastases(Title/Abstract)] OR [metastasis(Title/Abstract)]} AND {[breast(Title/Abstract)]} AND {[cancer(Title/Abstract)] OR [tumor(Title/Abstract)] OR [tumour(Title/Abstract)] OR [neoplasm(Title/Abstract)]}. The search was limited to papers in English language. Of the 3,086 papers found, 757 were excluded as reporting animal models, 378 were not in English language, 1 was a duplicate of the same research, 1,414 did not report on BC metastases, 108 were previous review reviews on BC or tumour to tumour metastases; 428 papers were included in this review. Despite the improvements in BC management, most deaths from cancer result from metastases that are resistant to conventional therapies. In general, it is uncommon to find isolated rare metastases, the vast majority of these develops together with metastases in other sites, thus highlighting a worsening systemic disease. However, the early detection of even rare metastases represents the only chance to control the disease and prolong survival while waiting for the development of more effective systemic therapies.