Cancer metastasis occurs when malignant cells detach from the original tumor and travel through the bloodstream or lymphatic system to establish a new growth in a distant organ. Although the breast is a common site for cancer development, it is a rare location for cancer to spread from outside the breast. It is possible, however, for lung cancer to metastasize to the breast tissue. This article explores this unusual occurrence, how medical professionals distinguish it from a primary breast tumor, and the different treatment approach required for this secondary cancer.
Understanding Metastasis to the Breast
Metastasis from an organ outside the breast to the breast tissue is extremely rare. Reported incidences of lung cancer spreading to the breast fall between 0.2% and 1.3% of all breast malignancies. The breast is not a common destination for metastatic disease compared to organs like the liver, bone, or brain. Consequently, a new lump in the breast is overwhelmingly more likely to be a benign growth or a primary breast cancer.
The spread of lung cancer cells to the breast most commonly occurs through the bloodstream, known as hematogenous dissemination. Less frequently, cells may travel through the lymphatic system, especially if the initial lung tumor involves the lymph nodes in the chest and armpit area. Lung adenocarcinoma is the type of lung cancer most frequently linked to this metastasis, though small cell and squamous cell lung cancers can also spread this way. While a breast nodule usually appears months or years after the initial lung cancer diagnosis, in rare cases, the breast lesion is the first sign of the underlying lung malignancy.
Clinical Differences Between Primary and Secondary Breast Tumors
Metastatic tumors in the breast often present with features that differ from primary breast cancer, offering initial clues to medical professionals. Primary breast cancers frequently arise in the upper-outer quadrant, but secondary tumors from the lung are more often found in the central or peripheral areas. These metastatic lesions typically appear as a single, firm, and often painless mass that may grow rapidly.
Unlike many primary breast cancers, metastatic tumors are less likely to be associated with skin changes, nipple discharge, or nipple retraction. On imaging, such as an ultrasound, lung metastases often appear as a well-defined, hypoechoic mass—meaning it is darker than the surrounding tissue. They typically lack the fine, clustered calcifications common in primary breast carcinoma. Some secondary tumors may mimic inflammatory breast cancer, presenting with skin edema and redness, which can lead to initial misdiagnosis.
How Doctors Confirm the Origin of the Tumor
The definitive way to determine if a breast tumor originated in the lung or the breast is through a biopsy followed by specialized laboratory testing. Distinguishing the source of the cancer cells is necessary because the treatments for the two are drastically different. Pathologists examine the tissue sample under a microscope and perform Immunohistochemistry (IHC) staining.
IHC uses specific antibodies that bind to proteins unique to certain cancer types. To confirm a lung origin, pathologists test for markers such as Thyroid Transcription Factor-1 (TTF-1) and Napsin A. TTF-1 is a protein found in lung and thyroid cells, and its presence strongly suggests the cancer cells originated in the lung. Napsin A is frequently positive in lung adenocarcinoma cells, providing further evidence of a lung primary.
Conversely, the tissue is tested for markers associated with primary breast cancer: Estrogen Receptor (ER), Progesterone Receptor (PR), and Human Epidermal growth factor Receptor 2 (HER2). Metastatic lung cancer cells in the breast are usually negative for these breast-specific markers. They can sometimes mimic a triple-negative breast cancer, which is also negative for all three. The combination of a positive lung-marker stain and a negative breast-marker stain confirms the tumor is metastatic lung cancer.
Treatment Approach for Secondary Lung Cancer in the Breast
The fundamental principle guiding treatment for metastatic cancer is that the disease is treated based on its tissue of origin, not the site where it has spread. Therefore, secondary lung cancer in the breast is treated as advanced lung cancer. The primary goal is to control the systemic disease throughout the body, not just the breast lesion.
Treatment involves systemic therapies, such as chemotherapy, targeted therapy, and immunotherapy, which circulate throughout the body to attack cancer cells. The specific regimen is tailored to the molecular and genetic profile of the original lung tumor, including testing for mutations like EGFR or ALK rearrangements. Localized treatments, such as surgery or radiation therapy, are reserved for palliative purposes, such as controlling pain or preventing the mass from ulcerating. Local treatment is used only if systemic therapy is insufficient to control symptoms caused by the breast metastasis.