Breast cancer can spread to the lungs. This process, known as metastasis, occurs when cancer cells detach from the primary tumor and travel through the body to establish new tumors in distant organs. The lungs are one of the most common sites for breast cancer metastasis, alongside the bones, liver, and brain. Lung metastasis means the cancer cells found there are still breast cancer cells, not a new, separate lung cancer. This distinction is significant because it dictates the entire treatment approach and prognosis.
The Mechanism of Breast Cancer Metastasis
The journey of cancer cells from the breast to the lungs relies primarily on the body’s circulatory systems: the lymphatic system and the bloodstream. Cancer cells first detach from the original tumor mass and invade surrounding tissue.
The cells then enter the circulation, a process called intravasation, often using blood vessels or lymphatic channels. The lungs are particularly susceptible because they contain the first major capillary bed that breast cancer cells encounter after entering the general bloodstream. This dense network of small blood vessels easily traps circulating tumor cells.
Once in the lung tissue, the cells must exit the vessel (extravasation) and survive the new microenvironment to establish a new tumor colony. The primary tumor primes the lung for colonization by secreting factors and tiny vesicles called exosomes. These modify the lung tissue, helping the breast cancer cells survive and proliferate in their new location.
Recognizing Signs of Lung Involvement
When breast cancer cells colonize the lungs, a patient may experience respiratory symptoms, though the condition can be asymptomatic early on. A persistent cough that does not resolve is a frequently reported sign of lung involvement. This cough is often dry, but in some cases, it may involve coughing up blood, a symptom known as hemoptysis.
Shortness of breath (dyspnea) is another common concern, often noticeable during physical activity. This occurs due to tumor masses or a buildup of fluid between the lung and the chest wall, known as a pleural effusion. Other signs include wheezing, persistent chest pain or tightness, and recurring chest infections.
Distinguishing Breast Metastasis from Primary Lung Cancer
Differentiating a metastatic tumor from a newly developed primary lung cancer is essential for diagnosis, as treatment strategies differ significantly. Initial imaging, such as a CT or PET scan, identifies the location and pattern of the lung lesion. However, a biopsy of the tumor tissue is necessary for a definitive diagnosis, and this sample is examined by a pathologist.
The distinction relies on immunohistochemical (IHC) staining, which tests tumor cells for specific protein markers. Breast cancer cells often stain positive for markers like Estrogen Receptor (ER), Progesterone Receptor (PR), and GATA-3, and sometimes HER2. Primary lung adenocarcinomas frequently stain positive for Thyroid Transcription Factor-1 (TTF-1) and Napsin A, markers rarely found in breast cancer.
If the lung tumor cells express the same hormone receptors and HER2 status as the original breast tumor, it confirms the lesion is a metastasis. A panel of markers, including ER, TTF-1, and GATA-3, establishes the tumor’s origin with certainty. This precise identification ensures the patient receives treatment appropriate for breast cancer.
Systemic Treatment for Lung Metastasis
When breast cancer is confirmed to have spread to the lungs, it is classified as Stage IV disease. The focus of care shifts to systemic treatment tailored to the tumor’s molecular profile. Systemic therapies include:
- Hormone therapy
- Targeted drugs
- Chemotherapy
- Immunotherapy
If the cancer is Hormone Receptor-Positive, endocrine therapy is the primary approach, often combined with targeted therapies like CDK4/6 inhibitors. For HER2-Positive tumors, treatment involves targeted agents such as trastuzumab and pertuzumab, frequently combined with chemotherapy or antibody-drug conjugates. Chemotherapy remains an option for all subtypes, particularly for aggressive or Triple-Negative breast cancer.
Localized treatments like surgery or radiation are not curative for metastatic disease but manage specific symptoms or treat isolated lesions. Stereotactic body radiation therapy (SBRT) can precisely target small lung lesions to reduce their size and relieve symptoms like pain or obstruction. The goal of systemic treatment is to suppress cancer growth for as long as possible, maintaining the patient’s quality of life.