Practice Essentials
Secondary lung tumors are neoplasms that spread from a primary lesion. The primary tumor can arise within the lung or outside the lung, with the metastases traveling through the bloodstream or lymphatic system or by direct extension to reach their destination. The secondary tumors most typically appear as well-circumscribed, noncalcified nodules. [1]
These secondary cancers are identified by their site of origin. Thus, a colon cancer that metastasizes to the lung is still known as a colon cancer. In children, most lung cancers are secondary. [2]
Metastatic malignant neoplasms are the most common form of secondary lung tumors. Lung metastases are identified in 30-55% of all cancer patients, though prevalence varies according to the type of primary cancer. Benign neoplasms (eg, benign metastasizing leiomyomas) are uncommon exceptions.
In this article, the approach to secondary lung tumors is discussed, with an emphasis on clinical decision-making to determine whether tissue diagnosis would alter clinical management. Also discussed is the multidisciplinary approach to determine when continued systemic chemotherapy for metastatic disease should be accompanied by radiation, surgery, or both.
Almost any cancer has the ability to spread to the lungs, but the tumors that most commonly do so include bladder cancer, colon cancer, breast cancer, prostate cancer, sarcoma, Wilms tumor, and neuroblastoma. (Primary lung cancers most commonly metastasize to the adrenal glands, liver, brain, and bone.) [1]
Secondary lung tumor is a term that is also used for the malignancies that arise in the lungs as a consequence of therapy for cancer (eg, chemotherapy, radiotherapy, or bone marrow transplant). [3] This article is not intended to cover the description of such tumors.
Spread to the lungs is usually the marker of an advanced malignant disease, but spread can also occur as an isolated early event. In certain circumstances, surgical resection with curative intent can be performed, with a reported 5-year survival rate of as high as 30-40%, depending on the underlying primary malignancy and the selection criteria for surgery.
Pathophysiology
The mechanisms through which cancer spreads to the lungs are direct extension and true metastatic spread through the bloodstream, airway, or lymphatic system. Iatrogenic implantation of a primary tumor is exceedingly rare.
Direct extension
Cancer spread through direct extension is not frequently encountered and most commonly includes direct invasion by a primary neoplasm, involving a contiguous organ or structure (eg, thyroid, esophagus, thymus, or chest wall), or spread from a neoplasm metastatic to another intrathoracic structure (eg, rib or mediastinal lymph node, commonly causing an obstructive lesion of the trachea or bronchus).
Direct extension can also occur through a vascular route, such as the spread of renal cell cancer or testicular germ cell cancer as a tumor thrombus to the lung via the inferior vena cava and the right side of the heart.
Metastatic spread
True metastases occur via the pulmonary arteries or bronchial arteries, via the pulmonary lymphatics, across the pleural cavity, or, infrequently, via the airways.
Arterial
The pulmonary arteries are the most common route for metastases. Cancers most likely to metastasize to the lungs include those with a rich vascular supply draining directly into the systemic venous system. Spread via bronchial arteries may be responsible for some endobronchial metastases. (Other proposed modes of endobronchial spread include bronchial invasion from parenchymal lesions, spread via involved mediastinal or hilar lymph nodes, and extension along the proximal bronchus.)
Lymphatic
Lymphangitic spread can occur in association with hematogenous dissemination, which is subsequently followed by invasion of the adjacent interstitium and lymphatics, with subsequent tumor spread toward the hila or toward the periphery of the lung.
Lymphangitic spread can also occur via retrograde spread of a tumor from the originally affected mediastinal or hilar lymph nodes, with consequent obstruction of lymphatic flow.
Pleural
Pleural spread most frequently results in pleural metastases in the caudal and posterior parts of the pleural cavities.
Airway
Spread via airways is rare and difficult to prove, except in the case of bronchoalveolar carcinoma.
Etiology
Although any cancer can metastasize to the lungs, the following neoplasms are most likely to do so:
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Choriocarcinoma
Prognosis
Lung metastases commonly cause no symptoms, but in some cases they can be the major cause of morbidity. Symptoms include hypoxemia, dyspnea, cough, and hemoptysis. Hypoxemia and dyspnea are most commonly observed in patients with lymphangitic spread, and cough and hemoptysis are associated with endobronchial metastases. Palliative care to address symptoms or local treatment with curative or palliative intent may be indicated.
The presence of hypoxemia cannot be explained by a cancerous process in the absence of lymphangitic spread, major lung collapse, or massive pleural effusion. Thus, it is usually a finding in patients with advanced disease. The presence of hypoxemia in the absence of these conditions should prompt the search for causes such as the following:
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Tumor emboli syndrome
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An infectious etiology
The presence of metastasis indicates an advanced stage of the malignant process. In certain circumstances, however, depending on the underlying primary malignancy and the selection criteria for surgery, surgical resection with curative intent can be performed, with an expected 5-year survival rate of 30-40%.
The following 5-year survival rates have been reported after resection of single pulmonary metastasis of the metastatic cancers known to respond favorably to surgical treatment:
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Adenoid cystic carcinoma - 63%
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Testicular cancer - 60%
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Squamous cell carcinoma (SCC) of the head and neck - 40-50%
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Colon cancer - 40%
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Breast cancer - 30-50%
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Soft-tissue sarcomas - 38%
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Renal cell cancer - 30-35%
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Osteogenic sarcoma - 20-57%
Solitary lung metastasis has a significantly better prognosis than does metastasis at any other visceral site in metastatic malignant melanoma, with a median survival of 8.3 months and a 5-year survival rate of 4%. The other important independent outcome predictor in metastatic malignant melanoma is the disease-free interval prior to the identification of metastatic disease (< 12 months vs >12 months).
