Pulmonary Metastasis: Causes, Frequency, Mortality, Clinical Details and Treatment

It is the expansion of malignant tumors in the lungs, better known as extrathoracic neoplasms.

The lungs are the second most frequent site to reach a metastasis.

The development of pulmonary metastases in patients with malignant tumors implies an adverse prognosis and alters the management plan.

Causes

Malignant diseases can reach the lung through 5 different routes:

  • Hematogenous through the pulmonary or bronchial artery.
  • Lymphatics
  • Pleural space
  • Airway.
  • Direct invasion

The tumor initially disseminates hematogenously to pulmonary arterioles and capillaries with retrograde dissemination from hilar ganglion metastases or upper abdominal tumors.

But later it spreads through the vascular walls, invades the low resistance peribroncovascular lymphatic vessels and spreads throughout the lymphatic vessels.

Endobronchial dissemination of tumor cells occurs with tumors of the respiratory tract. It is more common in bronchoalveolar carcinoma, less common in other types of lung cancer, and even less common in tracheobronchial papillomatosis.

Frequency

The venous return that contains the lymphatic fluid from the tissues of the body flows into the lungs through the pulmonary vascular system; therefore, all tumors have the potential to affect the lungs.

Cancers of breast, colorectal, lung, kidney, head, neck and uterus are the most common primary tumors with lung metastases at autopsy.

Choriocarcinoma, osteosarcoma, testicular tumors, malignant melanoma, Ewing’s sarcoma, and thyroid cancer often metastasize to the lung, but the frequency of these tumors is low.

Colorectal cancer, which accounts for 10% of all cancers, accounts for 15% of all cases of lung metastases.

Mortality

The presence of pulmonary metastases usually indicates advanced disseminated disease. Occasionally, tumor dissemination may be an isolated event.

Mortality depends on the primary tumor; for example, in pancreatic and bronchogenic carcinomas, the 5-year survival rate in patients with pulmonary metastases is less than 5%.

Early diagnosis is essential to plan effective therapy in patients who can be cured. Depending on several factors, the metastasis can be resected, with 5-year survival rates of up to 30-40%.

Clinical details

While a large number of patients with pulmonary metastases are asymptomatic at the time of diagnosis, some patients develop symptoms such as hemoptysis , cough, shortness of breath, chest pain, weakness and weight loss.

Particularly, patients with lymphangitic carcinomatosis present with respiratory dysfunction, including severe dyspnea.

Other problems to consider

The most common pattern of pulmonary metastasis is the presence of multiple, well-defined nodules.

Differential diagnoses for multiple pulmonary nodules include infections such as:

  • Histoplasmosis.
  • Coccidioidomycosis in endemic areas.
  • Cryptococcal, nocardial and abscess infections.
  • Vascular diseases

Differential diagnoses for other patterns are discussed in detail on radiographs and CT scans.

Treatment

In specific circumstances, histopathological samples of the lung lesion are required.

Biopsy or fine needle aspiration is usually done under CT guidance. Complete descriptions of the procedure and its complications are beyond the scope of the article.

The definitive treatment for pulmonary metastases of extrathoracic neoplasms is surgical resection ( pulmonary metastasectomy ).

Surgery is performed if the primary tumor is controlled, if there are no extrathoracic lesions, if it is technically resectable and if the general and functional risks are tolerable.

The 5-year overall survival rate for patients with pulmonary metastasectomy is 15-48%, compared with 13% for patients without the procedure.

The average survival is 12-18 months. It has been shown that survival is better in patients with a lower number of metastases.

However, pulmonary metastasectomy can only be performed in 25-50% of patients, due to the presence of multiple metastatic lesions or the presence of comorbid conditions, including poor respiratory function or refusal to undergo surgery.

Recurrence after pulmonary metastasectomy also limits other surgical options.

In patients who are not physically fit to undergo pulmonary metastasectomy, alternative options available include stereotactic radiosurgery and thermal ablation procedures.

Thermal ablation procedures induce necrosis of the coagulation of the tumor cells and are typically performed with CT guidance. These include radiofrequency ablation, microwave ablation, laser ablation and cryoablation.

The main goal of all these tumor ablation procedures is to eradicate all malignant cells together with a normal tissue margin, but to cause minimal damage to normal lung disease.

By doing this, adequate tumor control is achieved and survival is prolonged. The main advantage of thermal ablation procedures is the selective and limited damage of lung tissue to cause minimal impact on lung function.