Diagnosis of Extrathoracic Metastases to the Lung
Reviewers: Leonard A. Farber, MD and John Chang, MD
Source: Oncology, March 1998, Volume 12, Number 3, 431-438
At present, the standard diagnostic imaging modalitiesfor detection of metastases to the lungs include the chest radiographand thoracic computed tomography (CT). Rates of detecting thoracicmetastases have great variability at autopsy. Early detectionof pulmonary metastases during a patient's clinical course mayultimately affect tumor staging and treatment planning. However,it is also important to avoid subjecting patients to unnecessaryprocedures on the basis of low likelihood of positive yield. For these reasons, it is reasonable to assess the utility of othermodalities that may be valuable in detecting metastases. Theseinclude positron emission tomography (PET) and magnetic resonanceimaging (MRI).
There are a variety of factors that may help determinewhich tests will be most useful in demonstrating pulmonary metastasesfrom extrathoracic primary tumors: mechanisms of hematogenoustumor spread, the likelihood of distant metastasis vs. spreadto nearby lymph nodes, and the probability of distant metastasiswith the presence of local invasion. Depending on the tumor typeand the extent of local disease, different radiologic modalitiesmay be necessary for optimal screening for detection of metastaticdisease.
Cancers That Metastasize to the Lung
Primary tumors that have a high propensity for pulmonary metastasesinclude melanomas, choriocarcinomas, sarcomas, and carcinomasof the head and neck, testis, and adrenal gland, as well as certaintypes of thyroid cancer. Primary tumors that are not as likelyto metastasize to the lung but are found in high prevalence inthe population include adenocarcinoma of the breast, colorectum,stomach, and pancreas.
Patterns of Metastases
Depending on the primary tumor type and location, the patternsof hematogenous spread follow a multi-step cascade process, thathelp explain the likelihood of metastasis to each organ system. The authors summarize this in a table, as shown below:
Radiographic Technologies and Their Utility in Screening.
Standard Chest Radiograph: The standard PA and lateralchest radiograph is the most fundamental screening tool for pulmonarymetastatic disease. It may be used for staging and follow-up.
Thoracic CT: Chest CT scans, especially spiral techniques,are highly sensitive for the detection pulmonary nodules, butwith less specificity. For malignancies that do not usually metastasizeto the lung, the yield of thoracic CT for metastatic disease withoutmetastatic disease elsewhere, is low. However, the chest CT ismore likely to demonstrate very small lesions (< 1 cm) thatare not detected by standard chest radiography.
Positron Emission Tomography: Unlike most radiographicimaging modalities which rely on morphologic features for thedetection of disease, PET is a functional imaging technique thatrelies on the metabolic characteristics of tissue for diagnosis. Furthermore, it has a relatively high spatial resolution in comparisonto other functional nuclear medicine techniques, making it especiallyuseful for assessing small, indeterminate pulmonary lesions. It may also be useful for selecting patients for resection, whenwhole-body PET is utilized.
Magnetic Resonance Imaging: MRI has high-contrastresolution and sensitivity to flowing blood. It is especiallyuseful in assessing the extent of tumor invasion of the mediastinumand great vessels, for studying vascular lesions within the chest,and in evaluation fibrosis or atelectasis. It is, however, oflimited value in imaging lung parenchyma, and is its resolutionis further compromised by respiratory motion. An advantage ofMRI over CT is multiplanar imaging capabilities, which may beespecially helpful in the assessment of vascular structures, thechest wall, or the brachial plexus.
The choice of imaging modalities in the staging, detection, andsurveillance of cancer patients should be governed by the naturalbehavior of the primary tumor and how the results would impacton management of the disease. Such an approach should be individuallymade, relying on tumor behavior, treatment options, and utilityof the diagnostic modality. Finally, cost-effectiveness considerationswhen the techniques are utilized in the appropriate settings shouldhelp to yield a balanced treatment approach for the patient.