Radiologic Diagnosis of Extrathoracic Metastases to the Lung

The article by Woodard et al reviews the imaging modalities available for evaluating metastatic disease in the lungs. It is important to understand the disease processes that metastasize to the lungs in order to use the correct imaging technique, and the authors have provided a concise review of these processes. They have also included a description of the situations in which the utility of certain imaging modalities is unclear, such as the usefulness of computed tomographic (CT) imaging in differentiating metastasis from a second primary in head and neck carcinoma patients. The authors' Table 1 includes a breakdown of the patterns of hematogenous spread of various primary tumors. The table can serve as a guide for selecting those patients who would benefit most from further evaluation of the lungs for metastatic disease.

As the authors state, the chest x-ray and CT examination are the main modalities utilized for metastatic evaluation, and this trend will continue until better methods are developed. Positron emission tomography (PET) and magnetic resonance imaging (MRI) have improved our ability to characterize some chest abnormalities, but not well enough to supplant CT. Magnetic resonance imaging has helped in evaluating the chest wall, heart, and diaphragm, but remains less than optimal for the lungs. Positron emission tomographic imaging, although useful, is not readily available. In addition, reimbursement for PET imaging is not universally accepted.

Assigning a Diagnosis to Indeterminate Lesions

I agree with the authors that the detection of an indeterminate lesion in a patient with a known malignancy presents a dilemma. The options are biopsy or follow-up imaging. As Dr. Woodard and colleagues state, this decision is determined by such factors as patient age, type of primary malignancy, patient desires, and evidence of other metastasis. However, I do not agree totally with the rationale that small lesions detected by CT are most likely to be benign. The reference for this rationale is an article written by Chalmers and Best,[1] which stated that 80% of small lesions detected on CT examinations but not seen on chest x-rays were benign in patients with a known malignancy.[1] However, this study was limited by small sample size (16 patients) and by the diagnosis of benign disease via follow-up x-ray over a range of 4 to 29 months. Only 2 of the 16 patients were followed for 2 or more years, and because of this short follow-up interval, benign disease cannot be confirmed.

We recently reported that 58% of lesions that were indeterminate by CT and were 1 cm or smaller represented a malignancy.[2] These lesions were removed by thoroscopic surgery, and malignancy was histologically confirmed. Other authors have reported a similarly higher incidence of malignancy in indeterminate lesions detected by CT when compared to indeterminate lesions detected by chest x-rays.[3-5] The tendency for indeterminate CT lesions to more likely be malignant is thought to be due to the elimination of benign lesions by CT criteria.

Imaging Other Areas of the Chest

Other areas of the chest not mentioned in the article by Woodard et al, including the pleura, mediastinum, and pulmonary airways, are also well imaged by CT. Extrathoracic tumors commonly metastasizing to the mediastinum include breast, testicular, and renal cell tumors, as well as melanomas, sarcomas, and lymphomas. Metastasis to the pulmonary bronchi is seen in melanoma, renal cell, breast, and colon carcinomas, and Kaposi's sarcoma. Pleural metastasis from breast carcinoma, melanoma, and sarcomas is not uncommon. These extrathoracic malignancies, however, also tend to metastasize to the lungs, making CT examination even more valuable in these patients.

Cost Considerations

Decisions regarding which radiologic screening approach is appropriate are also very important because of the expense to the patient for these procedures. CT, MRI, and PET are costly imaging examinations, and it is inappropriate to incorrectly utilize these resources when more appropriate follow-up is indicated. Conversely, the appropriate use of radiographic studies allows clinicians to coordinate the most beneficial therapy, and in these days of managed care and cost containment, correct utilization of available resources is even more important. Therefore, using radiologic imaging “correctly” is critical to practicing “good medicine,” and the authors have provided a guideline for accomplishing these goals.

References:

1. Chalmers N, Best JK: The significance of pulmonary nodules by CT but not by chest radiography in tumour staging. Clin Radiol 44:410-412, 1991.

2. Munden RF, Pugatch RD, Liptay MJ, et al: Small pulmonary nodules detected at CT: Clinical importance. Radiology 202:105-110, 1997.

3. Zerhouni EA, Stitik FP, Siegelman SS, et al: Computed tomography if the pulmonary nodule: A national cooperative study. Radiology 160:319-327, 1986.

4. Siegelman SS, Zerhouni EA, Leo FP, et al: CT of the solitary pulmonary nodule in Donner MW, Heuck F, (eds): Radiology Today: Multinational Series. pp 113-120. New York, Springer Verlag, 1981.

5. Proto AV, Thomas SP: Pulmonary nodules studied by computed tomography. Radiology 156:149-153, 1985.