Thoracic Metastatic Disease

Introduction

Metastatic disease is the most common chest malignancy, and the chest acquires more metastases than any system. In autopsy series, pulmonary metastases are present in 20% to 54% of patients with a primary malignancy. The most common extrathoracic malignancies to metastasize to the chest include breast cancer, gastrointestinal (GI) malignancies (colon, pancreatic, and gastric cancer), melanoma, head and neck tumors, and renal cell cancer. Rarely, metastatic disease to the thorax can be the initial presentation of a malignancy. Tumor spread to the chest can occur via hematogenous, lymphatic, or endobronchial routes. Chest radiographs and computed tomography (CT) are the main modalities used to assess thoracic metastatic disease. Other modalities currently serve complementary roles and include magnetic resonance imaging (MRI), ultrasound (US), and positron emission tomography (PET)/CT. Technique and typical and atypical CT imaging manifestations of pulmonary, pleural, and cardiac metastases are addressed.

Technique

In the evaluation of patients with a primary tumor, imaging serves to stage patients and to detect or exclude metastatic disease. Achieving this goal with the appropriate technique is essential. Chest radiographs (posteroanterior and lateral views) generally serve as the initial evaluation tool in patients with possible metastatic disease. In patients whose radiographs demonstrate obvious metastatic disease, additional imaging modalities may not be necessary, and radiographic follow-up may be sufficient. However, chest radiographs lack sensitivity, and CT has been shown to be superior to radiography in the detection of pulmonary nodules. CT is now the state-of-the-art modality for both the detection and characterization of intrathoracic metastatic disease. The evolution of CT as the standard began with several improvements in CT technology: the ability to obtain thinner collimation and the development of spiral, and subsequently multidetector row CT (MDCT), scanners. Thinner collimation enabled the development of high-resolution CT to assess diffuse lung disease and improved anatomic assessment and characterization of small lesions. Current MDCT scanners acquire a continuous volume and thus decrease misregistration artifacts and improve nodule detection by eliminating interslice gaps. This aids assessment of diffuse lung diseases, increases sensitivity for small lung nodules, and improves multiplanar reconstructions. Other benefits of CT include its ability to quantify disease in the case of possible metastectomy, assess response to therapy, and provide a detailed roadmap to guide biopsy.

Our present CT chest protocol for assessing patients with possible metastatic disease includes MDCT scan parameters that allow image review in both standard and lung algorithms at 2.5- and 1.25-mm image thickness. Images are also reconstructed in both coronal and sagittal planes and reviewed at workstations with cine mode capabilities.

Intravenous contrast (100 mL) is used in patients in whom there is no contraindication, at a rate of 3 mL/sec after a 30-second delay. The rationale for intravenous contrast is multiple:

• 1.

  Contrast enhancement permits better delineation of the mediastinum and hilar regions and is also useful for detecting subtle pleural metastatic disease.

• 2.

  Patients with primary malignancy are at increased risk for developing pulmonary emboli. In retrospective reviews of CT scans of oncologic patients, between 3.3% and 4.0% of patients had incidentally discovered pulmonary emboli, with higher risks associated with inpatients, advanced disease, gynecologic malignancy, and melanoma.

• 3.

  Patients undergoing chemotherapy often have indwelling catheters that may lead to thrombotic occlusion of vessels and collateral vessel formation.

Although sensitive, CT is not specific in the assessment of metastatic disease. False-positive nodules are often caused by intraparenchymal lymph nodes and noncalcified granulomas. Image processing can improve image interpretation. Maximum intensity projection images increase sensitivity to small lung nodules by demonstrating vascular structures as tubular branching structures.

Other modalities such as US, MRI, and PET serve complementary roles in evaluating thoracic metastatic disease. Electrocardiogram (ECG)-gated cardiac MRI (CMRI) is useful in evaluating the heart and surrounding structures for findings on either CT or echocardiography that may suggest metastatic disease. CMRI is a cardiac-gated study that eliminates cardiac motion and has excellent temporal resolution and excellent soft tissue contrast. ECG-gated CT is an alternative to CMRI in the evaluation of cardiac tumors in patients who are unable to undergo CMRI, for example, those patients who have an implanted ferromagnetic device or who experience discomfort lying on the MRI table for prolonged periods of time.

Patterns of Metastatic Disease

Simple Nodules

Most metastases to the chest are spread hematogenously. The characteristic appearance of these metastatic lesions includes multiple bilateral spherical or ovoid sharply marginated nodules of variable sizes predominantly affecting the periphery of the lungs and the lower lobes (where the majority of the blood flow is directed) ( Fig. 31.1A ). The typical sharply marginated edges of metastatic lesions help differentiate these tumors from primary lung cancers, as well as their characteristic ill-defined, spiculated margins that extend into the adjacent lung parenchyma. Growth of metastatic lesions is extremely variable, and volume doubling times (an increase in the diameter of a nodule by 26%) have been reported to range from 1 to 2 weeks for rapidly growing lesions such as sarcomas, melanomas, and germ cell tumors to months for thyroid malignancy. Hematogenous metastases can also simulate a miliary pattern, typical of medullary thyroid malignancy. Occasionally, imaging metastatic nodules with contrast-infused CT demonstrates areas of enhancement with dilated, tortuous, tubular enhancing vessels. This feature can be seen in sarcomas, particularly alveolar soft part sarcoma or leiomyosarcoma ( Fig. 31.1B ).

Benign tumors rarely metastasize, and benign tumors that can metastasize to lung include leiomyoma of the uterus, hydatidiform mole, giant cell tumor of bone, chondroblastoma, pleomorphic adenoma of the salivary gland, and meningioma. Slow growth of these solid benign nodules is typical, but the appearance is indistinguishable from other malignant metastatic nodules.

Metastatic nodules that remain stable on serial exams may represent sterilized metastatic disease. These residual nodules are indistinguishable from viable tumor on CT scans. Biopsied materials show areas of necrosis and/or areas of fibrosis. Testicular cancer, breast cancer, and choriocarcinoma are common tumors that present in this manner. In some cases, following serum markers such as beta–human chorionic gonadotropin or alpha-fetoprotein or, alternatively, PET scanning can be helpful in following and assessing viability. In addition, growth of metastatic nonseminomatous germ cell lesions with negative serum markers frequently represents a conversion to a benign mature teratoma.

Cavitary Nodules

Cavitation in metastases is less frequent than in primary lung cancers. Metastatic squamous cell lesions, from any source, were thought to have the highest rate of cavitation, at approximately 10%. However, a similar rate of cavitation was demonstrated in a series of metastatic adenocarcinoma. Typical cavitary metastases have thick, irregular walls ( Fig. 31.2A ); less frequently, they can have thin walls, and this feature is particularly noted in patients with metastatic adenocarcinoma or sarcoma. Chemotherapy can induce cavitation, and this is more common in sarcomatous metastases than in other cancers ( Fig. 31.2B and C ).