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Findings: Chest radiograph (not shown) shows diffuse interstitial lung disease with the presence of extensive septal lines and peripheral air space opacities. The heart size is normal. Images from a high resolution CT scan (HRCT) shows extensive interlobular septal thickening, some of which is nodular in appearance. There is mediastinal lymphadenopathy and pleural effusions.
Diagnosis: Lymphangitic spread of tumor in a patient with lung cancer.
Discussion: The anatomy of the secondary pulmonary lobule must be understood in the interpretation of HRCT. The secondary pulmonary lobule is the basic building block of the microscopic anatomy of the lung. 1-2.5 cm in size and supplied by 3-5 respiratory bronchioles, the secondary lobule contains the important interstitial spaces in the lung. The lobule, which is pentagonal in shape is bounded peripherally by the pleura and subpleural interstitium and laterally by the interlobular septa. The interlobular septa contain connective tissue, lymphatics and pulmonary veins. Any disease process that affects these structures will cause thickening and nodularity of the interlobular septa. These processes include edema, pulmonary fibrosis, sarcoidosis, and alveolar proteinosis. However, the most important etiology is lymphangitic tumor, and should be considered in any patient with an underlying malignancy and the finding of interlobular septal thickening.
Lymphangitic spread of cancer has been described in a number of primary tumors, the most important being lung, breast, GI and prostatic neoplasms. A number of etiologies have been established to explain the pattern of lymphangitic spread. It is felt that hematogenous metastases deposit in the lungs and then travel centrally along the lymphatics and venous system. In lung cancer, there can be direct extension to the lymphatics, while some tumors do appear to spread from central lymph nodes peripherally into the lymphatic system.
HRCT is the modality of choice to diagnose lymphangitic spread of tumor. The CXR is normal in 50%, and conventional CT may also be insensitive to the diagnosis. In 50% of patients, the distribution of lymphangitic spread is often asymmetric, and can be unilateral. There is thickening of the interlobular septa, and involvement of the peribronchovascular interstitium is also often seen. Unlike patients with pulmonary fibrosis, the intervening lung parenchyma tends to be normal in appearance, without significant distortion or honeycombing. The pattern of interlobular thickening can be helpful in differentiating benign from malignant causes. In an autopsy study in patients with proven lymphangitic spread of tumor, 19/22 showed a "beaded" pattern of thickening. Pulmonary edema tends to demonstrate uniform thickening, though there is a great degree of overlap. Clearly, correlation with the patient's symptoms and medical history is important in determining the possible etiologies in any patient who presents with interlobular septal thickening.
References:
Webb,WR, Muller, NL and Naidich, DP High Resolution
CT of the Lung Lippincott-Raven, Philadelphia, 1996.
Munk PL, Muller NL, Miller RR, Ostrow DN. Pulmonary Lymphangitic
Carcinomatosis: CT and Pathologic Findings. Radiology 1988; 166:705-709.
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