Hilum Overlay Sign in Anterior Mediastinal Tumor

Fever of 1 month's duration was this 28-year-old man's only complaint. He had smoked marijuana in the past but denied intravenous drug use. His temperature was 39.4°C (103°F), but no other abnormalities were noted on physical examination.

A lateral chest film showed an anterior mediastinal tumor (A, arrow). A posteroanterior film demonstrated the hilum overlay sign (B, arrow), in which the configuration of the anterior mediastinal tumor simulates an enlarged heart; the lungs appeared normal. Contrast-enhanced CT of the chest revealed mediastinal lymphadenopathy (C, arrow) with foci of necrosis.

Results of all laboratory investigations were negative. These included an HIV antibody test, a tuberculin skin test with 5 tuberculin units of purified protein derivative, and an examination of induced sputum smears and cultures for acid-fast bacilli. A CT-guided needle biopsy revealed a poorly differentiated large cell carcinoma.

Despite extensive diagnostic evaluation, the source of a primary tumor could not be identified. The patient responded well to combined radiation and chemotherapy, and his condition was stabilized within 6 months.

Surprisingly, an anterior mediastinal mass may closely resemble a dilated heart or pericardial effusion. The hilum overlay sign may be helpful in making this distinction. It is best shown on the anteroposterior Bucky film, in which the tumor is magnified more than the heart behind it, thus accentuating the discrepant position of the left pulmonary artery.