A 71-year-old woman presented to the Emergency Department with a 2-week history of productive cough, fevers and general deterioration. Her past medical history was non-contributory other than hypertension, dyslipidemia and remote cholecystectomy. General examination demonstrated an oxygen saturation of 92% on room air and decreased air entry over the left lung field. A chest radiograph was subsequently performed.

The frontal radiograph demonstrates the presence of a “veiling” opacity extending over the left hemithorax (arrow Fig. 1a) with the presence of the luftsichel sign, which is German for “air crescent” (arrowheads Fig. 1a). [1, 2] These radiographic findings are specific for the presence of left upper lobe collapse. Anatomically, the air crescent represents a hyper-expanded superior segment of the left lower lobe which lies between the aortic knuckle and medial border of the collapsed lobe [2]. Unlike pneumonia, which has a more patchy distribution, there is no significant silhouetting of the left cardiomediastinal contour.

Fig. 1
figure 1

a A frontal posteroanterior (PA) radiograph demonstrating the veiling opacity over the left hemithorax (arrow) and the luftsichel sign (arrowheads). b A coronal contrast-enhanced computed tomography of the thorax image demonstrating a large left upper lobe mass with associated lobar collapse (arrow)

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Left upper lobe collapse presents with distinctive radiological features, not to be misinterpreted with other commonly encountered chest radiograph abnormalities. Prompt identification of lobar collapse is important, as it should elicit further work-up with cross-sectional imaging to exclude an underlying endobronchial lesion. Computed Tomography of the thorax revealed a large left upper lobe mass with complete occlusion of the upper lobe bronchus (arrow Fig. 1b). Further evaluation with bronchoscopy and endobronchial biopsy yielded squamous cell carcinoma. Although not all cases of lobar collapse are attributable to malignancy, it remains important to exclude it as a potential etiology. The patient was referred to pulmonology for work-up and treatment.