Small-cell carcinoma of the lung resembling a brachial plexus tumour.

A small-cell carcinoma of the lung was identified in a six-year-old female German shepherd dog with a history of chronic lameness of the left forelimb, Horner's syndrome and sensory deficits on the caudal portion of the left forelimb below the elbow. A mass, the exact location of which was difficult to ascertain, was identified during radiographic examination of the thorax. It was easily identified, using magnetic resonance imaging, as an apical tumour of the left lung with dorsal extension and involvement of paraspinal structures, such as spinal nerve roots C8 to T1 and the sympathetic trunk. Postmortem examination confirmed a mass in the left apical lobe of the lung, compatible with a diagnosis of small-cell carcinoma by histopathology and immunohistochemistry. This clinical presentation is similar to Pancoast syndrome described in humans.