The mass is a squamous cell carcinoma of the lung. These neoplasms tend to arise centrally. As a result, this one has obstructed the main bronchus and produced distal atelectasis, reducing the overall size of the lung. Squamous cell carcinoma of the lung can produce a bulkly mass, and this one shown here is quite large, extending into the periphery of the lung. Squamous cell carcinomas, along with small cell carcinomas of the lung, are strongly associated with cigarette smoking. Note the changes of centrilobular emphysema in upper lung fields in this image, also typical for smoking.