Two cases (a cat and a dog) illustrating computed tomographic features distinguishing an accessory lung lobe mass from a caudal mediastinal origin

A seven‐year‐old female Chinchilla cat and a 13‐year‐old male Cocker Spaniel were presented to Dick White Referrals for respiratory difficulties and cough, respectively. Based on radiographic and CT findings, including the midline location of the lesion and its mass effect on mediastinal structures, a caudodorsal mediastinal mass was considered more likely than a pulmonary origin in both cases. However, further investigations in the form of thoracic ultrasound and ultrasound‐guided fine needle aspirates in the first case and histopathology in the second, revealed the presence of an accessory lobe pulmonary carcinoma in both cases. Retrospective review of these cases allowed identification of CT features that can be used to distinguish mediastinal and accessory lung lobe masses in future cases, including absence of part or all of the accessory lobe and its lobar bronchus, presence of gas in the lesion and the shape of the lesion mimicking a lung lobe.