Normal thymus evaluation using radiographs and computed tomographic images in young and adult dogs

The thymus generally presents as a sail-shaped soft-tissue opacity on thoracic radiographs in young, prepubescent dogs. After puberty, lymphatic tissue is replaced by adipose tissue and can usually no longer be identified on radiographs; however, thymic remnants have often been observed in adult dogs. The objective of this retrospective descriptive study was to investigate the incidence of thymic remnants using thoracic radiographs and to analyze the incidence in dogs by age, sex and breed. We analyzed the data from 1,464 dogs that underwent both thoracic ventrodorsal and right lateral radiography, of which 40 underwent both radiography and computed tomography (CT). The frequency of thymus identification on thoracic radiographs decreased with age in 184 dogs aged < 1 year (P = 0.024). Of 1,280 dogs aged > 1 year, thymic remnants were observed in 81 dogs (6.33%), with a greater frequency in intact female dogs than intact male dogs (P = 0.015). Thymic remnants were observed more frequently in castrated male dogs than in intact male dogs, with borderline significance (P = 0.051). The frequency of thymic remnants was not related to age, body size or breed. Thymic remnants were identified in 20 out of 40 dogs that underwent CT examination, of which two showed thymic remnants on both thoracic radiographs and CT images, and 18 showed thymic remnants only on CT images. In conclusion, although the thymus involutes with aging, thymic remnants may be observed in adult dogs, at an incidence of approximately 6.3%. The frequency of thymic remnants on thoracic radiographs was higher in intact females than in intact males but was not related to age, body size or breed. Additionally, CT could be more useful than radiography for detecting thymic remnants.