CT Characteristics of the Thymus in Adult Dogs with Non-Thymic Neoplasia Compared to Young Dogs

SIMPLE SUMMARY: The thymus is a lymphatic organ located in the cranial mediastinum. Both in humans and dogs, the thymus is largely changing with age, becoming smaller with time and also undergoing replacement of the active tissue with fat. Computed tomography is the imaging modality of election for the visualization and characterization of the thymus in human patients, and the characteristics of this organ with regard to the age of the patients are well described. On the contrary, in dogs, detailed description of the computed tomographic appearance of the thymus in adult and young patients is so far lacking. The results of this study show the different appearance of the thymus in two populations: adult dogs with diagnosed neoplasia and young dogs. ABSTRACT: The thymus is a lymphatic mediastinal organ that is largely subject to changes with age. In human patients, the CT characteristics of the thymus in children and adults is well described. Furthermore, it is known in human medicine that stress can lead to a reduction in the size of the thymus, followed by a phase of hyperplasia (called the ‘rebound effect’). The visualization of thymic tissue in the cranial mediastinum of adult dogs with neoplasia is possible and could be related to a similar effect. In this study, we aimed to describe the CT characteristics of the thymus in adult dogs with neoplasia and to compare the aspect of the thymus in these dogs to juvenile dogs with a presumed normal thymus. A total of 11 adult dogs with neoplasia and 20 juvenile dogs were included. Several CT features of the thymus were evaluated, including the size, shape, and pre- and post-contrast attenuation values. The overall appearance was lobulated in all of the adult dogs and homogeneous in all of the juvenile dogs; it was left-sided in all of the adult dogs, while it was located in the midline in a few of the juvenile dogs (right-sided only in one). The thymus was less attenuating in adult dogs, in some cases with negative minimum pre-contrast attenuation values. In some dogs with neoplasia, the thymus can be detected at CT examination despite their age.