From the Guidelines
Thymus hyperplasia is diagnosed through a combination of imaging studies, including chest CT scan or MRI, which can reveal an enlarged thymus gland that maintains its normal shape and tissue characteristics, and clinical evaluation, with consideration of the patient's age and associated conditions such as myasthenia gravis, as noted in the study by 1. The diagnostic criteria for thymus hyperplasia involve:
- Localization to the anterior mediastinal compartment
- Texture, including cystic versus solid components
- Evaluation of edge contours, such as invasion or not, as outlined in the study by 1
- A soft tissue mass conforming to the shape of the thymic gland, especially in young patients, is typically thymic hyperplasia, as mentioned in the study by 1 Some key points to consider in the diagnosis of thymus hyperplasia include:
- The use of chemical shift (opposed-phase) MR imaging to differentiate thymic hyperplasia from thymoma, as thymic hyperplasia demonstrates loss of signal on out-of-phase imaging due to suppression of microscopic fat, as noted in the study by 1
- The consideration of the patient's age, as the thymus normally involutes with age, making hyperplasia more significant in adults than in children, where it may represent normal development, as discussed in the study by 1
- The importance of clinical evaluation, including blood tests to check for associated conditions, particularly myasthenia gravis, by testing for acetylcholine receptor antibodies, as recommended in the study by 1 In cases where imaging cannot definitively distinguish between hyperplasia and thymoma, a tissue biopsy may be necessary, either through CT-guided needle biopsy or surgical approaches like mediastinoscopy or video-assisted thoracoscopic surgery (VATS), as suggested in the study by 1.
From the Research
Diagnostic Criteria for Thymus Hyperplasia
The diagnostic criteria for thymus hyperplasia can be summarized as follows:
- True thymic hyperplasia is characterized by an increase in the size of the thymus due to an increase in the number of thymic cells, without the presence of lymphoid follicles 2.
- Follicular hyperplasia is characterized by the presence of lymphoid follicles in the thymus, which can be associated with autoimmune disorders 3, 4.
- The diagnosis of thymic hyperplasia is based on microscopic features, including the presence of well-developed corticomedullary architecture, scattered Hassall corpuscles, and a thin fibrous capsule 2, 5.
- Immunohistochemical studies can help distinguish thymic hyperplasia from other anterior mediastinal masses, such as thymoma, by showing a normal pattern of distribution for keratin-positive thymic epithelial cells and CD3/TdT/CD1a+ lymphocytes 2.
Key Features of Thymic Hyperplasia
Some key features of thymic hyperplasia include:
- Enlargement of the thymus, which can cause symptoms such as chest discomfort or shortness of breath 2, 5.
- Presence of a mass lesion in the mediastinum, which can be suspicious for malignancy on imaging studies 2.
- Absence of cytologic atypia, confluence of lobules, or evidence of lymphoid follicular hyperplasia in true thymic hyperplasia 2.
- Normal clinical outcome after surgical excision, with all patients being alive and well after a mean follow-up of 9 years 2.
Differential Diagnosis
Thymic hyperplasia should be considered in the differential diagnosis of anterior mediastinal masses, particularly in cases where the diagnosis is uncertain or where there are symptoms or worrisome imaging findings 2, 5. The criteria for distinguishing thymic hyperplasia from lymphocyte-rich thymoma are important to consider in these cases 2.