Thymushyperplasie

A diagnostic

approach to the mediastinal masses. a–c Thymic hyperplasia in a 43-year-old woman (arrows). a Non-contrast-enhanced CT scan reveals a lobulated lesion with smooth margins in anterior mediastinum. b Lesion appears slightly hyperintense on in-phase gradient-echo T1-weighted MR image. c Opposed-phase gradient-echo T1-weighted MR image shows decreased signal intensity within the lesion, confirming presence of fat. d–f Stage II thymoma (WHO type B2) in a 62-year-old woman (arrows). d Non-contrast-enhanced CT scan shows an anterior mediastinal soft tissue mass. e The lesion shows an intermediate signal intensity on in-phase gradient-echo T1-weighted MR image. f There is no substantial decrease in signal intensity relative to in-phase MR image on opposed-phase sequence

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Thymic

hyperplasia • Thymic rebound hyperplasia - Ganzer Fall bei Radiopaedia

Rogalskyi, V. Thymic rebound hyperplasia. Case study, Radiopaedia.org. (accessed on 21 Oct 2022) https://doi.org/10.53347/rID-66896 • CC by-nc-sa 3.0 (modified)

Role of

different imaging modalities in the evaluation of normal and diseased thymus. Axial MRI chest in a 28-year-old male revealed anterior mediastinal mass that is iso-intense to muscle on in-phase images (a) and b demonstrating a decrease in signal intensity on opposed-phase images indicating microscopic fat, CSR = 0.4. These findings are compatible with thymic hyperplasia. MRI diagnosis and histopathological confirmation revealed “thymic hyperplasia”

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Thymic hyperplasia is a disorder whereby there is hyperplasia of the thymus gland.

Pathology

Thymus hyperplasia can be subdivided into two forms:

Both true thymic hyperplasia and lymphoid hyperplasia manifest as diffuse symmetric enlargement of the thymus so that it is difficult to distinguish between the two types on the basis of imaging findings alone.

It is important that radiologists be able to distinguish thymic hyperplasia from neoplasm, which tends to present as a focal mass.

True thymic hyperplasia

True thymic hyperplasia associations include:

rebound hyperplasia to chemotherapy/steroids (see: thymic rebound hyperplasia)
radiation therapy
burns
other severe systemic stresses

Lymphoid hyperplasia

This is also known as lymphoid follicular thymic hyperplasia or autoimmune thymitis. Lymphoid hyperplasia associations include

Radiographic features

Diffuse symmetric enlargement of the gland is the key morphologic feature of hyperplasia (neoplasm tends to manifest as a focal mass).

MRI

Chemical shift artifact may be useful in differentiating from other tumors :

chemical shift MRI helps differentiate thymic hyperplasia and thymus gland tumors in patients 16 years of age or older
chemical shift MRI depicts no decrease in signal intensity of thymic tumors, unlike the decreased signal intensity of thymic hyperplasia
- this can be formally calculated using the chemical shift ratio (CSR)

Differentiating normal from hyperplastic thymus can be difficult and guidelines for making this distinction and verifying the presence of normal thymus include :

presence of rounded soft-tissue masses 7 mm
presence of a convex contour of the thymus beyond 19 years of age
presence of soft-tissue lobulation
presence of increased thymic thickness (should be ≤1.3 cm beyond age 20 years)
presence of a diagnosis associated with thymic enlargement or hyperplasia, e.g. Graves disease

Nuclear medicine

FDG PET is often performed in patients with malignancy; however, differentiation between thymic hyperplasia and thymic involvement by malignancy is difficult because the thymus demonstrates normal physiologic uptake.