IMAGING IN THYMIC LESIONS radiology presentation

IMAGING IN THYMIC LESIONS

1. ROLE OF IMAGING IN THYMIC LESIONS PRESENTATION BY :DR MIRZA SANAULLA MODERATOR:DR MALLIKARJUNAPPA B

2. THYMUS The thymus is a lymphoid organ in the anterior mediastinum for the production and maturation of T-cells until puberty. It is a vital component of the immune system and plays a role in prevention of cancer 12, infection and autoimmunity

3. THYMUS Structure : Composed of two lobes: right and left. Fused superiorly near the thyroid gland. Molded to the anterior aspect of great vessels and heart. Occupies the thyropericardic space. Ectopic Locations : Rarely found outside normal position, primarily in the neck. Lobe Size : Left lobe usually larger than the right. Weight Changes : Average weight at birth: 25 g . Maximum weight at puberty: 35-50 g . Involution : Begins after puberty. Continues for 5-15 years . Thymic follicles atrophy and are replaced by fat. Fat-to-thymic tissue ratio increases until age 60 . Age 60 and Beyond : Minimal thymic tissue remains.

4. RADIOGRAPH In adults, the thymus is usually invisible on chest radiographs. In children, the thymus may be quite prominent, mimicking a mass.

5. THYMIC SAIL SIGN The thymus may project to one or both sides of the mediastinum, showing a sharply marginated undersurface, having the appearance of a sail

6. THYMIC WAVE SIGN The edgeof the thymus may have a wavy appearance because of indentation by the anterior ribs or costal cartilage

7. THYMIC NOTCH SIGN Interruption of the cardiac silhouette forms a notch, which may be seen on either side, but more frequently is seen on the left side.

8. CT Anatomy in Children : In children, the thymus occupies the prevascular space, wrapping around the great vessels and the heart. It extends upward beyond the innominate vein and typically reaches the level of the pulmonary arteries in infancy. Shape and Size : In infants and young children, the thymus has a quadrilateral shape on CT, which transitions to a triangular or bilobed appearance as they grow. Each lobe is generally 1 to 2 cm thick and maintains smooth, sharp margins. Imaging Characteristics : On unenhanced CT, the thymus has an attenuation similar to that of muscle (approximately 36 HU). After contrast injection, it shows a homogeneous enhancement of 20 to 30 HU. Changes with Age : From puberty to around 25 years, the thymus gradually transforms from a more prominent structure to one that is often triangular or bilobed. The left lobe is usually larger and is positioned lateral to the aortic arch, while the right lobe may be less visible. Involution : After 25, the thymus undergoes progressive fatty involution, becoming less recognizable on CT. By about 40 years of age, it may still appear as islands of soft tissue within a fatty matrix, and the anterior mediastinum is largely filled with fat. CT Density : The thymic remnant after complete involution may have a CT density slightly higher than subcutaneous fat.

9. Thymic Measurement Measurement Dimensions : Length : Measured in the cephalocaudal dimension. Width : Measured in the transverse dimension. Thickness : Measured perpendicular to the pleura. Average Thickness : In children aged 5 years or less, the average thickness is 1.4 cm . Maximum Normal Thickness : Under 20 years : Maximum thickness is 1.8 cm . Older adults : Maximum thickness is 1.3 cm .

10. MRI Appearance : Normal thymus appears homogeneous with intermediate signal intensity on T1-weighted images. Signal intensity is less than surrounding mediastinal fat but greater than muscle. Age Dependence : Thymic appearance varies with age due to progressive involution. In patients over 30, differentiating the thymus from adjacent mediastinal fat may become challenging. T2 Relaxation Times : T2 relaxation times of the thymus are similar to fat at all ages.

11. THYMIC LYMPHOID FOLLICULAR HYPERPLASIA LFH is characterized by hyperplastic lymphoid germinal centers in the thymic medulla. Associated with a lymphocytic and plasma cell infiltrate . Associated Conditions : Commonly linked to: Myasthenia gravis Connective tissue disease Pure red blood cell hypoplasia HIV infection May also occur in some normal young subjects . Imaging Characteristics : Plain Radiographs : Usually normal. CT Findings : Normal-appearing thymus : 45% Enlarged thymus with normal shape : 35% Focal thymic mass : 20%

12. THYMIC HYPERPLASIA Thymic hyperplasia is characterized by an increase in size of the thymus with normal gross and histologic appearance . Associated Conditions : Commonly associated with: Hyperthyroidism (e.g., Graves’ disease) Sarcoidosis Red blood cell aplasia Recovery from chemotherapy , stress , or burns (known as thymic rebound ). Imaging Characteristics : Chest Radiographs : Usually normal in adults. Thymic enlargement may be seen in children. CT Findings : Shows increased thickness of the thymic lobes while maintaining a normal appearance.

13. Thymic Rebound Thymic Involution : Occurs during periods of stress (e.g., illness, burns, chemotherapy). Results in significant size reduction, particularly in children and young adults. Decrease in thymic volume can exceed 40% . Recovery and Thymic Rebound : Thymus typically regrows to its original size within several months after stress. May exhibit “rebound,” growing to a size larger than the original. Increased thymic size is often visible on plain radiographs, especially in children. More frequently detected via CT imaging . Differentiation in Imaging : In patients with extrathoracic malignancies, thymic rebound is not typically a diagnostic issue. In lymphoma patients, distinguishing between thymic rebound and recurrent mediastinal tumors can be challenging. Lymph node enlargement alongside thymic enlargement suggests recurrent tumor. Isolated thymic enlargement indicates thymic rebound. MRI Findings : Enlargement of the thymus noted, with signal intensity comparable to normal thymus. Incidence : Thymic rebound occurs in 10% to 25% of children and young adults post-chemotherapy. Enlargement can be observed shortly after stress or from 1 to 9 months afterward.

14. Thymoma

15. RADIOGRAPH Location and Distribution Thymomas typically arise near the junction of the heart and great vessels. Can be seen superior to the clavicles and inferior to the costophrenic angles. Frontal radiograph appearance resembles an upside-down horseshoe embracing the heart. Visibility Thymomas may be subtle or invisible on chest radiographs. When visible, they appear sharply marginated and smooth or lobulated. Size and Contour Typically range from 5 to 10 cm in diameter. May obscure right or left heart borders based on size and location.

16. Calcification Dense calcification may be present throughout the mass or at its periphery. Lateral Radiograph Findings Can show distinct opacity in the inferior aspect of the retrosternal clear space. The space is lucent and located posterior to the sternum, anterior to the aortic arch and heart. Normal Findings Lack of lucency or poor definition of the anterior margin of the ascending aorta or pulmonary artery may be normal. Mass Characteristics A mass may overlie the heart and cardiophrenic angle; differentiation from fat pads or normal cardiac anatomy is essential. Invasion Indicators Invasive thymomas may show pleural involvement with signs like pleural thickening, nodularity , or effusion.

17. CT Location Thymomas typically occur in the prevascular space. Displaces great vessels (aorta, superior vena cava, main pulmonary artery) posteriorly. Differentiation from Thymic Hyperplasia Asymmetric enlargement of the thymus. Lobular contour or visible focal rounded lesion. Follicular thymic hyperplasia can mimic a mass up to 5 cm. Common Characteristics Approximately 80% of thymomas found at the base of the heart. Masses are sharply demarcated: oval, round, or lobulated shapes. Asymmetrical growth toward one side of the mediastinum . Ectopic Thymic Tissue Found in up to 20% of subjects; may occur in the neck or thoracic inlet. Can mimic thyroid masses .

18. Imaging Appearance Homogeneous attenuation, with or without contrast. Large thymomas may appear cystic or necrotic. Calcification may be present in the capsule or within the tumor . Invasion Assessment Difficulty distinguishing between invasive and non-invasive thymomas . Defined fat planes suggest no local invasion; obliteration may indicate invasion. Invasion indicators: Pericardial/thoracic thickening. Pleural nodules or effusion. Fat infiltration. Irregular tumor -lung interface. Invasive thymomas may extend to the posterior mediastinum and retroperitoneum . Associated Conditions Myasthenia gravis often linked with thymic pathology: 65% of patients have thymic hyperplasia. 10-30% present with thymoma . Normal-appearing thymus on CT may indicate normal histology or focal hyperplasia.

19. MRI The role of MR in diagnosing thymic masses is limited. On MR, thymomas typically have a low signal intensity on T1-weighted images, which increases with T2 weighting they may appear homogeneous in intensity or inhomogeneous with or without cystic components or may show nodules or lobules of tumor separated by relatively low-intensity septations . MR has proven valuable in identifying the presence or absence of vascular invasion in patients with thymoma , especially in patients to whom intravenous contrast cannot be administered

20. FDG PET FDG PET At present, the role of FDG PET in the evaluation of thymic neoplasms is unclear. Thymic hyperplasia may appear similar to thymoma in activity, and high-grade and low-grade thymomas are not easy to distinguish. However, PET may be valuable in detecting metastases in some patients with aggressive tumors .

21. THYMIC CARCINOMA

22. Arises from thymic epithelial cells; less common than thymoma . Accounts for about 20% of thymic epithelial tumors. Diagnosis Can be diagnosed as malignant based on histologic criteria. Includes various cell types: squamous cell, basaloid , adenocarcinoma , mucoepidermoid carcinoma. Classified as type C in the WHO classification of thymic epithelial tumors. Aggressiveness and Metastasis More aggressive than invasive thymoma ; higher likelihood of distant metastases. Distant metastases at diagnosis in 50-65% of patients; only 5% in invasive thymoma . Common metastasis sites: lungs, liver, brain, and bone. Prognosis Poor prognosis with a 5-year survival rate of 30%. Average age at presentation is 50 years.

23. Symptoms Symptoms related to mediastinal mass; common invasion of mediastinal structures. Superior vena cava syndrome may be present. Paraneoplastic Syndromes Rarely associated with thymic carcinoma compared to thymoma (e.g., myasthenia gravis, pure red cell aplasia). Imaging Characteristics Mass typically measures 5 to 15 cm; may have low attenuation areas. Calcification is uncommon. Cannot be distinguished from thymoma on CT; however, enlarged lymph nodes and irregular margins are more common in carcinoma. Less likely to have pleural implants. MRI Appearance Appears higher in signal intensity than muscle on T1-weighted images. Increased signal on T2-weighted images. Heterogeneous signal may indicate necrosis, cystic regions, or hemorrhage . Thymoma tends to show a multinodular appearance more than thymic carcinoma.

24. Thymic Neuroendocrine Tumor

25. Thymic Neuroendocrine Tumor Rare tumors, accounting for 2% to 5% of thymic neoplasms. Arise from thymic cells of neural crest origin (APUD cells). Classification Classified similarly to pulmonary neuroendocrine tumors: Low-grade carcinoid tumor Intermediate-grade atypical carcinoid tumor High-grade large cell neuroendocrine carcinoma or small cell neuroendocrine carcinoma Most cases represent atypical carcinoid tumors.

26. Clinical Presentation Symptoms often due to compression of mediastinal structures. 25% to 40% of patients may have Cushing’s syndrome from tumor secretion of ACTH. Approximately 20% associated with multiple endocrine neoplasia (MEN) syndromes I and II. Imaging Characteristics CT may show dense contrast enhancement, but appearance is similar to thymoma . Mediastinal mass may not be visible on CT despite endocrine abnormalities. MRI findings are nonspecific and resemble those of thymoma . Aggressiveness More aggressive than thymoma ; higher incidence of superior vena cava obstruction. 5-year survival rate is about 65%. Metastasis Mediastinal lymph node or distant metastases may be present. Tumor behavior correlates with cell type and differentiation.

27. T hymolipoma

28. Rare, benign, well-encapsulated thymic tumor . Primarily consists of fat with variable amounts of thymic tissue. Can arise within the thymus or be connected by a pedicle. Epidemiology Accounts for less than 5% of thymic tumors . 80% of patients present within the first four decades of life. Clinical Presentation Typically asymptomatic; detected incidentally on chest radiographs. Often large, averaging nearly 20 cm in diameter. May extend into both hemithoraces .

29. Chest Radiography: Drapes over the heart and extends into cardiophrenic angles without obscuring the cardiac margin. Can simulate cardiac enlargement, lower lobe collapse, or elevation of a hemidiaphragm . CT Imaging: Appears predominantly fat with wisps, whorls, or small nodules of soft tissue. Uncommonly may present primarily as soft tissue attenuation. Connection to the thymic bed is always visible. MRI: High signal intensity on T1-weighted images, similar to subcutaneous fat. Intermediate signal intensity areas reflect soft tissue presence.

30. Behavior and Management Does not invade surrounding structures. May compress mediastinal structures in about 50% of cases. No recurrence following resection. Associations No known association with myasthenia gravis.

31. Thymic Cyst Thymic cysts are uncommon. They can be either congenital or acquired. Congenital thymic cysts are rare; acquired thymic cysts have been reported following radiation therapy, in association with thymic tumors , and following thoracotomy . Their attenuation is usually that of water but can be higher or lower depending on the presence of hemorrhage or fat. One should be cautious in making the diagnosis of thymic cyst; cystic regions can be seen in a variety of thymic tumors , including thymoma and lymphoma . CT can suggest the diagnosis of thymic cyst if the lesion (a) appears thin walled, (b) is unassociated with a mass lesion, (c) contains fluid with a density close to that of water, and (d) remains unopacified following contrast infusion. Calcification of the cyst wall can also be seen. MR characteristics are similar to those of other cystic lesions.

32. Thymic Lymphoma Predilection: Thymus involvement is common in conjunction with mediastinal lymph node enlargement. Incidence: Thymic enlargement observed in 30% of patients with intrathoracic HL. Imaging Features: Typically results in homogeneous thymic enlargement. Lobulation or nodular appearance may be noted. Cystic areas of necrosis visible on CT in 20% of adults. Non-Hodgkin’s Lymphoma (NHL) Involvement: Less commonly affects the thymus. Types: Large B-cell lymphoma Extranodal marginal zone lymphoma (MALT lymphoma) T-cell lymphoblastic lymphoma/leukemia may arise in the thymus.

33. Imaging Features: Thymic lymphoma usually appears as a thymic mass. Associated mass or lymph node enlargement in the mediastinum is suggestive of lymphoma. Radiological Characteristics CT Findings: Thymic lymphoma generally exhibits homogeneous enlargement. Calcification is uncommon without prior radiation or chemotherapy. MRI Findings: Low intensity on T1-weighted images. Variable signal intensity on T2-weighted images.

34. Thymic Metastases Metastatic tumors , particularly lung and breast carcinomas, can also involve the thymus. Involvement of mediastinal lymph nodes is also typically present. CT and MRI appearances of thymic metastases are nonspecific.

35. DIFFERENTIALS

36. Teratoma

37. Seminoma

38. Nonseminomatous Germ-Cell Tumors

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