Clinical and imaging manifestations of normal and abnormal thymus

Clinical and imaging manifestations of normal and abnormal thymus

In the imagery, the shape and size of the thymus gland are diverse, even in the same individual. As we age, it gradually subsides and may shrink rapidly during periods of physical stress. During the recovery period, it will return to its original size, or even larger, a phenomenon called thymus reactive hyperplasia.

These anatomical variations and dynamic changes appear to be the main cause of confusion with pathological conditions. Conversely, these misconceptions can also lead to prolongation or alteration of chemotherapy regimens, or to unnecessary radiotherapy, biopsies, or thymus. Familiarity with the embryology, anatomy, and dynamic physiology of the thymus is essential to avoid unnecessary imaging or invasive procedures.

Radiologists play an important role in distinguishing between normal thymus variants, ectopic thymus tissue and non-neoplastic thymus disorders (e.g., reactive hyperplasia), and neoplastic diseases. Understanding the imaging presentation of thymic tumors helps radiologists make the correct diagnosis.

embryology

Ectopic and parathymic tissue can occur anywhere in the descending pathway (thymopharyngeal duct) and can result from failure of descent, isolation, or degeneration. Ectopic or ectopic thymus tissue is seen near the superior vena cava, cephalic vessels, and aorta. Rarely seen in the posterior mediastinum or even on the skin.

Ectopic thymus tissue may present as a lump in the neck, which may be mistaken for a pathological process. At the same time, when the thymphangeal duct atrophises, the remnants of the thymus can develop into cysts. Since the parathyroid glands also originate in the third and fourth pharyngeal sacs, ectopic parathyroid glands, parathyroid adenomas can appear near or inside the thyroid or thymus gland.

Boys 7 weeks old, parapharyngeal thymus ectopic, right mandibular angle area has a "lump". MR coronary enhancement T1WI shows a strengthened parapharyngeal mass (arrow), consistent with mediastinal thymus (arrow) signals. Puncture biopsy confirms that the mass is ectopic thymus tissue.

66-year-old female, hypercalcemia and kidney stones. Thymus intrathytopic parathyroid adenoma . (a) CT scan showing a partial cystic mass (arrow) between the superior vena cava and the aorta. (b) 99 m-sestamibi coronary scan showing uptake of the mass (arrow).

dissect

The thymus gland is located in the anterior mediastinum and covers the pericardium, aortic arch, left unnamed vein, and trachea. The thymus gland can extend upward to the inferior thyroid pole and down to the diaphragm. The thymus gland attaches to the thyroid gland through the thymus ligament of the thyroid gland.

12-year-old boy with normal thymus gland with neck component. (a) Ultrasound images show the mediastinum and neck components of the thymus gland (black arrow) connected to the lower thyroid pole (white arrow) by the thyroid thymus ligament (arrow). Note the "starry"-like manifestations of the thymus gland; this is due to the formation of high echo fats against the background of the remaining low echo lymphoid tissue. (b) Corresponding anatomical schematic showing the mediastinal (arrow) and neck (arrow) components of the thymus gland.

3-year-old boy with thymus neck component, the mother has a family history of papillary thyroid cancer. The neck thymus is mistaken for an "exogenous tumor of the thyroid gland". Contrast-enhanced CT scan shows a normal mediastinal thymus gland (Figure a-arrow) and a cervical part (Figure B-arrow). Follow-up after 2 years showed no change and the patient was asymptomatic.

Imaging of normal thymus

Given the variability of the shape and size of the thymus, familiarity with the multiple imaging manifestations of the normal thymus gland is required.

On orthostatic chest x-rays in infants and toddlers, the thymus glands are surprisingly large and sometimes difficult to distinguish from the heart shadow. The thymus usually has a smooth border and can still be seen on x-rays at age 3 years. Thymus wavy signs refer to their fan-shaped or wavy contours, caused by indentations in the anterior ribs (figure). About 5% of children can see thymic sail sign, the right lobe of the thymus triangle is slightly convex, and the bottom is clearly demarcated, caused by a horizontal fissure adjacent to the right lung (figure).

The thymus gland is very soft and does not cause compression or displacement of adjacent structures, and this manifestation may be a particularly important part of real-time ultrasound, as heart beats and respiratory movements affect the shape of the thymus gland. Conversely, solid tumors or diffuse infiltrates are less malleable and harder.

The normal thymus glands of 11-year-old boys were misdiagnosed as metastatic. Patients have a history of lung metastases from osteosarcoma. Coronary T1WI images show a normal thymus gland with components of its mediastinum (white arrow) and neck (black arrow).

5 months old girl with mild respiratory distress. Thymus wavy signs (arrows) and sail signs (arrows) may be seen on orthostatic chest x-rays, which are formed due to the imprint of the anterior rib on the normal thymus gland and the horizontal fissure of the right lobe of the thymus gland.

7-year-old girl, right aortic arch, suspected "supraclavicular mass" on CT. Sagittal ultrasound images of the lower neck show normal echoes of the cervical components of the thymus. Normal thymus (arrow), the thymus gland has an uneven low echo compared to the thyroid gland. SCV = subclavian vein.

Healthy 1-year-old boy, chest contrast-enhanced CT scan shows a quadrangular appearance of the thymus at the pulmonary artery level and an outer margin bulge (arrow).

5-year-old girl, normal thymus, with Burgett lymphoma of the maxilla and central nervous system. On PET/CT, the thymus is mistaken for recurrent lymphoma and a puncture biopsy is performed, which shows normal thymic tissue. (a) Enhanced CT scan of the level of the aortic arch shows a normal triangle or arrow-shaped thymus gland (arrow) relative to the patient's age. (b) Sagittal PET images showing the thymus (arrow) did not show increased FDG metabolism.

Thymus lesions

Thymus hyperplasia

Thymic Hyperplasia

Histologically, thymic hyperplasia can be divided into two different types: true hyperplasia and lymphoid tissue hyperplasia (follicular hyperplasia).

True Thymic Hyperplasia: Characterized by an increase in the size and weight of the thymus gland, which retains its tissue microscopic characteristics. Although the hypertrophic thymus gland can maintain its normal shape, it is more common to lose its pronounced diplodocus-like appearance and take an oval shape.

Clinically, patients with true thymic hyperplasia can be divided into three groups: patients without associated disease; patients recovering from recent stress events such as pneumonia (pictured), corticosteroid therapy, radiation therapy, chemotherapy (pictured), surgery or burns; and other conditions such as hyperthyroidism, sarcoidosis, or red blood cell aplastic disorders.

Thymic rebound hyperplasia typically presents as diffuse enlargement, a mixture of fat and lymphoid tissue, smooth appearance, and normal blood vessels; in contrast, thymic tumors are often accompanied by a nodular contour, often with necrotic foci (figure) or calcification foci.

Thymic Lymphoid Hyperplasia: characterized by the presence of hyperplastic lymphatic germination centers in the thymus medullary gland, associated with lymphocyte and plasma cell infiltration. Unlike true hyperplasia, lymphatic hyperplasia may or may not be accompanied by enlarged thymus. Thymic lymphoid tissue hyperplasia is often associated with autoimmune diseases such as myasthenia gravis, thyroid poisoning, and connective tissue disease, and is reported to occur in the early stages of human immunodeficiency virus infection.

Although thymic lymphoid tissue hyperplasia usually presents as normal thymus on conventional x-rays, it may present as normal (45%), enlarged (35%), or localized thymus mass (20%) on CT.

18-month-old girl, convalescent period of pneumonia, thymic reactive hyperplasia. (a) Chest x-ray showing air retention in both lungs and increased peri-bronchial density (arrows) around the hilars of both lungs is consistent with viral pneumonia. (b) Follow-up x-ray showing thymus reactivity (arrow), pneumonia disappearance (arrow).

11-year-old girl, Hodgkin lymphoma, thymus reactive hyperplasia. (a) Contrast-enhanced CT scan obtained at the time of diagnosis shows enlarged lymphadenopathy (arrows) on the right side of the mediastinum. Arrow = thymus gland. (b) CT scan after chemotherapy shows enlarged lymph node resolution (arrow). Early thymus reactive hyperplasia (arrowhead). (c) CT scan 1 month after chemotherapy shows enlarged lymph nodes almost disappearing (arrows) and thymic reactive hyperplasia is more pronounced (arrows).

A 14-year-old boy who completed chemotherapy for primary mediastinal T-cell lymphoma 6 months ago showed thymic reactive hyperplasia as a pronounced mediastinal "mass" that was repeatedly misdiasected as a recurrence of lymphoma. (a) Enhanced CT scan showing a denser enlarged thymus gland in front of the aortic arch. (b) Ultrasound images of suprasternal transverse segmentation show a normal thymus gland (arrow) with smooth edges. AA = aortic arch, PA = pulmonary artery.

In a 13-year-old boy, acute lymphoblastic leukemia relapses. Enhanced CT scan shows multiple enlarged lymph nodes in the mediastinum and a large thymic mass, with necrosis areas (arrows) seen in the center.

Thymus cyst

Thymic Cysts

Congenital thymic cysts originate from thymic remnants and may be seen in the thymic catheter walking area, extending from the upper neck to the anterior mediastinum, and rarely occurs in the posterior mediastinum or near the diaphragm.

Acquired thymic cysts have been reported to occur before and after NHL or HL chemotherapy (figured), after thyrotomy, and in about 40% of patients with thymomas.

Thymus cysts usually appear as uniform, rounded masses with calcifications at the edges. Density is usually close to water and may vary due to bleeding or the presence of fat. On CT, thymus cysts are visible with thin walls, no solid components, no reinforcement.

Female 61 years old, thymus cyst, accidentally found. Irregular, cystic, no reinforcement, mild mass effect.

Males aged 19 years, patients with HL, cystic changes in the thymus and lymph nodes. Enhanced CT scan shows solid (arrows in a) and cystic (arrows in b) thymus masses. When the patient completes chemotherapy, the cystic lymph nodes recede, but the cystic regions in the thymus remain unchanged.

28-year-old male, 3 years ahead of HL treatment, cystic changes in the thymus. Enhanced CT scan shows thymic cysts (arrows) and thymic reactive hyperplasia (arrows).

Tumors of the thymus

Thymic Tumors

Thymus tumors are divided into: epithelial tumors (including thymoma and thymus carcinoma), lymphoma (including HL and NHL), Langerhans cell histiocytosis, thymoid lipoma, carcinoid, germ cell tumor, sarcoma, metastatic tumor.

In adults, thymoma is the most common primary tumor of the thymus, lymphoma is the second most common, followed by germ cell tumors. In children, lymphoma is the most common primary tumor of the thymus; Germ cell tumors are the second most common. Thymomas are rarely seen in children.

Diseases of the thymus and lymph nodes of Hodgkin lymphoma disappear after most patients undergo appropriate treatment. Residual areas of soft tissue density during imaging usually indicate fibrosis, while an emerging "mass" suggests reactive hyperplasia.

27-year-old female, thymoma. CT enhancement scan at the left ventricular level shows a solid mass of the thymus gland, strengthened. The left ventricle is depressed by compression.

13-year-old boy with invasive thymoma. Contrast-enhanced CT aortic arch horizontal images show a large unevenly strengthened solid tumor (black arrow) in the left thoracic cavity, originating in the thymus gland (white arrow), growing along the pleura.

12-year-old boy, intermittent fever and shortness of breath, thymic cancer. Contrast-enhanced CT images of aortic arch levels show a large thymic mass, a nodular contour at the edges, and an encapsulated nameless vein.

A 51-year-old woman with Hodgkin lymphoma and thymus and lymph node involvement. (a) Enhanced CT scan showing left lobe of the thymus (arrow) and subproduction lymph nodes (arrows) involvement. (b) Coronary PET images show asymmetric increased FDG metabolism in the left lobe of the thymus (arrow) and sub-lobe lymph nodes (arrow).

In 18-year-old men, HL involves the thymus gland. Axial PET/CT and coronary PET images show multiple enlarged lymph nodes (arrows) in the neck and axillaries and increased asymmetric FDG metabolism of the thymus (arrows).

4-year-old boy, thymic T-cell acute lymphoblastic leukemia. Orthostatic chest x-ray showing a large thymic mass with lobes at the edges, and the mass is not affected by the second to fourth anterior rib, which can be compared with a normal thymic wave sign.

5-month-old girl, histiocytosis of the Langerhans cell of the whole body, thymus gland involvement. The orthostatic chest x-ray shows a large lobulate-like mass (arrow) that is not clearly demarcated from the contours of the heart and is not affected by the anterior ribs. The lump completely subsided after chemotherapy, but recurs after 11 months.

14-year-old girl, cough and fever for 2 weeks, thymos lipoma. (a) Contrast-enhanced CT scan showing a large fat-dominated mass in the left thoracic cavity with a slight displacement of the heart. (b) Coronary T1WI shows that the mass is bright and high-signal, there are fibers separated inside, the left lung is crushed and collapsed, and the left lung is completely reasserted after the mass is removed.

22-year-old male, thymic carcinoid, persistent dry cough for 3 months. (a) Enhanced CT showing uneven strengthening of the thymus mass (arrows). (b) PET images show strong uptake of lump FDG. Note the nodular contours of the masses in a and b, which differ from the manifestations of reactive thymus hyperplasia.

27-year-old male, thymus teratoma. Coronary T1WI shows a mediastinal mass (arrow) in the left lobe of the thymus (arrow). The mass is highly signaled on both T1WI and T2WI, suggesting fat content.

9 years old girl, asymptomatic, thymic sarcoma. T1WI shows a distinctly unevenly strengthened mass (arrow) originating in the right lobe of the thymus gland, which is close to the diaphragm and slightly under pressure on the right atrium. After chemotherapy failed, the tumor was surgically removed, but the patient eventually died of metastatic disease.

In 10-year-old boys, thymus metastases develop after 2 years of striated muscle tumors in the thighs. Contrast-enhanced CT scans showed a solid mass (arrow) of the anterior mediastinum, almost replacing the thymus gland.

Source: Panda Radiation

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