Imaging Pearls ❯ Chest ❯ Thymus
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- “Thymic imaging is challenging because the imaging appearance of a variety of benign and malignant thymic conditions are similar. CT is the most commonly used modality for mediastinal imaging, while MRI and fluorine 18 fluorodeoxyglucose (FDG)PET/CT are helpful when they are tailored to the correct indication. Each of these imaging modalities has limitations and tech-nical pitfalls that may lead to an incorrect diagnosis and mismanagement. CT may not be sufficient for the characterization ofcystic thymic processes and differentiation between thymic hyperplasia and thymic tumors. MRI can be used to overcome these limitations but is subject to other potential pitfalls such as an equivocal decrease in signal intensity at chemical shift imaging, size limitations, unusual signal intensity for cysts, subtraction artifacts, pseudonodularity on T2-weighted MR images, early im-aging misinterpretation, flow and spatial resolution issues hampering assessment of local invasion, and the overlap of apparent diffusion coefficients between malignant and benign thymic entities.”
Thymic Imaging Pitfalls and Strategies for Optimized Diagnosis
Maximiliano Klug, Chad D. Strange, Mylene T. Truong, et al.
RadioGraphics 2024 44:5 - The thymus is a small gland located in the prevascular medi-astinum that has a role in the development and maturation ofT lymphocytes, the key elements of the immune system. Imag-ing of the thymus can be performed for a myriad of indications,including the evaluation of incidentally discovered prevascularmediastinal masses in asymptomatic patients or screening inpatients with multiple endocrine neoplasia syndromes. Imag-ing can also be initiated in patients who present with symp-toms such as chest pain or cough and in those with variousparaneoplastic syndromes, with myasthenia gravis being themost common, even without chest symptoms. Finally, imagingcan be used for staging and posttreatment surveillance in pa-tients with a known thymic epithelial malignancy
Thymic Imaging Pitfalls and Strategies for Optimized Diagnosis
Maximiliano Klug, Chad D. Strange, Mylene T. Truong, et al.
RadioGraphics 2024 44:5 - “Indications for imaging of the thymus include evaluation ofan incidental imaging abnormality, local symptoms, para-neoplastic symptoms, screening in patients with multiple endocrine neoplasia syndromes, and surveillance after treatment of a thymic epithelial neoplasm. Classically, a thymic mass is incidentally discovered as a mediastinal abnormality at chest radiography, occasionally with an elevation of a hemidiaphragm due to phrenic nerve involvement. With the increased use of CT for surveillance and screening, the most common indication for thymic imaging investigation is an in-cidentally discovered nodule in the thymic bed. When symp-tomatic, the local symptoms leading to imaging include chestpain, hoarseness, dysphagia, shortness of breath, coughing,wheezing, and arrhythmia, especially if these symptoms persist or worsen over time.”
Thymic Imaging Pitfalls and Strategies for Optimized Diagnosis
Maximiliano Klug, Chad D. Strange, Mylene T. Truong, et al.
RadioGraphics 2024 44:5 - “Paraneoplastic syndromes that require imaging to exclude a thymic epithelial neoplasm include myasthenia gravis,Lambert-Eaton syndrome, pure red cell aplasia, hypogam-maglobulinemia, lichen planus, Good syndrome, limbic en-cephalitis, and others (neuromuscular, endocrine, and con-nective tissue disorders). Paraneoplastic syndromes are often linked to favorable characteristics such as younger age,type B1 thymoma, earlier stage, and a higher rate of complete resection status, but their presence does not independently predict recurrence-free survival or overall survival in patients with thymic epithelial neoplasms.”
Thymic Imaging Pitfalls and Strategies for Optimized Diagnosis
Maximiliano Klug, Chad D. Strange, Mylene T. Truong, et al.
RadioGraphics 2024 44:5 - CT has low contrast resolution compared with that of MRI,sometimes creating difficulties in distinguishing among thymic hyperplasia, thymic cysts, and thymomas. One of the most common pitfalls in evaluation of a thymic lesion at CT is misinterpreting a simple thymic cyst as a solid lesion. Thymic cysts may contain proteinaceous or hemorrhagic fluid .This results in attenuation measurements greater than thosexpected for simple fluid (>20 HU), which are reported to beup to 97 HU. In a large series, Ackman et al reported a mean attenuation value of 25 HU for individual cysts witmore than 5 years of follow-up. Therefore, when a homo-geneously smooth well-defined saccular, oval, or round lesionis detected in the thymic area, even if it measures higher than 20 HU, it is advisable to conduct further evaluation using MRI.
Thymic Imaging Pitfalls and Strategies for Optimized Diagnosis
Maximiliano Klug, Chad D. Strange, Mylene T. Truong, et al.
RadioGraphics 2024 44:5 - “Another pitfall encountered with CT is misinterpretationof thymic hyperplasia as thymic malignancy. At CT, the pres-ence of a diffusely enlarged thymus, exhibiting nodularity and a pyramidal configuration, with increased length anthickness of each lobe relative to age-specific measurementsshould raise suspicion for thymic hyperplasia. This phenomenon is often seen in patients after undergoing a stress-or such as chemotherapy, radiation therapy, or steroid treatment; after recovery from burns or injuries; or in conjunction with systemic diseases such as myasthenia gravis or collagen vascular disease. The presence of intralesional fat intercalating between thymic tissue is diagnostic of thymic hyperplasia.”
Thymic Imaging Pitfalls and Strategies for Optimized Diagnosis
Maximiliano Klug, Chad D. Strange, Mylene T. Truong, et al.
RadioGraphics 2024 44:5 - “There are some typical interpretation pitfalls and blind spots for metastatic involvement in the assessment of patients with thymic malignancies. Thymic malignancies have a predilec-tion for pleural involvement. Pleural thickening, nodules,or masses are the most common radiologic signs of pleural dissemination; pleural effusion is rare. Pleural metas-tases can be lenticular shaped and relatively flat and maybe missed when they are small. Thin-section CT is superior to thick-section CT for evaluation of pleural metastases .The thinner 1.25-mm sections, with the improved spatial resolution of the lung kernel, can help to identify early pleural nodularity in comparison with the usually thicker 2.5-mmsections used with the soft-tissue kernel. Moreover,coronal images may be helpful for detecting subtle diaphragmatic surface pleural metastases.”
Thymic Imaging Pitfalls and Strategies for Optimized Diagnosis
Maximiliano Klug, Chad D. Strange, Mylene T. Truong, et al.
RadioGraphics 2024 44:5 - Most mediastinal germ cell tumors are found incidentally in imaging studies. At the time of diagnosis, approximately 20% to 40% of patients are asymptomatic. Generally, initial symptoms are associated with the enlargement of the mediastinal cavity and protrusion into surrounding structures. Common clinical symptoms include chest pain, cough, dyspnea, fever, night sweats, and weight loss. Manifestations are dependent on the size of the tumor and its histological subtype. Treatment involves either surgical resection or cisplatin-based chemotherapy followed by surgical resection depending on the histological subtype.
Mediastinal Germ Cell Tumors.
Kang J, Mashaal H, Anjum F.
[Updated 2023 Apr 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 - “Germ cell tumors of the mediastinum are a rare entity, making up only 15% of all tumors located in that area. They are the most common type of germ cell tumors outside the gonads but only account for approximately 1% to 3% of all germ cell malignancies. Mediastinal germ cell tumors are predominantly seen in men (97%) and occur in younger patients with a mean age of 31 years.[10] Mediastinal germ cell tumors have been associated with Klinefelter syndrome, especially for non-seminomatous variants.”
Mediastinal Germ Cell Tumors.
Kang J, Mashaal H, Anjum F.
[Updated 2023 Apr 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 - “Mediastinal teratomas are usually located near or within the thymus gland in the anterior space of the mediastinum. Biochemical markers are typically not elevated in benign masses. The contents of a mediastinal mature teratoma include at least two of the three embryonic cell layers (ectoderm, mesoderm, endoderm) developed into well-differentiated histologic elements. Mesodermal elements are represented by bone, cartilage, and muscle. Endodermal elements include gastrointestinal, respiratory, and endocrine gland tissue. Ectodermal tissue can exhibit skin, dermal appendages, and cystic structures lined by squamous epithelium."
Mediastinal Germ Cell Tumors.
Kang J, Mashaal H, Anjum F.
[Updated 2023 Apr 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024
- First described by Dr. William Halsted in Transactions of the American Surgical Association in 1914, thymic hyperplasia in Graves' disease is not uncommon, although the true incidence is unclear because chest imaging is not routinely obtained in every patient who is newly diagnosed with Graves' disease. The pathogenesis is thought to be related to a direct trophic effect from excess thyroid hormones, as well as autoimmunity from thyrotropin receptor antibody, leading to increased proliferation of cortical epithelial cells and medullary lymphoid follicles.
- Thymic hyperplasia in Graves' disease is a benign condition and often reversible to various degrees with treatment. Various studies have reported a significant decrease (33% to 90%) in thymic volume after successful treatment of hyperthyroidism using anti-thyroid drugs with a follow-up ranging from six weeks to two years. Therefore, a repeat chest image six months after achieving euthyroidism is recommended. If there is less than 50% regression of the mass, biopsy or thymectomy should be reconsidered to rule out malignancy.
- - Thymic hyperplasia in Graves' disease is related to excess thyroid hormones and thyrotropin receptor antibody.
- It is a benign condition and usually improves after successful treatment of Graves' disease.
- Biopsy and thymectomy should be deferred in the setting of thyrotoxicosis and be reconsidered if thymus fails to regress on repeat imaging.
- “The average long-axis dimension was 17.50 ± 6.00 mm, the CT value range across the 24 cases was 5–81 HU, and the average CT value of the noncontrast enhanced scans was 39.75 ± 20.66 HU. The CT value in the noncontrast enhanced scan was >20 HU in 79% of the sample cases.”
Special Computed Tomography Imaging Features of Thymic Cyst.
He ZL, Wang ZY, Ji ZY.
Int J Clin Pract. 2022 Oct 11;2022:6837774 - “Thymic cysts are considered to be a relatively rare type of anterior mediastinal mass; they are reported to account for approximately 1%–3% of cases. With the growing popularity of low-dose chest computed tomography (CT) screening, the detection rate of thymic cysts is also increasing. The density of thymic cysts is often not the typical liquid density, and the CT diagnosis rate is not high, leading to thymic cysts often being misdiagnosed as thymomas or other types of solid mass and being removed by resection.”
Special Computed Tomography Imaging Features of Thymic Cyst.
He ZL, Wang ZY, Ji ZY.
Int J Clin Pract. 2022 Oct 11;2022:6837774 - “The thymic cysts in the present study were all located in the anterior mediastinum. The average long-axis dimension measurement was 17.50 ± 6.00 mm. All 24 cases had uniform density. The average noncontrast enhanced scan CT value was 39.75 ± 20.66 HU, and the lowest and highest noncontrast enhanced scan CT values were approximately 5 and 81 HU, respectively. Among the sample, 19/24 cases (79%) had a noncontrast enhanced scan CT value of > 20 HU.”
Special Computed Tomography Imaging Features of Thymic Cyst.
He ZL, Wang ZY, Ji ZY.
Int J Clin Pract. 2022 Oct 11;2022:6837774 - “Most of the existing literature reports that thymic cysts are typically triangular or round in shape, with smooth edges, rare lobes, and smooth junctions with the pleura. Some thymic cysts can display changes in shape between different scan phases. Most cases in the present study were triangle/chestnut-shaped (50%) or round (29%), and the majority of the cysts had smooth edges.”
Special Computed Tomography Imaging Features of Thymic Cyst.
He ZL, Wang ZY, Ji ZY.
Int J Clin Pract. 2022 Oct 11;2022:6837774 - Due to the issue of atypical density, the main concern in the differential diagnosis of thymic cysts is thymoma. Larger thymomas can more easily be distinguished from thymic cysts due to their uneven internal density, progression, and invasion of the surrounding structures, combined with more obvious enhancement. However, the density of smaller thymomas is relatively uniform, and there exist significant image overlaps between thymic cysts and some mildly enhanced tumors. This is also the reason for most cases of misdiagnosis of thymic cysts as thymoma
Special Computed Tomography Imaging Features of Thymic Cyst.
He ZL, Wang ZY, Ji ZY.
Int J Clin Pract. 2022 Oct 11;2022:6837774
- Ectopic Cushings Syndrome
- Ectopic Cushing syndrome is a form of Cushing syndrome in which a tumor outside the pituitary gland produces a hormone called adrenocorticotropic hormone (ACTH).
- ACTH is usually made by the pituitary in small amounts and then signals the adrenal glands to produce cortisol. Sometimes other cells outside the pituitary can make large amounts of ACTH. This is called ectopic Cushing syndrome. - Ectopic Cushing syndrome: Causes
- Benign carcinoid tumors of the lung
- Islet cell tumors of the pancreas
- Medullary carcinoma of the thyroid
- Small cell tumors of the lung
- Tumors of the thymus gland
- “Thymic masses are comprised of hyperplasia, cysts, thymic epithelial tumors (TETs), lymphomas, malignant germ cell tumors or metastatic cancers and so on. True thymic hyperplasia is usually regarded as a rebound phenomenon and characterized by an increase in mass of the gland after a stressor, such as chemotherapy, radiation, steroid treatment, burns or surgery. Thymic cysts are relatively uncommon lesions that can be found at any age and can be congenital or acquired. Although TETs are rare neoplasms as to the whole tumors, they are the most common mediastinal tumors in adults, including thymomas and thymic carcinomas.”
“Comparison between CT and MRI in the Diagnostic Accuracy of Thymic Masses.”
Li, Hao-Ran et al.
Journal of Cancer vol. 10,14 3208-3213. - “True thymic hyperplasia is usually regarded as a rebound phenomenon and characterized by an increase in mass of the gland after a stressor, such as chemotherapy, radiation, steroid treatment, burns or surgery.”
“Comparison between CT and MRI in the Diagnostic Accuracy of Thymic Masses.”
Li, Hao-Ran et al.
Journal of Cancer vol. 10,14 3208-3213. - Thymic Hyperplasia: Etiology
- Graves disease
- Response following chemotherapy
- Myasthenia gravis - Masses of Thymic Origin
- Benign
--- Thymic hyperplasia
--- Thymic cyst
--- Thymolipoma
--- Thymoma (Masaoka-Koga stage I/II)
- Malignant
--- Thymoma (Masaoka-Koga stage III/IV)
--- Thymic carcinoma
--- Thymic lymphoma
--- Thymic carcinoid
- Rare lesions
--- Metastatic disease
--- Langerhans cell histiocytosis
--- Thymic germ cell tumour
--- Follicular dendritic cell sarcoma - “Two distinct histologic types of thymic hyperplasia exist:true thymic hyperplasia and lymphoid follicular hyperplasia. In true thymic hyperplasia, the gland is enlarged(ie, increased mass of tissue) with preserved microscopic and histologic architecture. In lymphoid follicular hyperplasia,enlarged lymphoid germinal centers account for the increased size of the gland. These 2 entities are indistinguishable from one another at imaging.”
A Tour of the Thymus: A Review of Thymic Lesions With Radiologic and Pathologic Correlation
Goldstein AJ et al.
Canadian Association of Radiologists Journal, Volume 66, Issue 1,2015,Pages 5-15 - "True thymic hyperplasia is usually seen as a rebound phenomenon, characterized by an increase in mass of the gland after a stressor, such as chemotherapy, radiation, steroid treatment, burns, or surgery. Lymphoid follicular hyperplasia is associated with numerous chronic inflammatory and autoimmune disorders, including myasthenia gravis, Graves disease, systemic lupus erythematosus, rheumatoid arthritis, scleroderma, and other autoimmune conditions.”
A Tour of the Thymus: A Review of Thymic Lesions With Radiologic and Pathologic Correlation
Goldstein AJ et al.
Canadian Association of Radiologists Journal, Volume 66, Issue 1,2015,Pages 5-15 - "Although the 2 types of thymic hyperplasia are impossible to differentiate at imaging, thymic hyperplasia can often be differentiated from neoplasm on the basis of key imaging findings. Thymic hyperplasia is usually manifested by diffuse, symmetric thymic enlargement, a smooth contour, interspersed fat and soft-tissue elements, normal vessels, and preserved adjacent fat planes. Alternatively, a neoplasm may demonstrate a focal mass, a nodular contour, heterogeneity (ie, hemorrhage or necrosis), or calcifications.”
A Tour of the Thymus: A Review of Thymic Lesions With Radiologic and Pathologic Correlation
Goldstein AJ et al.
Canadian Association of Radiologists Journal, Volume 66, Issue 1,2015,Pages 5-15 - "Thymolipoma is a rare, benign, predominantly fatty mass with interspersed fibrous septae and normal thymic tissue. Due to their pliability, thymolipomas can become very large and have been reported to occupy almost the entire hemithorax. On imaging, they appear as large, fatty masses with scattered soft-tissue components, and can mimic mediastinal or epicardial lipomatosis. On histologic examination, they are characterized by mature adipose tissue with non-neoplastic thymic epithelial cells and fibrous bands.”
A Tour of the Thymus: A Review of Thymic Lesions With Radiologic and Pathologic Correlation
Goldstein AJ et al.
Canadian Association of Radiologists Journal, Volume 66, Issue 1,2015,Pages 5-15 - "Thymoma is the most common anterior mediastinal mass in adults. These tumours are often asymptomatic and incidentally discovered but may present with cough, hoarseness, dysphagia, chest pain, or symptoms related to vascular compression. CT imaging usually reveals a homogenous soft-tissue mass with sharp borders and a lobulated, round, or oval shape. Occasionally, calcifications or low-density components that represent necrosis can be seen (especially in large tumours) . Thymoma is classically associated with myasthenia gravis, and approximately 30%–50% of patients with thymoma have myasthenia gravis; alternatively, only 10%–20% of patients with myasthenia gravis have an underlying thymoma.”
A Tour of the Thymus: A Review of Thymic Lesions With Radiologic and Pathologic Correlation
Goldstein AJ et al.
Canadian Association of Radiologists Journal, Volume 66, Issue 1,2015,Pages 5-15
- “Thymic neoplasms are rare tumors that account for less than 1% of all adult malignancies, with reported incidences of one to five cases per 1 million people per year.The primary epithelial neoplasms of the thymus are thymoma and thymic carcinoma, with thymoma being more common. Thymic carcinoma is a more aggressive disease that is often diagnosed with needle biopsy before treatment planning; it has been discussed elsewhere.”
Role of Imaging in the Diagnosis, Staging, and Treatment of Thymoma
Benveniste MFK et al.
RadioGraphics 2011; 31:1847–1861 - Thymomas typically occur in patients older than 40 years of age, are rare in children, and affect men and women equally. Most thymomas are solid neoplasms that are encapsulated and localized to the thymus. Approximately one-third exhibit necrosis, hemorrhage, or cystic components, and approximately one-third invade the tumor capsule and the surrounding structures.
Role of Imaging in the Diagnosis, Staging, and Treatment of Thymoma
Benveniste MFK et al.
RadioGraphics 2011; 31:1847–1861 - “Thymomas are slow-growing neoplasms that may exhibit aggressive behavior such as invasion of adjacent structures and involvement of the pleura and pericardium, but distant metastases are rare”.
Role of Imaging in the Diagnosis, Staging, and Treatment of Thymoma
Benveniste MFK et al.
RadioGraphics 2011; 31:1847–1861 - “Myasthenia gravis associated with thymoma occurs most frequently in women. Between 30% and 50% of patients with a thymoma have myasthenia gravis, whereas 10%–15% of patients with myasthenia gravis have a thymoma.Ten percent of patients with a thymoma have hypogammaglobulinemia, whereas 5% have pure red cell aplasia.Thymomas are also associated with various autoimmune disorders such as systemic lupus erythematosus, polymyositis, and myocarditis”.
Role of Imaging in the Diagnosis, Staging, and Treatment of Thymoma
Benveniste MFK et al.
RadioGraphics 2011; 31:1847–1861 - “The role of imaging is to initially diagnose and properly stage thymoma, with emphasis on the detection of local invasion and distant spread of disease, to identify candidates for preoperative neoadjuvant therapy (ie, those with stage III or IV disease). Imaging of treated patients is di- rected at identifying resectable recurrent disease, since patients with completely resected recurrent disease have similar outcomes as those without recurrence.
Role of Imaging in the Diagnosis, Staging, and Treatment of Thymoma
Benveniste MFK et al.
RadioGraphics 2011; 31:1847–1861 - Thymomas may result in vascular invasion , pleural involvement, or pericardial dis- semination. Direct signs of vascular involvement include (a) an irregular vessel lumen contour, (b) vascular encasement or obliteration, and(c) endoluminal soft tissue, which may extend into cardiac chambers (3). Pleural dissemination (“drop metastases”) manifests at CT as one or more pleural nodules or masses, which can be smooth, nodular, or diffuse and are almost always ipsilateral to the anterior mediastinal tumor.
Role of Imaging in the Diagnosis, Staging, and Treatment of Thymoma
Benveniste MFK et al.
RadioGraphics 2011; 31:1847–1861 - “The differential diagnosis for anterior mediastinal tumors includes other primary thymic malignancies (eg, thymic carcinoma, thymic carcinoid tumor), nonthymic tumors (eg, lymphoma, germ cell tumor, small-cell lung cancer), and mediastinal metastasis.”.
Role of Imaging in the Diagnosis, Staging, and Treatment of Thymoma
Benveniste MFK et al.
RadioGraphics 2011; 31:1847–1861