Thymic epithelial tumors

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Although thymic epithelial tumours are rare, they are the most common primary neoplasm of the thymus and of of the antero-superior ~~mediastium~~ ²mediastinum. This article discusses thymomas, invasive thymomas and thymic carcinoma.

Epidemiology

Typical presentation is in the 5^th to 6^th decades, without gender predilection ².

Clinical presentation

Thymic epithelial tumours are in, in the vast majority of cases, located in the anterior ~~mediastium~~mediastinum, although cases in the neck and posterior ~~mediastium~~mediastinum have been reported ^2,6. As such they can present with venous obstruction, dysphagia or stridor.

Associations

Over 30 conditions have been associated with thymomas ⁵. Those with the strongest correlation and most frequently encountered include:

myasthenia gravis: most common association ⁵
- 10-20% of patients with myasthenia gravis have a thymoma
- 30-50% of patients with a thymoma have myasthenia gravis
pure red cell aplasia
- 50% of patients with pure red cell aplasia have a thymoma
- 5% of patients with a thymoma have pure red cell aplasia
hypogammaglobulinaemia
- 5% of patients with hypogammaglobulinaemia have a thymoma
- 10% of patients with a thymoma have hypogammaglobulinaemia
systemic lupus erythematosus (SLE)
rheumatoid arthritis
non-thymic cancers

Pathology

Macroscopically thymic tumours are of variable shape, with thymomas typically rounded with a bosselated outer surface. The cut surface is tan or grey-pink with lobulated architecture, separated by fibrous septae ¹¹.

Both non-invasive and invasive thymomas may appear to have an intact capsule, and microscopic examination is required. Obvious macroscopic invasion may be evident ¹¹.

Larger tumours are more likely to demonstrate cystic changes as well as haemorrhage and calcification^8,11.

Numerous histologic classification systems for thymic epithelial neoplasms have been proposed over the years. A widely used, and simple division is into ⁵:

non-invasive thymoma
invasive
- invasive thymoma
- thymic carcinoma

The WHO classification scheme for thymic epithelial tumours divides them according to histological appearance, which correlates with likelihood of invasiveness (from least to most likely) and thus correlate with staging ^5,9. The Masaoka staging system is commonly adopted to assess invasion, and is assessed at surgery ^4-5.

Location

Vast majority of thymomas arise in their expected anterior mediastinal region. Very rarely they can arise at other sites in around 4% of cases (e.g. cervical deeper mediastinal region) - see ectopic thymoma ^12-13.

Radiographic features

Plain filmradiograph

Presents as an anterior mediastinal mass. On plain ~~film~~radiographs they are often seen as a well ~~defined~~-defined, lobulated soft tissue density slightly towards one side of the mediastinum. Can demonstrate associated calcification (commonly amorphous/flocculent).

CT

Thymic epithelial tumours are usually of soft tissue attenuation, and are usually located between the sternum and great vessels. Of interest CT findings can predict histology as described by the WHO classification scheme for thymic epithelial tumours and thus by extension prognosis ^5-6.

type A: tumours (medullary histology thymomas), are typically rounded, smooth or somewhat lobulated masses of soft tissue attenuation
type B: tumours more frequently demonstrate calcification, although calcification is also frequently see in thymic carcinoma ⁶
type C:
- tumours (thymic carcinoma) usually demonstrate invasion of mediastinal fat or mediastinal structures, and are usually much larger than type A or B tumours
- mediastinal lymph node enlargement may be present, although the reported frequency of this finding varies widely (13-44%) ⁶

Pleural seeding is seen in invasive thymoma or thymic carcinoma.

MRI

T1: isointense to slightly hyperintense signal compared to muscle ^5-6,8
T2
- heterogeneous signal, slightly higher than muscle ^1,5-6
- cystic areas may be seen, especially in larger tumours ¹
- fibrous septa ~~crossing~~ crossing the mass are of low signal intensity ¹
in/out phase imaging: typically no signal drop out, i.e. no chemical shift
T1 C+ (Gd): linear regions of enhancement may be seen coursing though the mass, thought to represent fibrous septae ⁶

FDG PETNuclear medicine

Thymic carcinoma has higher FDG-18 PET-CT uptake than other, better differentiated thymic epithelial tumours, as well as normal or hyperplastic thymus. Using an SUV ~~cut-off~~cutoff point of 5.0 high sensitivity (84.6%) and specificity (92.3%) can be achieved when trying to distinguish between thymic carcinoma and thymoma ⁵.

Treatment and prognosis

Treatment depends on the stage as well as presence of myaethenia gravis, which dictates earlier surgical excision ⁹. For thymoma local excision usually suffices, and complete excision usually results in cure. For invasive thymoma, surgery may still be contemplated if complete excision is thought possible. If not, then down-staging with preoperative chemotherapy may be employed ¹⁰.

Surgical resection can be achieved via a number of approaches ⁹:

sternotomy
video assisted thoracoscopic thymectomy
trans cervical thymectomy (limited exposure)

Radiotherapy is not usually employed for stage I thymomas, but has a role in both postsurgical management of resected invasive thymoma or for inoperable invasive tumours, including thymic carcinoma ¹⁰.

Prognosis is significantly influenced by histological type and surgical staging (e.g. Masaoka staging system).

type A, AB and B1 tumours have the best survival (up to 85% 5 year survival) ¹⁰
type B2 and B3 tumours have ~~the~~ intermediate survival
type C tumours (thymic carcinoma) have the worst survival (35% 5 year survival) ¹⁰

Recurrence rates after surgical excision varies between 8-29% depending on the series and type of tumour ⁶.

Differential diagnosis

The differential for plain radiography is that of an anterior mediastinal mass.

On cross sectional imaging (CT/MRI), the differential is usually narrowed down to:

primary tumours of the thymus
thymic rebound:, e.g. postchemotherapy
thymic hyperplasia
lymphoma: primary thymic lymphoma or involvement from systemic lymphoma
mediastinal germ cell tumours

-<p>Although <strong>thymic epithelial tumours</strong> are rare, they are the most common <a href="/articles/primary-neoplasms-of-the-thymus">primary neoplasm of the thymus</a> and of the antero-superior mediastium <sup>2</sup>. This article discusses thymomas, invasive thymomas and thymic carcinoma.</p><h4>Epidemiology</h4><p>Typical presentation is in the 5<sup>th</sup> to 6<sup>th</sup> decades, without gender predilection <sup>2</sup>.</p><h4>Clinical presentation</h4><p>Thymic epithelial tumours are in the vast majority of cases located in the anterior mediastium, although cases in the neck and posterior mediastium have been reported <sup>2,6</sup>. As such they can present with venous obstruction, dysphagia or stridor.</p><h5>Associations</h5><p>Over 30 conditions have been associated with thymomas <sup>5</sup>. Those with the strongest correlation and most frequently encountered include:</p><ul>
+<p>Although <strong>thymic epithelial tumours</strong> are rare, they are the most common <a href="/articles/primary-neoplasms-of-the-thymus">primary neoplasm of the thymus</a> and of the antero-superior mediastinum. This article discusses thymomas, invasive thymomas and thymic carcinoma.</p><h4>Epidemiology</h4><p>Typical presentation is in the 5<sup>th</sup> to 6<sup>th</sup> decades, without gender predilection <sup>2</sup>.</p><h4>Clinical presentation</h4><p>Thymic epithelial tumours are, in the vast majority of cases, located in the anterior mediastinum, although cases in the neck and posterior mediastinum have been reported <sup>2,6</sup>. As such they can present with venous obstruction, dysphagia or stridor.</p><h5>Associations</h5><p>Over 30 conditions have been associated with thymomas <sup>5</sup>. Those with the strongest correlation and most frequently encountered include:</p><ul>
-</ul><p>The <a href="/articles/thymic-epithelial-tumours-classification-who">WHO classification scheme for thymic epithelial tumours</a> divides them according to histological appearance, which correlates with likelihood of invasiveness (from least to most likely) and thus correlate with staging <sup>5,9</sup>. The <a href="/articles/masaoka-staging-system">Masaoka staging system</a> is commonly adopted to assess invasion, and is assessed at surgery <sup>4-5</sup>.</p><h5>Location</h5><p>Vast majority of thymomas arise in their expected anterior mediastinal region. Very rarely they can arise at other sites in around 4% of cases (e.g. cervical deeper mediastinal region) - see <a href="/articles/ectopic-thymoma">ectopic thymoma</a> <sup>12-13</sup>.</p><h4>Radiographic features</h4><h5>Plain film</h5><p>Presents as an <a href="/articles/anterior-mediastinal-mass">anterior mediastinal mass</a>. On plain film often seen as a well defined, lobulated soft tissue density slightly towards one side of the mediastinum. Can demonstrate associated calcification (commonly amorphous/flocculent). </p><h5>CT</h5><p>Thymic epithelial tumours are usually of soft tissue attenuation, and are usually located between the sternum and great vessels. Of interest CT findings can predict histology as described by the <a href="/articles/thymic-epithelial-tumours-classification-who">WHO classification scheme for thymic epithelial tumours</a> and thus by extension prognosis <sup>5-6</sup>. </p><ul>
+</ul><p>The <a href="/articles/thymic-epithelial-tumours-classification-who">WHO classification scheme for thymic epithelial tumours</a> divides them according to histological appearance, which correlates with likelihood of invasiveness (from least to most likely) and thus correlate with staging <sup>5,9</sup>. The <a href="/articles/masaoka-staging-system-of-thymoma">Masaoka staging system</a> is commonly adopted to assess invasion, and is assessed at surgery <sup>4-5</sup>.</p><h5>Location</h5><p>Vast majority of thymomas arise in their expected anterior mediastinal region. Very rarely they can arise at other sites in around 4% of cases (e.g. cervical deeper mediastinal region) - see <a href="/articles/ectopic-thymoma">ectopic thymoma</a> <sup>12-13</sup>.</p><h4>Radiographic features</h4><h5>Plain radiograph</h5><p>Presents as an <a href="/articles/anterior-mediastinal-mass">anterior mediastinal mass</a>. On plain radiographs they are often seen as a well-defined, lobulated soft tissue density slightly towards one side of the mediastinum. Can demonstrate associated calcification (commonly amorphous/flocculent). </p><h5>CT</h5><p>Thymic epithelial tumours are usually of soft tissue attenuation, and are usually located between the sternum and great vessels. Of interest CT findings can predict histology as described by the <a href="/articles/thymic-epithelial-tumours-classification-who">WHO classification scheme for thymic epithelial tumours</a> and thus by extension prognosis <sup>5-6</sup>. </p><ul>
~~-<li>fibrous septa crossing the mass are of low signal intensity <sup>1</sup>~~
+<li>fibrous septa crossing the mass are of low signal intensity <sup>1</sup>
~~-<strong>in/out phase imaging:</strong> typically no signal drop out i.e. no chemical shift</li>~~
+<strong>in/out phase imaging:</strong> typically no signal drop out, i.e. no chemical shift</li>
-</ul><h5>FDG PET</h5><p>Thymic carcinoma has higher uptake than other better differentiated thymic epithelial tumours, as well as normal or hyperplastic thymus. Using an SUV cut-off point of 5.0 high sensitivity (84.6%) and specificity (92.3%) can be achieved when trying to distinguish between thymic carcinoma and thymoma <sup>5</sup>.</p><h4>Treatment and prognosis</h4><p>Treatment depends on the stage as well as presence of myaethenia gravis, which dictates earlier surgical excision <sup>9</sup>. For thymoma local excision usually suffices, and complete excision usually results in cure. For invasive thymoma, surgery may still be contemplated if complete excision is thought possible. If not, then down-staging with preoperative chemotherapy may be employed <sup>10</sup>.</p><p>Surgical resection can be achieved via a number of approaches <sup>9</sup>:</p><ul>
+</ul><h5>Nuclear medicine</h5><p>Thymic carcinoma has higher FDG-18 PET-CT uptake than other, better differentiated thymic epithelial tumours, as well as normal or hyperplastic thymus. Using an SUV cutoff point of 5.0 high sensitivity (84.6%) and specificity (92.3%) can be achieved when trying to distinguish between thymic carcinoma and thymoma <sup>5</sup>.</p><h4>Treatment and prognosis</h4><p>Treatment depends on the stage as well as presence of myaethenia gravis, which dictates earlier surgical excision <sup>9</sup>. For thymoma local excision usually suffices, and complete excision usually results in cure. For invasive thymoma, surgery may still be contemplated if complete excision is thought possible. If not, then down-staging with preoperative chemotherapy may be employed <sup>10</sup>.</p><p>Surgical resection can be achieved via a number of approaches <sup>9</sup>:</p><ul>
-</ul><p>Radiotherapy is not usually employed for stage I thymomas, but has a role in both postsurgical management of resected invasive thymoma or for inoperable invasive tumours, including thymic carcinoma <sup>10</sup>.</p><p>Prognosis is significantly influenced by histological type and surgical staging (e.g. <a href="/articles/masaoka-staging-system">Masaoka staging system</a>). </p><ul>
+</ul><p>Radiotherapy is not usually employed for stage I thymomas, but has a role in both postsurgical management of resected invasive thymoma or for inoperable invasive tumours, including thymic carcinoma <sup>10</sup>.</p><p>Prognosis is significantly influenced by histological type and surgical staging (e.g. <a href="/articles/masaoka-staging-system-of-thymoma">Masaoka staging system</a>). </p><ul>
~~-<a href="/articles/thymic-epithelial-tumours-who-classification-scheme">type B2 and B3</a> tumours have the intermediate survival</li>~~
+<a href="/articles/thymic-epithelial-tumours-who-classification-scheme">type B2 and B3</a> tumours have intermediate survival</li>
~~-<a href="/articles/thymic-rebound-hyperplasia">thymic rebound</a>: e.g. postchemotherapy</li>~~
+<a href="/articles/thymic-rebound-hyperplasia">thymic rebound</a>, e.g. postchemotherapy</li>

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