Marom, Edith M. MD*; Rosado-de-Christenson, Melissa L. MD†‡; Bruzzi, John F. FFRRCSI§; Hara, Masaki MD∥; Sonett, Joshua R. MD¶; Ketai, Loren MD#
There is a growing demand for structured reporting in radiology and for the formulation of standard terms to be used by clinicians and radiologists alike. A study focusing on computed tomography (CT) reporting of lung nodules found inconsistency of the reporting of their margins and presence of calcifications.1 Another study, comparing imaging terms listed in the Fleischner Society Glossary2 with their use in medical lexicons including the International Classification of Diseases, 9th revision, Clinical Modification (Center of Medicare and Medicaid Services, Washington, DC), Systematized Nonmenclature of Medicine–Reference Terminology (SNOMED-RT) (College of American Pathologists, Northfield, IL), and the Unified Medical Language System (National Library of Medicine, Bethesda, MD), found low rates of utilization of Fleischner terms ranging from 3 to 36%.3 On the other hand, when standardized reporting is implemented for imaging a specific disease, such as seen in the screening for breast cancer, this leads to improved patient care. It is expected that the creation of a standardized terminology for the description of an anterior mediastinal mass suspicious for thymoma and the promotion of its use will result in improved communication between the clinician and the radiologist and will ultimately positively impact patient care.
In addition to developing a common language, including pertinent information in the radiologic report that will influence therapy requires knowledge of the disease and is often disease specific. Because of this, standardized reporting has been created for specific diseases in a few organs and has been proven useful. The American College of Radiology developed the Breast Imaging Reporting and Data System4 to characterize breast lesions seen on mammography and breast ultrasound in a standardized manner that correlates with the underlying histologic findings. The Breast Imaging Reporting and Data System assigns a percentage probability of malignancy to each category and has gained worldwide acceptance in its use for guiding clinical management.5 A similar system has been developed for the ultrasound evaluation of thyroid nodules, stratifying them into those with imaging findings more likely to be malignant and those less likely to be malignant with suggestions for appropriate clinical management called Thyroid Imaging and Reporting Data System.6
Although there is preliminary evidence to suggest that certain imaging findings in thymoma are important for staging or prognosis, further validation will require the prospective acquisition of data, which will be facilitated by the creation of a structured radiology report. The following guidelines contain descriptors that should be included in CT reports of patients with anterior mediastinal masses in whom thymoma is considered in the differential diagnosis. Corresponding representative images are included for clarity and consistency. Pertinent negatives with regard to these descriptors are just as important. Consistent use of these descriptors will help establish a common reporting language that will be used and accepted by clinicians as they make management decisions regarding their patients with thymoma and facilitate further research.
The process used in development of this document was designed to represent a broad consensus within the community of clinicians and researchers interested in thymic diseases. A core workgroup (Edith M. Marom, Melissa L. Rosado-de-Christenson, John F. Bruzzi, Masaki Hara, Joshua R. Sonett, and Loren Ketai) reviewed the existing literature as well as existing standards for chest radiology that applied or could be adapted to achieving consistency in reporting imaging findings in anterior mediastinal masses. This group drafted proposed standard report terms and definitions. These were refined at an International Thymic Malignancy Interest Group (ITMIG) Definition and Terminology workshop on November 16, 2010, which was supported by the International Association for the Study of Lung Cancer. After distribution to all ITMIG members for comment, the final document was approved and adopted by ITMIG members in February 2011.
Review of Existing Studies
The Masaoka staging system and its variants7,8 have been shown to strongly correlate with prognosis, 8–10 yet staging is often only established postoperatively. Patients with locally advanced thymoma may receive neoadjuvant chemotherapy to enable effective resection, 9,11 as complete resection, even of advanced disease, improves survival.7,12 Currently, it is recommended that patients with stage III and IV thymoma should receive neoadjuvant therapy.13–18 Historically, mediastinal imaging has been considered to have limited value in the staging of patients with thymoma. This may have been due to the limitations of older imaging techniques and the rarity of thymoma. These factors likely influenced scientific publication and limited the number of published case studies on the imaging of thymoma, which in turn decreased the statistical power of these studies.
CT is currently considered the preferred imaging modality for the initial assessment and follow-up of patients with thymoma.19 In the last decade, CT technique has improved dramatically resulting in routine rapid acquisition of thin-section slices enabling high quality image reformations in multiple planes. This resulted in improved visualization of these tumors, allowing assessment of internal lesion characteristics as well as detailed visualization of the tumor's relationship to surrounding structures.
Promising studies have been published in the last decade, taking advantage of modern CT imaging techniques. These studies have shown that some CT characteristics correlate with aggressive tumor behavior and higher stage. To the best of our knowledge, only two have correlated the CT appearance of thymoma with Masaoka staging.20,21,21a One study assessed 50 patients with thymoma21,21a and found that invasive thymomas were more likely to be larger and have low attenuation regions, calcifications, and lobulated and irregular contours when compared with low-stage thymomas. A later study assessed 99 patients with thymoma20 and found multiple factors associated with advanced disease (stages III and IV): large size, lobulated contours, heterogeneous attenuation, calcifications, infiltration of surrounding mediastinal fat, tumor abutting ≥50% of a mediastinal structure, adjacent lung abnormalities, and pleural effusion. However, after performing multivariate analysis, larger tumor size, lobulated contours, and fatty infiltration surrounding the tumor were the only imaging findings that were likely to correlate with higher stage disease, i.e., stage III or IV.20
Thymomas are classified histologically by the World Health Organization (WHO) classification. Although the clinical use of the WHO classification is debatable because of lack of adequate reproducibility and clinical predictive value that was found in some studies, 10 several CT studies have correlated CT appearance with the WHO histologic classification. Two studies that evaluated 45 and 76 patients with thymoma22,23 found that lobulated and irregular tumor contours were associated with more aggressive disease, although this was not confirmed by a third study24 that evaluated 48 patients with thymoma.
The above imaging studies are promising. In fact, some clinicians already use large tumor size, tumor heterogeneity, and tumor lobulation to help identify patients who should receive neoadjuvant therapy before attempted resection. However, much larger studies correlating the relationship of imaging findings of thymoma with its biologic behavior are needed. Such studies will have to be performed on an international basis as despite thymoma being the most common primary neoplasm of the anterior mediastinum, it only accounts for less than 1% of all adult malignancies.25
We believe that each CT report of a mediastinal mass suspicious for thymoma or of newly diagnosed thymoma should include the following data about the primary mass and its surrounding structures: lesion location and size in the x, y, and z axis, description of the lesion contour (smooth or lobulated), presence or absence of heterogeneous attenuation, calcifications, infiltration of surrounding mediastinal fat, and tumor abutting ≥50% of an adjacent mediastinal structure and direct invasion into a vessel lumen. The following information regarding the surrounding structures must also be included: diaphragmatic elevation (consistent with phrenic nerve involvement), adjacent lung abnormalities, pleural effusion, pleural nodule or nodules, lymph node enlargement, and findings suggestive of distant metastatic disease (i.e., lung, liver, adrenal, or peritoneal nodules). If these variables are prospectively and consistently recorded, they can be used to create a table or drop down menu to be used in future structured reports (Tables 1 and 2). In addition, if all these data are routinely captured in radiologic reports of cases of thymoma, retrospective studies could be performed using the information contained in these reports.
Definitions of Thymoma Report Terms
Primary Tumor Size
We recommend documenting the three axes of tumor size to mirror information that is consistently contained in pathology reports of excised thymomas. The axial slice chosen for measurement is that which demonstrates the longest tumor dimension. The short axis is perpendicular to the long axis on the same slice (Figure 1). Because tumor orientation does not always conform to strict sagittal or coronal reformats, the superior-inferior dimension of the tumor should be obtained by subtracting the lowest from the highest bed position in which the primary tumor is seen (Figure 1).
It is expected that most of these lesions will be located in the prevascular anterior mediastinum. Some of these lesions are unilateral, whereas others cross the midline involving both sides of the mediastinum.
A lesion contour is considered smooth in the absence of spiculation, ill-defined borders, or lobulation. Smooth lesions are typically spherical or ovoid in shape, but lesion contours may also conform to the shape of the adjacent mediastinum. A lobulated contour is one that exhibits one or more lobulations, characterized as convex tumor contours with adjacent notches between tumor lobules (Figure 2).
Thymomas may demonstrate homogeneous or heterogeneous attenuation. Heterogeneous attenuation often manifests as areas of low attenuation within the tumor and should be assessed on soft tissue or mediastinal windows. Administration of contrast will help demonstrate tumor heterogeneity and is recommended if not contraindicated (Figure 3). Cystic thymomas exhibit intrinsic low attenuation manifesting as homogeneous water attenuation surrounded by the soft tissue tumor capsule. These lesions may also exhibit internal soft tissue septa. The presence of mural soft tissue nodules in a cystic anterior mediastinal mass is one of the characteristic manifestations of cystic thymoma.
Calcifications of any pattern, including curvilinear, punctate, or coarse, have been associated with more advanced disease and should be described and characterized. Viewing the same image at a different window level, such as with bone window, may accentuate the differences between intravascular contrast and tumor calcification (Figure 4).
Infiltration of Surrounding Mediastinal Fat
For a tumor to be characterized as infiltrating the surrounding fat, it needs to only infiltrate the fat in one location and not necessarily along its entire circumference. Such neoplasms may exhibit irregular borders. Tumors that abut the mediastinal vessels without an intervening fat plane are not considered as infiltrating of surrounding fat, as the mediastinal fat that typically surrounds the vessel cannot be evaluated for infiltration if it cannot be visualized (Figure 5).
Tumor Abutting ≥50% of Mediastinal Structure
To maintain consistency in imaging reports, vessel abutment should be described as the percentage of the vessel circumference that is touched by the adjacent tumor without an intervening tissue plane (Figure 6).
Direct Vascular Invasion
Vascular invasion is rarely seen but may manifest as direct extension of the tumor into a vessel lumen (Figure 7). When present, any appreciable narrowing or deformity of the vessel lumen should be described as well.
Mediastinal Lymph Node Enlargement
The presence and nodal stations of mediastinal lymph node enlargement should be noted in the report, 26 as removal of any enlarged lymph nodes at surgery is recommended.27 The definition of mediastinal lymph node enlargement is a short-axis diameter of a lymph node greater than 1 cm on an axial image (Figure 8).
Adjacent Lung Abnormalities
Adjacent lung abnormalities such as intrapulmonary extension of tumor are rarely appreciated on CT and are usually detected intraoperatively. The most common pulmonary abnormality seen on chest CT is compressive atelectasis by the adjacent tumor, but this may be difficult to differentiate from direct extension of tumor into lung (Figure 9).
Pleural effusions are not common in patients with thymoma. However, presence or absence of pleural effusions should be documented on the report as they are more frequently associated with thymic carcinoma and metastatic pleural involvement by primary neoplasms other than thymoma (Figure 9).
Inclusion of the phrenic nerve in the resection may compromise pulmonary function and may lead to serious postoperative complications.28 Preoperative documentation of phrenic nerve involvement is of utmost importance as affected patients may receive preoperative chemotherapy before surgery to allow complete tumor resection of disease without resection of the phrenic nerve, thus leading to a favorable functional and survival benefit. Therefore, elevation of the hemidiaphragm should be documented in the report. In addition, tumor abutting the anatomic location of the course of the phrenic nerve should also be mentioned. The phrenic nerve courses over the brachiocephalic artery, posterior to the subclavian vein, and then crosses anterior to the hilum, over the pericardium covering the right atrium (right phrenic nerve) or left ventricle (left phrenic nerve) to the diaphragm, where it divides into branches which pierce that muscle and are distributed to its under surface (Figure 10).
Metastatic thymoma typically involves the pleura and manifests as soft tissue pleural nodules that range from small lentil-shaped nodules to large pleural masses and can progress to circumferential nodular pleural thickening with involvement of the interlobar fissures. Solid pleural metastases (stage IVa) should be distinguished from pulmonary parenchymal metastases (stage IVb). Pleural nodules are disposed along the anatomic location of the pleural surfaces and are best assessed on the lung window in cases of early disease. Intraparenchymal pulmonary nodules29 are completely surrounded by lung parenchyma (Figure 11).
Distant metastases are uncommon at presentation and constitute stage IVb disease. The most common site is the lung followed by the liver, lymph nodes, and bone.30
This article provides a lexicon of terms that should be consistently used for describing mediastinal masses suspected of representing a thymoma. Knowledge of the imaging features of high-stage thymomas as well as the correct use of these terms will add value to the CT report, will facilitate communication between clinicians and radiologists, and will allow the radiologist to play an important role in helping the clinician make management decisions for their patients with thymoma including the use of preoperative therapy. Documentation of the above imaging features of thymoma provides the basis for structured reporting and a database for collected newly diagnosed thymoma cases that permits further research.
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