Extended abstract: imaging workup of prevascular mediastinal tumors

Anterior (prevascular) mediastinal tumors can present with diverse characteristics on imaging that allow them to be differentiated and classified, subsequently benefitting therapy planning (1-3). When anterior mediastinal tumors present as diffuse enlargements, they can be benign, as seen in cases of thymic hyperplasia, or malignant, as often seen in cases of lymphoma. When anterior mediastinal tumors present as discrete masses, they can also be benign, as seen in cases of thymic cyst and thymolipoma, or malignant, as seen in cases of thymoma, thymic carcinoma, and thymic carcinoid. In this context, various aspects of imaging have been explored for the differentiation and classification of anterior mediastinal tumors. Moreover, the need for improved differentiation and classification sets the stage for novel imaging assessment methods like radiomics.

Currently, chest X-rays (CXR) and contrast-enhanced computed tomography (CT) are considered essential for identifying and evaluating anterior mediastinal tumors. CXR can identify 45–80% of mediastinal tumors, which usually present as a well-defined lobulated soft tissue density in the prevascular space, towards one side of the mediastinum. For the evaluation and characterization of anterior mediastinal tumors, contrast-enhanced CT is considered the best initial imaging modality. Magnetic resonance (MR) may be preferred if iodinated contrast is contraindicated, for differentiating cystic from solid masses, and for detecting intralesional fat. Fluorodeoxyglucose positron emission tomography/computed tomography (FDG PET/CT) is not typically used for routine evaluation due to its lack of specificity but can be valuable for staging and assessing disease extent.

We review the presentation of several different anterior mediastinal tumors on imaging below:

• Thymic hyperplasia, which can develop in response to stress or autoimmune conditions, can present as diffuse thymic enlargement with an arrowhead like morphology on CT and sometimes with intralesional fat. The differentiation of thymic hyperplasia, which is benign, from thymic tumors can be achieved using chemical shift sequences on magnetic resonance imaging (MRI), as the former is characterized by the loss of signal intensity on out-of-phase imaging stemming from the presence of microscopic fat (4).
• Thymic cysts are characterized by low density lesions on CT. However, a significant number of cysts do not present as hypodense, and MR provides a certain diagnosis by exhibiting T2-hyperintensity, sometimes with a fluid-fluid level suggesting a hemorrhagic or mucinous component.
• Thymomas appear as well-defined, rounded, or lobular masses with heterogeneous or cystic areas due to hemorrhage and necrosis, sometimes outlined by fat and containing punctate or coarse curvilinear calcifications.
• In contrast to thymomas, thymic carcinomas usually lack a well-defined capsule; can present as necrotic, cystic, or calcified lesions; and exhibit irregular contours. Distinguishing between these entities is not possible on imaging alone.
• Thymic neuroendocrine tumors may be large, invasive masses within the anterior mediastinum and are difficult to differentiate from other invasive thymic epithelial tumors. The use of gallium-68 DOTATATE (⁶⁸Ga-DOTATATE) PET/CT is valuable for imaging neuroendocrine tumors that express somatostatin receptors, aiding in tumor localization, detection of metastatic disease, monitoring of treatment effects, and the selection of patients for specific therapies.
  • Thymic lymphomas may involve the thymus as part of disseminated disease or manifest as an isolated mass, with the majority representing Hodgkin lymphoma.

For presurgical planning, differentiating benign thymic tumors from early-stage thymic malignancies on imaging is particularly crucial to avoid unnecessary invasive diagnostic procedures (5). CT is the imaging modality of choice for the initial evaluation of anterior mediastinal tumors. Features such as intralesional fat, midline location, and a triangular thymic shape can suggest a benign etiology. In contrast, the infiltration of mediastinal fat, especially in older patients or those with large masses, increases the likelihood of malignancy.

Additionally, predicting the invasiveness and the completeness of resection is crucial. Preoperative CT characteristics, such as a lobulated tumor contour, extensive vessel abutment, thoracic lymphadenopathy, adjacent lung changes, and pleural nodularity, can help determine the likelihood of successful surgical resection and identify patients who may benefit from neoadjuvant chemotherapy (6).

Radiomics, an evolving field, involves the computerized extraction of quantitative features from radiologic images, subsequently allowing for the analysis of various extracted quantitative features, including tumor size, tumor location, tumor shape, tumor vascularity, necrosis, and more. Several radiomic features have been associated with malignancy and have shown promise for the prediction of prognosis and response to therapy.

We recently published an article on the value of CT radiomic features to predict the pathologic classification of anterior mediastinal lesions (7). For the differentiation of benign from malignant lesions, the predictive model based on radiomic features slightly outperformed the predictive model based on conventional imaging features [area under the curve (AUC) of 0.678 and 0.605, respectively]; the predictive model that combined the best-performing conventional imaging and radiomic features demonstrated the best accuracy, with a moderate AUC of 0.715. For the differentiation between thymoma and thymic carcinoma, the predictive model based on conventional imaging features had a limited AUC of 0.558. On the other hand, the predictive model based on radiomic features demonstrated a moderate AUC of 0.774, and the predictive model based on the combination of both radiomics and conventional imaging features demonstrated a good AUC of 0.810.

In summary, this presentation covered various aspects of imaging for the differentiation and classification of anterior mediastinal tumors, emphasizing the importance of differentiating benign thymic tumors from early-stage thymic malignancies on imaging, in particular for presurgical planning, with CT as the imaging modality of choice for the initial evaluation of anterior mediastinal tumors. It also highlighted the potential of radiomics for improved tumor characterization and prognostication, offering a promising avenue for future research in the field of thoracic oncology.

Acknowledgments

Funding: This research was supported by the National Institutes of Health/National Cancer Institute Cancer Center Support Grant P30 CA008748 (to M.S.G.).

Footnote

Provenance and Peer Review: This article was commissioned by the Guest Editors (Malgorzata Szolkowska, Chul Kim, Mohammad Ashraghi, and Claudio Silva) for “The Series Dedicated to the 13th International Thymic Malignancy Interest Group Annual Meeting (ITMIG 2023)” published in Mediastinum. The article has undergone external peer review.

Peer Review File: Available at https://med.amegroups.com/article/view/10.21037/med-23-60/prf

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://med.amegroups.com/article/view/10.21037/med-23-60/coif). “The Series Dedicated to the 13th International Thymic Malignancy Interest Group Annual Meeting (ITMIG 2023)” was commissioned by the editorial office without any funding or sponsorship. M.S.G. reports grant support from the National Institutes of Health/National Cancer Institute Cancer Center Support Grant P30 CA008748. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

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