Thymoma

Diagnosis

Thymoma

Diagnosis

Thymoma

Findings

Hypermetabolic intrathoracic lesions involving the anterior mediastinum ( 17x 12 mm, SUVmax = 8), right paracardiac region (45 x 40 mm, SUVmax = 15.8), right cardiophrenic region (22 x 16 mm, SUVmax = 14.4). Discrete hypermetabolic pretracheal and precarenal lymph nodes (SUVmax 2.2 -3.3).
Seven millimeters lung nodule in the right lower lung field ( SUVmax = 0.8).
Activated brown fat.

Discussion

Malignant thymic lesions include thymoma, lymphoma, germ cell tumors, soft tissue lesions of the mediastinum, dendritic cell and myeloid neoplasms, carcinoid lesions and r other types of carcinomas. In children, lymphomas and germ cell tumors are the most common and thymomas infrequent.
The WHO histological classification of thymic tumors is based on the Masaoka-Koga system and subtypes are A, AB, B1-B3 and thymic carcinoma (C). A,AB and B1 lesions were termed low risk (stage I), B2-B3 high risk (stage II) With the exception of micronodular thymoma and some rare subtypes , all thymomas are now classified as malignant using the International Classification of Disease for Oncology (ICD-O). There is however deterioration of prognosis when progressing from A to C.
In addition to the histopathological classification, different staging system for local and distant disease exist: the French Groupe d'Etude des Tumeurs Thymiques system, the Masaoka-Koga system with the International Thymic Malignancy Interest Group (ITMIG) modifications and serial TNM based classification. The last TNM classification was recently endorsed by the WHO organization, ITMIG, the International Association for the Study of Lung Cancer, the American Joint Committe on Cancer and the Union for International Cancer Control.
Lesions invading the pericardium, mediastinal pleura, thoracic wall are staged as T3. Lymph nodes metastasis in location other than the anterior mediastinum are N2.
Those findings are in keeping with a TNM stage IVb. The advantage of the TNM classification is it can be applied to any thymic epithelial tumors and studies have found TNM to be superior to histomorphologic classification for prognosis.
Differential diagnosis of lesions in the anterior mediastinum includes thymic hyperplasia, thymoma, thymic carcinoma, lymphoma and germ cell tumors, all of those showing various degrees of FDG uptake. Global sensitivity to classify malignant disease in the anterior mediastinum has been reported to be 83% and specificity 58%. The majority of malignant lesions have SUV max superior to 2.0.
Most of the studies concerning the use of FDG PET/CT in the evaluation of thymic lesions use SUVmax for quantification but some use tumor to mediastinum ratios.
Studies have shown significantly higher FDG uptake in thymic carcinoma (TC) when compared to thymoma and thymic hyperplasia. Significant differences were found between thymoma and thymic hyperplasia. In some groups, FDG uptake was similar in low risk (A, AB, B1) and high risk (B2, B3) thymoma while in others the difference was significant. Homogenous uptake, presence of mediastinal fat invasion and SUVmax superior to 7 suggest thymic carcinoma. However SUVmax cut-off values as low as 4.58 have been reported for differentiation between thymoma and TC.
A recent meta-analysis confirmed significant differences between the various subgroups of thymic lesions but no SUVmax cut-off values to separate them.
FDG PET/CT allows identification of lymph nodes and distant metastasis.
Thymomas and thymic carcinomas are extremely rare in children. The retrospective analysis of the European database of rare tumors between 2000 and 2012 identified 16 children with thymoma and 20 with thymic carcinoma. In adults, myasthenia gravis (MG) is found in about half the patients with thymoma but in this cohort only two children had MG.
Only one case of pediatric type B1 localized thymoma imaged with FDG PET/CT has been reported. Uptake was low ( SUV max: 2.6).

Key points

Differential Diagnosis:
Lymphoma
Teratoma
Thyroid neoplasm
Thymoma
Sarcoma

References

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