Applied Radiology

Prepared by Jyotsna E. Rao, MD; José R. Barreras, MD; Stephen J. Pomeranz, MD; and Parshan S. Ramsingh, MD, Department of Nuclear Medicine, The Christ Hospital, Cincinnati, OH.

CASE SUMMARY:

A 48-year-old woman with no significant medical history presented with a cough. A chest x-ray was performed, revealing a mass in the left hilum extending into the anterior mediastinum. Computed tomography (CT) and a fluorine-18-fluorodeoxyglucose (FDG) positron emission tomography (PET) scan of the chest were done. What is the most likely diagnosis?

DIAGNOSIS:

Surgical excision of the mass showed a grade 1 minimally invasive malignant thymoma.

IMAGING FINDINGS:

The chest CT revealed a 5.5 * 3 cm mass in the left anterior mediastinum, along the lateral margin of the main pulmonary artery. The PET scan showed a moderate sized focal, tumor-grade, hypermetabolic focus in the left anterior mediastinum at the level of the left hilum, corresponding with the abnormality noted on the chest x-ray and CT scan. The tumor/background ratio was calculated at 2.00, compared to 1.4 to 1.6 in the normal tissue/background. No other tumor-grade hypermetabolic focus was noted.

The differential diagnosis of malignant thymoma includes, among nonthymic tumors, thyroid or parathyroid tumors, parathymic lymphomas, aneurysms, myxomas, lipomas, paraganglionomas, bronchogenic cysts, hemangiopericytomas, and giant lymph node hyperplasias. Thymic tumors include thymic cancer, lymphomas, germ cell tumors, thymic carcinoid tumors, thymolipomas, thymic cysts, and metastases to the thymus.

DISCUSSION:

The thymus, a central lymphoid organ, arises embryologically from the third and fourth branchial clefts and migrates to the anterior mediastinum as a bilobed organ. It enlarges until late puberty and involutes by the fifth and sixth decades of life. The cortex is rich in lymphocytes and the medulla in epithelial cells. ¹

Thymomas are rare epithelial tumors seen in the anterosuperior mediastinum; fewer than 1000 cases have been reported. These lesions are slow growing. Incidence, which generally peaks in the fourth and fifth decades of life, is equal in men and women. In children, however, the lesions are invasive and aggressive.

Thirty percent of patients with thymomas are asymptomatic; their lesions are discovered incidentally on a chest x-ray. Thirty to forty percent present with symptoms such as cough, chest pain, dyspnea hoarseness, neck mass, and SVC syndrome. ¹ Thymomas can be associated with paraneoplastic syndromes such as myasthenia gravis (as is the case in one-third to one-half of patients), or pure red cell aplasia (occurring in 5%), especially in patients with the spinal cell type and hypogammaglobulinemia (5 to 10%). Most metastases are intrathoracic. However, extrathoracic metastases to bone, liver, and lymph nodes have been reported.

When a chest x-ray abnormality is detected, CT imaging is the next step. The use of thallium-201, gallium-67, or technetium sodium pertechnetateium also has been reported in detecting thymomas. Treatment in all stages is by total thymectomy; radiation therapy and chemotherapy also can be used, the latter including corticosteroids. ²

Normal thymic uptake of FDG may be seen in patients who are between age 2 and 13. ³ However, as most patients with thymomas are between forty and fifty, this may not be a problem in diagnosis. Review of the literature shows that thymic cancer and invasive malignant thymoma showed high FDG uptake, while noninvasive thymomas showed low uptake. As FDG uptake is higher in Go/G1 and G2 phases of the cycle, uptake can be used as biological marker of clinical behavior. Classification by PET agrees with clinical classification, which may prove useful because invasive behavior is the deciding factor of benignity or malignancy and helps in staging. ⁴ Diffuse, low grade uptake has been noted in thymic hyperplasia, focal and localized uptake in stages I and II of thymoma, and multiple discrete foci in the mediastinum and thoracic structures in stages II and IV. ⁵

We wish to emphasize that FDG PET can complement the role of CT in presurgical evaluation of thymomas in deciding the invasiveness and staging of a thymoma, and is also useful in distinguishing between thymomas and thymic hyperplasia.

References

1. Cecil RL, Bennett F, Claude J (eds): Diseases of the thymus. In: Cecil's Textbook of Medicine, ed 19, pp 1485-1487. Philadelphia, WB Saunders Co., 1988.

2. Chahiniam AP: Thymomas. Cancer Medicine, ed 3, pp 1385-1361. Lea & Febriger.

3. Patel PM, Alibazoglu H, Ali A, et al: Normal thymic uptake of FDG in PET imaging. Clin Nucl Med 21(10):772-775, 1996.

4. Kubota K, Yamada S, Kondo T, et al: PET imaging of primary mediastinal tumors. Br J Cancer 73(7):882-886, 1996.

5. Use of fluorine-18-fluorodeoxyglucose positron emission tomography in the detection of thymoma: A preliminary report. Eur J Med 22(12):1402-1407, 1995.