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.
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.
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?
Surgical excision of the mass showed a grade 1 minimally
invasive malignant thymoma.
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.
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.
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.
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
Normal thymic uptake of FDG may be seen in patients who are
between age 2 and 13.
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
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.
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.
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 &
3. Patel PM, Alibazoglu H, Ali A, et al:
Normal thymic uptake of FDG in PET imaging. Clin Nucl Med
4. Kubota K, Yamada S, Kondo T, et al:
PET imaging of primary mediastinal tumors. Br J Cancer
Use of fluorine-18-fluorodeoxyglucose positron emission tomography
in the detection of thymoma: A preliminary report. Eur J Med