AJR 2004; 182:511-513
© American Roentgen Ray Society
Positron Emission Tomography with FDG to Show Thymic Carcinoid
Ashley M. Groves1,
Hosahalli K. Mohan1,
Eva A. Wegner2,
Sharon F. Hain1,
John B. Bingham3 and
Susan E. M. Clarke1
1 Department of Nuclear Medicine, Guy's and St. Thomas' Hospital NHS Trust, St.
Thomas St., London SE1 9RT, England.
2 PET Centre, Guy's and St. Thomas' Hospital NHS Trust, St. Thomas St., London
SE1 9RT, England.
3 Department of Radiology, Guy's and St. Thomas' Hospital NHS Trust, St. Thomas
St., London SE1 9RT, England.
Received May 29, 2003;
accepted after revision July 14, 2003.
Address correspondence to A. M. Groves.
Introduction
Thymic carcinoid was first recognized in 1972
[1] as a result of advances in
microscopic and histologic techniques. Since then, thymic carcinoids have been
reported periodically [2]. The
imaging appearances have been described on CT
[3] and on scintigraphy using
agents such as iodine-123 metaiodobenzylguanidine (MIBG)
[4]. We are unaware of any
reports of imaging thymic carcinoid using FDG positron emission tomography
(PET). We describe a case of carcinoid of the thymus imaged on FDG PET and
provide correlative images from radiography, CT, MRI, technetium-99m methylene
diphosphonate (99mTc MDP) bone scintigraphy, and
123I-MIBG SPECT (Table
1).
Case Report
A 35-year-old man presented with a 1-year insidious history of weakness,
bloating, generalized aches, back pain, and mood disturbance. He also reported
easy bruising, a darkened mouth, altered taste, and bloating of the face.
Clinical examination revealed that the patient had a typical cushingoid
appearance. He had tachycardia and was hypertensive at admission. His blood
biochemistry findings were notable for a high level of basal
adrenocorticotropic hormone (ACTH) that did not improve after administration
of a high dose of dexamethasone.
A chest radiograph revealed mediastinal widening. MRI of the brain showed a
normal pituitary fossa. Body CT showed a mediastinal mass
(Fig. 1A) and bilateral adrenal
enlargement. Under CT guidance, the mass was biopsied; histology revealed an
atypical carcinoid of thymic origin, with strong staining for ACTH receptors.
Radiographs of the spine showed sclerotic vertebral bodies at C5 and L4 (Figs.
1B and
1C). On spinal MRI, low-signal
marrow in the corresponding vertebrae was observed on all sequences
(Fig. 1D). The L4 vertebra was
also associated with a soft-tissue mass that extended posteriorly into the
spinal canal with thecal compression. The patient then underwent an FDG PET
examination. Local attenuation-corrected views of the chest revealed high FDG
uptake in the anterior mediastinum. On the nonattenuated whole-body PET views
(Figs. 1E,
1F,
1G), additional increased FDG
uptake was observed in the area corresponding to the soft-tissue mass
posterior to L4, although no abnormal accumulation of FDG appeared in the
vertebrae. Lower-grade FDG uptake was also seen bilaterally in the region of
the adrenals, which would be consistent with adrenal hyperplasia.
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Fig. 1A. —35-year-old man with cushingoid appearance. Axial thoracic CT
image obtained after IV contrast medium administration reveals anterior
mediastinal mass in front of ascending aorta (arrow).
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Fig. 1D. —35-year-old man with cushingoid appearance. T1-weighted
sagittal image of cervical spine shows reduced signal in marrow of C5.
Anterior mediastinal mass (arrow) appears as intermediate-signal
structure seen behind sternomanubrial joint and anterior to great vessels.
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Fig. 1E. —35-year-old man with cushingoid appearance. Orthogonal images
from non–attenuation-corrected whole-body positron emission tomography
examination (ECAT 951R, Siemens, Erlangen, Germany) show increased FDG uptake
in anterior mediastinum. Increased FDG uptake is also seen in soft tissue
posterior to L4 vertebra on sagittal view (G).
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Fig. 1F. —35-year-old man with cushingoid appearance. Orthogonal images
from non–attenuation-corrected whole-body positron emission tomography
examination (ECAT 951R, Siemens, Erlangen, Germany) show increased FDG uptake
in anterior mediastinum. Increased FDG uptake is also seen in soft tissue
posterior to L4 vertebra on sagittal view (G).
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Fig. 1G. —35-year-old man with cushingoid appearance. Orthogonal images
from non–attenuation-corrected whole-body positron emission tomography
examination (ECAT 951R, Siemens, Erlangen, Germany) show increased FDG uptake
in anterior mediastinum. Increased FDG uptake is also seen in soft tissue
posterior to L4 vertebra on sagittal view (G).
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Whole-body 123I MIBG SPECT was performed to assess therapeutic
options, and it confirmed tumor radiotracer uptake in the mediastinum and in
L4 but no uptake in C5. Bone scintigraphy using 99mTc MDP was
performed in an attempt to resolve the discrepancy between the different
spinal imaging results. Bone scintigrams confirmed the lesions in C5 and L4
(Figs. 1H and
1I) and revealed additional
lesions, some of which were outside the MRI field of view and some of which
were not apparent on CT. Finally, biopsy of L4 revealed an atypical metastatic
carcinoid with the same histologic findings as those in the mediastinal
biopsy.
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Fig. 1H. —35-year-old man with cushingoid appearance. Whole-body
technetium-99m methylene diphosphonate–labeled bone scintigrams show
increased radiotracer uptake in region of C5 and L4 vertebral bodies. Multiple
other lesions are identified.
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Fig. 1I. —35-year-old man with cushingoid appearance. Whole-body
technetium-99m methylene diphosphonate–labeled bone scintigrams show
increased radiotracer uptake in region of C5 and L4 vertebral bodies. Multiple
other lesions are identified.
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Discussion
Carcinoid is a rare neoplasm accounting for up to 2% of intrathoracic
tumors [5]. Imaging
neuroendocrine neoplasms such as carcinoids with FDG PET is usually of limited
value because they are frequently indolent tumors that metabolize slowly
[5]. For this reason,
receptor-targeted imaging using agents such as indium-111 octreotide has been
tested and is claimed to be superior to FDG PET
[6]. The fact that FDG was
markedly taken up in this case suggests that the carcinoid was behaving
aggressively. Indeed, it is recognized that thymic carcinoids are more
malignant and metastasize more readily than carcinoids originating elsewhere
[2].
The failure of both 123I MIBG SPECTand PET to detect the
cervical and other skeletal abnormalities is consistent with a recent
investigation that claimed that MRI and conventional bone scintigraphy are the
most sensitive methods for detecting skeletal carcinoid metastases
[7]. Such discrepant findings
are also observed after 123I MIBG treatment, implying that there
may be heterogeneity of tumor differentiation at distal sites
[8] or that small-volume
disease is not visualized well on 123I MIBG SPECT.
The uptake pattern on 123I MIBG has an important therapeutic
implication. The areas that show uptake may respond to subsequent
131I MIBG therapy. In areas such as the C5 vertebra that showed no
123I MIBG uptake, a significant regional response is unlikely
[8]. Such tumor sites will be
metabolically inactive, and the patient should achieve a reasonably long
period of remission because of the indolent nature of most carcinoids.
This case shows imaging of a rare tumor using FDG PET and confirms that the
use of more expensive techniques does not necessarily improve the imaging of
certain tumors and may even be misleading because PET images failed to detect
the skeletal abnormalities.
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Introduction
Case Report
