British Journal of Radiology (2007) 80, e72-e74
© 2007 British Institute of Radiology
doi: 10.1259/bjr/74258606
Detection of an occult thymoma on 99Tcm-Tetrofosmin myocardial scintigraphy
M Hawkins, BSc, MSc, MRCSEd
1
A F Scarsbrook, FRCR
1
S Pavlitchouk, MD, PhD
2
N R Moore, MA, FRCCP, FRCR
1 and
K M Bradley, MA, FRCCP, FRCR
1
1 Department of Radiology and 2 Department of Nuclear Cardiology, John Radcliffe Hospital, Headley Way, Headington, Oxford OX3 9DU, UK
Correspondence: Dr Kevin Bradley, Consultant Radiologist & Nuclear Medicine Physician, Department of Radiology, John Radcliffe Hospital, Headley Way, Headington, Oxford OX3 9DU, UK. E-mail: kevin.bradley{at}orh.nhs.uk
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Abstract
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Thymoma is an uncommon tumour, which is often asymptomatic and may be detected coincidentally on cross-sectional imaging performed for other reasons. We report a case of thymoma detected on myocardial scintigraphy performed for suspected coronary artery disease. Extracardiac pathology may be suggested by abnormal non-cardiac tracer uptake on myocardial scintigraphy. An awareness of the potential causes of this is essential to allow potentially life threatening non-cardiac disorders to be recognized and treated promptly.
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Introduction
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Thymomas are rare tumours arising from the epithelial cells of the thymus gland. They are often asymptomatic or have a non-specific presentation. We report the case of a 72-year-old man with symptoms suggestive of coronary artery disease, who had extracardiac intrathoracic tracer uptake on a Technetium-99m (99Tcm) Tetrofosmin myocardial perfusion scintigram (MPS) which was subsequently found to be due to a thymoma. A discussion and brief review of the literature follows emphasising the importance of recognizing extracardiac findings on myocardial perfusion scintigraphy.
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Case history
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A 72-year-old male was referred from his general practitioner to the cardiology outpatient clinic with a 5 month history of shortness of breath on exertion. There was no associated chest pain, cough, orthopnoea or paroxysmal dyspnoea. Clinical examination was unremarkable. Electrocardiography (ECG) showed sinus rhythm with T wave inversion in leads I, aVL and V4 to V6 consistent with left ventricular strain. A treadmill exercise ECG was equivocal. Stress and rest MPS using 99Tcm-Tetrofosmin showed no evidence of inducible myocardial ischaemia, but a single abnormal focus of tracer uptake was noted within the mediastinum (Figure 1
). CT of the thorax demonstrated a lobulated 5.6 cmx3.4 cm homogeneous soft tissue mass within the anterior mediastinum with no evidence of local invasion (Figure 2). There was no significant hilar or mediastinal lymphadenopathy and normal appearances of the liver and lungs.
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Figure 1. A 72-year-old man with non-specific shortness of breath. Coronal images from the source data of a 99Tcm-Tetrofosmin myocardial scintigram show an abnormal focus of uptake within the mediastinum (arrows). Physiological tracer uptake is also seen within the heart, stomach, bowel and gallbladder.
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Figure 2. A 72-year-old man with non-specific shortness of breath. Non-contrast-enhanced CT showing the superior component of a lobulated homogeneous soft tissue mass within the anterior mediastinum. A percutaneous CT guided cutting needle biopsy of the mass confirmed this to be a thymoma (arrow).
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The mass was biopsied percutaneously under CT guidance via a left parasternal approach (Figure 2
). Histology revealed thick bands of collagenous material separating lobules of small to medium sized lymphoid cells consistent with a predominantly cortical thymoma (Type B1), although the samples were noted to be too small to categorically classify the histological subtype. The patient subsequently underwent thoracotomy and complete excision of the tumour and had an uneventful recovery. Final histology showed a mixed thymoma (Type AB).
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Discussion
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Thymic neoplasia is the most common cause of an anterior mediastinal mass [1]. A number of benign and malignant lesions arise from the three thymic embryonic germ cell layers. Of these thymoma is the most common and has an epithelial origin. Thymoma has distinct clinicopathological features and the World Health Organization (WHO) divides them into six categories. The first two types: Type A (medullary thymoma) and Type AB (mixed thymoma) are considered benign. The remainder: Type B1 (predominantly cortical thymoma), Type B2 (cortical thymoma), Type B3 (well differentiated thymic carcinoma) and Type C (thymic carcinoma), demonstrate malignant features. Other rarer thymic lesions include congenital or acquired cysts, lymphoid hyperplasia and carcinoid tumours which may have a similar appearance as may mediastinal lesions of a non-thymic origin [1]. Cross-sectional imaging helps differentiate thymic tumours from other causes of an anterior mediastinal mass and is essential for accurate staging prior to treatment. Although CT has a limited value in differentiating histological subtypes of thymic tumour, certain findings such as local invasion or pleural seeding are associated with a high recurrence rate [2, 3]. It is important to emphasise that a thymic tumour may be coincidentally detected on other imaging modalities during investigation due to their often asymptomatic presentation.
Like many patients with a thymoma our patient presented with non-specific symptoms and minimal clinical signs. Between 20% and 50% of patients with a thymoma are asymptomatic and the condition is often coincidentally detected during cross-sectional imaging [1, 4]. In patients who are symptomatic, these are usually non-specific and can include chest pain (23%), symptoms attributable to myasthenia gravis (17%) or a cough (15%) [1, 4].
MPS is widely used to aid decision making in the investigation of suspected coronary artery disease following indeterminate exercise ECG testing. The positive and negative predictive value of exercise ECG testing is poor compared with MPS and scintigraphy has proven cost effectiveness when used as the initial investigation in patients with suspected coronary artery disease [5, 6] and has a high sensitivity (91%) and specificity (89%) [5, 7].
Incidental scintigraphic detection of a thymoma has been described on MPS performed using Thallium-201 and 99Tcm-2-methoxy-isobutyl-isonitrile (MIBI) tracers [8, 9]. In the current case a different tracer, 99Tcm-1, 2-bis (bis (2-ethoxyethyl) phosphino) ethane (Tetrofosmin) was taken up by the thymoma. Other causes of abnormal mediastinal Tetrofosmin uptake on myocardial scintigraphy have been reported including lymphoma, metastatic lymphadenopathy from non-small cell lung cancer, metastatic seminona and small cell carcinoma [11, 12].
The radiopharmaceuticals most commonly used for MPS include Thallium (Thallium-201) and the technetium labelled tracers 99Tcm-MIBI and 99Tcm-Tetrofosmin. There is little difference in the accuracy between the different tracers [10], although the radiation dose is significantly lower from technetium labelled tracers. The exact mechanism of tracer uptake within tissues is unknown but is dependent on both cell and mitochondrial membrane potentials [11]. Both MIBI and Tetrofosmin are thought to localize within mitochondria [11, 12] which may explain the increased uptake within tumours. The mid and lower thorax and upper abdomen are included within the acquisition field of view during single photon emission computed tomography (SPECT) MPS and the source data should always be reviewed as occasionally valuable additional information on non-cardiac pathology may be suggested by extracardiac uptake, as in this case. A number of intrathoracic, non-cardiac pathologies have been reported as incidental findings during MPS including mediastinal tumours [13], benign and malignant breast lesions [14, 15] and focal lung disorders [15, 16]. Tetrofosmin is predominantly cleared via biliary excretion and abnormalities of the diaphragm, liver, gallbladder, kidney, spleen and bone marrow have all been detected on MPS [17].
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Conclusion
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Extracardiac, intrathoracic tracer uptake on MPS is an infrequent but often significant finding. Radiologists who interpret myocardial scintigrams should be aware of the causes of non-cardiac uptake so that potentially serious pathologies can be promptly diagnosed and treated accordingly. Asymptomatic, intrathoracic malignancy may be detected coincidentally and early recognition may be life saving.
Received for publication October 13, 2005.
Accepted for publication April 3, 2006.
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References
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Abstract
Introduction
