British Journal of Radiology (2006) 79, e78-e80
© 2006 British Institute of Radiology
doi: 10.1259/bjr/26663397
High resolution MRI in the detection of an intrathymic parathyroid adenoma
G Abikhzer, MDCM1,
M Levental, MDCM, FRCPC2 and
C Rush, MDCM, FRCPC3
1Department of Nuclear Medicine, McGill UniversityDepartments of, 2Radiology and 3Nuclear Medicine, Sir Mortimer B. Davis - Jewish General Hospital, 3755 Côte Ste. Catherine Road, Montreal, Quebec, H3T 1E2, Canada
Correspondence: Dr Mark Levental
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Abstract
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The most common cause of primary hyperparathyroidism is parathyroid adenoma. Of these, up to 20% have been reported to be in ectopic locations. Multiple imaging techniques have been utilized in the detection of ectopic adenoma and are discussed. We report on a case of an intrathymic parathyroid adenoma that was detected through the novel use of a breast coil in performing a high resolution MRI. The accurate localization permitted minimally invasive surgery, obviating morbidity associated with a sternotomy.
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Introduction
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Primary hyperparathyroidism is an important cause of morbidity and has been shown to increase the incidence of cardiovascular abnormalities, diabetes mellitus and urogenital diseases, as well as to increase the risk of mortality [1]. Although 54% of patients are asymptomatic at diagnosis, 58% of all patients exhibit some signs of bone disease [2]. Of those who are symptomatic, the most common symptoms include fatigue and mental status changes, renal calculi and gastric ulcers.
We recently encountered a patient with primary hyperparathyroidism in whom a Technetium (Tc) 99m Sestamibi scan (MIBI) revealed increased uptake in the superior mediastinum. CT imaging failed to reveal the exact location of the lesion while standard MRI suggested an anterior mediastinal location. We report on the novel use of a breast coil in performing a high resolution MRI which accurately localized the lesion, allowing minimally invasive surgery.
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Case report
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A 38-year-old woman presented with a palpable thyroid nodule. Investigation revealed a papillary carcinoma of the thyroid. Patient underwent total thyroidectomy with preservation of the parathyroid (PTH) glands. Routine studies revealed hypercalcaemia of 2.87 (2.12–2.62) mmol l–1 and PTH level of 86 (10–65) pg ml–1. MIBI scan (Millenium VG; GE Medical Systems, Milwaukee, WI; Cardiolite; Bristol-Myers-Squibb, New York, NY) revealed no abnormal activity in the neck, but a 2 cm focus of intense uptake in the superior mediastinum (Figure 1
). The increased uptake was felt to represent either a parathyroid adenoma or a pathologic lymph node.
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Figure 1. Anterior planar99Tcm Sestamibi image of the neck and thorax taken 10 min post-tracer injection. There is no focal tracer uptake in the neck region. There is an intense focus of abnormal tracer uptake projecting in the plane of the superior mediastinum consistent with an ectopic parathyroid adenoma.
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In order to further localize the lesion pre-operatively, the patient underwent uninfused CT of the thorax, which failed to reveal any abnormality. A standard thoracic MRI (Intera; Philips Medical Systems, Eindhoven, The Netherlands) suggested a possible lesion in the anterior mediastinum (Figure 2).
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Figure 2. Axial fat saturatedT2 weighted (repetition time 1714, echo time 80, slice thickness 4 mm, skip 1 mm, matrix 159x256) image through mid thorax with respiratory and cardiac gating using the phased array coil. There is a subcentimetric focus of hyperintensity within the residual atrophic thymic tissue, lying just anterior to the ascending aorta. There is some failure of fat suppression and/or slow flow in veins in the subcarinal mediastinum.
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A final attempt was made to accurately localize the lesion pre-operatively by performing a high resolution MRI of the anterior mediastinum using a breast coil (Bilateral breast coil; Phillips Medical Systems). T1 weighted, T2 weighted, fat saturated T2 weighted and post-gadolinium infused T1 weighted fat saturated images were obtained. These revealed a sharply defined 9 mmx5 mm focus, isointense on T1 weighting, hyperintense on T2 weighing fat saturated images (Figure 3) and which showed enhancement with contrast administration. The lesion was in keeping with a parathyroid adenoma and was accurately localized in the anterior mediastinum fat in the area corresponding to residual thymic tissue. The lesion appeared to be 4.5 cm inferior to the sternal notch, abutting the anterior wall of the ascending aorta. This allowed the Cardiothoracic Surgeon to perform a small window overlying the third coastal cartilage, which was removed together with a small segment of the adjacent rib, permitting the resection of the lesion in toto with immediate intraoperative decline of PTH levels. Pathological diagnosis of fat depleted parathyroid admixed with fragments of thymus was made intraoperatively. The surgery lasted 85 min.
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Figure 3. Axial fat saturatedT2 weighted (repetition time 5043, echo time 90, slice thickness 3 mm, skip 0 mm, matrix 256x512) image through mid thorax using the breast coil. There is a 9 mm very clearly defined focus of hyperintensity in the residual thymus tissue of the anterior mediastinum, representing ectopic parathyroid adenoma.
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Pre-operative PTH levels were 174 pg ml–1 and Ca levels were up to 3.05 mmol l–1. The levels subsequently declined to 17 pg ml–1 and 2.05 mmol l–1, respectively, within 12 h post-surgery. The patient made an uneventful recovery and was discharged on post-operative day 1. She remains clinically and biochemically normal.
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Discussion
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Primary hyperparathyroidism is the most common cause of hypercalcaemia with an annual incidence of 27.7 per 100 000 [3]. Of these cases, 87% are attributable to a solitary adenoma, 3% to multiple adenomata, 9% to hyperplasia, 1% to parathyroid carcinoma [4]. 5% of adenomata have an ectopic location. Sites of ectopic localization include thymus, tracheo-oesophageal groove, carotid sheath, intrathyroidal and paraoesophageal locations [2]. This is explained by the common embryological origin of both the inferior parathyroid gland and thymus from the third pharyngeal pouch, both migrating caudally to their respective locations. [5].
Historically, 95% of cases primary hyperparathyroidism were diagnosed and cured by a bilateral cervical exploration. The remaining 5% of cases required additional surgery with associated cost and morbidity. Subsequent surgeries without pre-operative localization studies had a 38% failure rate [6]. Pre-operative localization has allowed for minimally invasive surgery to be performed with decreased mean operative time [4] and a 1.2% complication rate, compared with 3% when no imaging is used [7].
Current imaging strategies for detection of parathyroid adenomata include ultrasound, 99Tcm-MIBI, CT and MRI.
Ultrasound has a 56% [8] to 74% [9] success rate for localization of orthotopic adenomata, but has not been shown to be useful in locating ectopic glands in the mediastinum [10].
Many studies have shown the cost effectiveness of pre-operative localization with MIBI in all hyperparathyroid patients [2, 4]. It has a sensitivity of 75% [8] to 92% [2] with a positive predictive value of 91% [2], permits a total body scan, but suffers from poor spatial resolution and 9% false positive rate mostly due to concomitant thyroid abnormalities [2]. The poor anatomic information regarding the relationship of the adenoma with surrounding structures renders minimally invasive surgery difficult [9], a procedure used by 59% of endocrine surgeons according to a recent survey [11].
CT has 40% [12] to 86% sensitivity, depending on the size of the lesion, and is capable of detecting mediastinal adenomata [13]. MRI is preferable to CT scanning due to the greater sensitivity, lack of ionizing radiation, multiplanar capability and superior tissue contrast. On T1 weighted images an adenoma appears as low signal intensity, isointense relative to muscle. High signal intensity is most commonly observed on T2 weighted images and lesions are particularly visible when fat saturation is applied. Hyperfunctioning parathyroid glands characteristically show intense contrast enhancement on T1 weighted images. MRI has shown promise in patients with persistent or recurrent hyperparathyroidism after previous surgery or in patients where an initial MIBI scan revealed an ectopic gland. MRI has an overall sensitivity of 77% [14] to 82% and positive predictive value of 89% [15]. MRI sensitivity in correctly locating mediastinal ectopic adenomata is 88% and may reach 100% sensitivity in cases of intrathymic ectopic adenomata [15]. MRI has a 1.6% rate of false positive results, typically due to concomitant thyroid disease or enlarged lymph nodes [14], the latter mimicking the appearance of an adenoma. The combination of a MIBI scan and MRI can combine a functional imaging technique with a precise anatomical scan and has been shown to increase sensitivity to 94% with a positive predictive value of 98% in patients with persistent hyperparathyroidism [15].
In our patient, standard thoracic MRI performed with fat saturated T2 weighted imaging suggested a possible lesion in the anterior mediastinum. We attempted, however, to obtain a higher resolution assessment of the anterior mediastinum in order to provide a detailed roadmap for minimally invasive surgery.
Utilizing a breast coil, 256x512 matrix and volume shimming yielded homogeneous, high resolution fat saturated imaging of the anterior mediastinum and revealed a <1 cm lesion very accurately whereas traditionally, MRI has had difficulty in locating lesions less than 1.5 cm in diameter [10]. The advantage of using a breast coil in a woman is that it allows for close contact of the receiving antenna with the chest wall, whereas in a typical thoracic MRI using a phased array body coil breast tissue displaces the coil anteriorly and may interfere with homogeneous fat saturated image formation.
Given the morbidity of open sternotomy and surgical exploration, the ability to precisely localize this small lesion allowed a very limited, minimally invasive surgical intervention with a short operating time and improved cosmetic result. The patient was discharged 1 day post-surgery in contradistinction to the average hospital stay of 14 days when sternotomy is used for excision of ectopic mediastinal adenomata [16].
Our case report supports the conclusion of previous studies in the literature, that pre-operative imaging of hyperparathyroidism allows minimally invasive surgical intervention [2, 4]. We propose that in cases of anterior mediastinal lesions either not detected or not clearly delineated by routine thoracic MRI, that the study be performed with high resolution surface coil to increase chances of detecting a small lesion.
Received for publication February 11, 2005.
Revision received August 31, 2005.
Accepted for publication September 6, 2005.
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Abstract
Introduction
