Eur J Cardiothorac Surg 2002;22:479-481
© 2002 Elsevier Science NL
Appearance of thymoma 15 years after extended thymectomy for myasthenia gravis without thymoma
Hirohisa Hirabayashi*,
Mitsunori Ohta,
Meinoshin Okumura,
Hikaru Matsuda
Department of Surgery, E1, Osaka University Graduate School of Medicine, 2-2, Yamadaoka, Suita, Osaka, 565-0871, Japan
Received 13 March 2002;
received in revised form 21 May 2002;
accepted 23 May 2002.
* Corresponding author. Tel.:+81-668793152; fax: +81-668793163
e-mail: hhirabar{at}surg1.med.osaka-u.ac.jp
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Abstract
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Several cases of thymoma recurrence after resection have been reported. However, thymoma appearance following an extended thymectomy for non-thymomatous myasthenia gravis is very rare. We report a case of thymoma in a 48-year-old woman, 15 years after an extended thymectomy for non-thymomatous myasthenia gravis. The importance of a complete dissection of mediastinal adipose tissue during the extended thymectomy as well as careful follow-up for such patients is also noted.
Key Words: Myasthenia gravis • Extended thymectomy • Thymoma • Recurrence
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1. Introduction
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Thymectomy is an accepted therapeutic option for treating myasthenia gravis, while an extended thymectomy includes the dissection of anterior mediastinal fatty tissue for treatment of the disease, based on the frequent existence of thymic tissue in anterior mediastinal adipose tissue around the thymus [1]. In fact, several reports have noted that residual thymic tissue has caused the recurrence of myasthenia gravis, because of incomplete removal of thymus glands or ectopic thymic tissue.
However, there are a few reports of primary thymoma arising from ectopic thymic tissue following a thymectomy [2]. We report a case of thymoma that appeared 15 years after an extended thymectomy for myasthenia gravis without thymoma. Although we could not re-examine the entire resected section from the first operation nor concluded whether the later tumor originated from residual ectopic thymic tissue or a microscopic thymoma left at the initial operation, this kind of occurrence of thymoma is very rare.
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2. Case report
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A mediastinal mass was found in a 48-year-old woman during a routine chest X-ray. Fifteen years previously, after manifesting dysphagia, ptosis, and generalized weakness, she had been diagnosed with myasthenia gravis and underwent an extended thymectomy on February 2, 1980. At that time, a thymectomy was performed through a median sternotomy with dissection of mediastinal adipose tissue, which was described as complete, and pathological results revealed thymic hyperplasia with no evidence of thymoma. Although the patient had developed respiratory failure due to the myasthenic crisis, which required reintubation and mechanical ventilation for 3 days, she recovered successfully and had been free of medication for myasthenia gravis.
Fifteen years after thymectomy, abnormalities on a routine chest X-ray and computed tomographic scan (CT scan) images revealed a solitary anterior mediastinal mass in close apposition to the left pulmonary artery, however, no enlarged mediastinal lymph node was observed (Fig. 1
). Further, T1-weighted magnetic resonance images showed hyperintence inhomogeneously and T2 images were hypointence. However, since the tumor could not be reliably differentiated from other types, such as malignant lymphoma, using the magnetic resonance and CT scan images, we assumed it to be thymoma rather than malignant lymphoma and the patient underwent another operation without a diagnosis based on preoperative histology.
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Fig. 1. CT scan of the thorax demonstrating an anterior mediastinal mass in close apposition to the left pulmonary artery.
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Through a median sternotomy, a white round mass was observed close to the left phrenic nerve with invasion to the pericardium and left mediastinal pleura, and scant residual adipose tissue in the anterior mediastinum. A pathological examination determined the mass to be a WHO type AB thymoma that had invaded the pericardium and pleura, which was classified as Masaoka stage III (Fig. 2) . Postoperative recovery was uneventful, during which the patient had 40 Gy of irradiation applied to the mediastinum. At a follow-up examination 5 years later, the patient was found to be free from recurrence.
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Fig. 2. Hematoxylin and eosin staining of the resected specimen showing thymoma with prominent lymphocytic infiltration.
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The entire resected specimen from the prior operation is no longer available, however, the specimen slides were reexamined and we could not find any evidence of thymoma.
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3. Discussion
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Thymectomy is a widely accepted option for treatment of myasthenia gravis [3,4]. Masaoka et al. noted the frequent existence of thymic tissue in anterior mediastinal adipose tissue around the thymus [1]. The borders of adipose tissue dissection are the diaphragm caudally, the thyroid gland orally, and the phrenic nerve laterally. However, the need for a complete resection of mediastinal adipose tissue is the subject of current debate. To perform a more complete dissection of mediastinal adipose tissue, Jaretzki et al. have advocated ‘maximal thymectomy’, which includes an additional resection of fatty tissue in the cervical and hilar regions through a T-shaped cervical and sternal incision [5]. However, results from this aggressive approach have not been shown to exceed those of a conventional trans-sternal thymectomy.
Thymoma developed in our patient 15 years after resection of the thymus, which was unusual, and its origin still remains a matter of speculation, though we have considered two possible origins of thymoma. First, thymoma might have come from residual thymic tissue that remained following the initial operation. Since it was located close to the left phrenic nerve, it could have originated from adipose tissue beyond the lateral border of dissection of the extended thymectomy, which was left behind to avoid nerve injury after dissection, though it resulted in an incomplete removal. In a study of autopsied cases, Fukai et al. noted that 7.7% of the incidences of thymic tissues were found in hilar and posterior mediastinal adipose tissues [6]. Another report found that residual thymic tissue caused the recurrence of thymic hyperplasia after thymectomy, giving support to this hypothesis [7]. To our knowledge, there is only one reported case of thymoma after thymectomy for non-thymomatous myasthenia gravis, similar to our case, which concluded that the tumor had originated from residual thymic tissue [2]. A second possible origin may have been the presence of a microscopic thymoma during the initial thymectomy. The most common recurrence of thymoma is pleural dissemination or local recurrence, whereas hematogenous or lymphatic metastasis is not common, as compared to other malignancies [8], and several reports have shown recurrence 10 or more years after initial excision [9]. If there was a microscopic thymoma in the resected thymus at the initial procedure, the present tumor, found 15 years later, should be considered as a local recurrence or dissemination. With such a possibility in mind, we reexamined slides of the resected specimens from the initial operation and were able to confirm that there was no tumoral component; therefore, we speculate that the later thymoma was likely a new primary tumor originating from ectopic thymic tissue rather than a recurrence. However, since the total resected thymus from the initial operation was no longer available, we could not rule out entirely the presence of thymoma in the whole section or confirm the possibility of ectopic thymic tissue origin.
We stress the importance of both complete anterior mediastinal fatty tissue dissections during an extended thymectomy and long-term radiological follow-up, even for patients with non-thymomatous myasthenia gravis, due to the risk of new thymoma occurrence.
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References
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- Masaoka A., Nagaoka Y., Kotake Y. Distribution of thymic tissue at the anterior mediastinum. Current procedure in thymectomy. J Thorac Cardiovasc Surg 1975;70:747-754.[Abstract]
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- Masaoka A., Yamakawa Y., Niwa H., Fukai I., Kondo S., Kobayashi M., Fujii Y., Monden Y. Extended thymectomy for myasthenia gravis patients: a 20-year review. Ann Thorac Surg 1996;62:853-859.[Abstract/Free Full Text]
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Abstract
1. Introduction
