Rapid diffuse pleural thickening due to metastatic meningioma

A 64‐year‐old man was referred to our hospital because of persistent dyspnoea for the past 1 month. He had recurrent brain anaplastic meningioma after two operations and irradiation. He suffered from right pleural effusion in the previous few months and was diagnosed with malignant mesothelioma via pleural biopsy 1 month prior to coming to our hospital. At his first visit to our hospital, thoracic computed tomography demonstrated rapidly developed large inhomogeneously enhancing pleural thickening up to 3 cm, which surrounded the right hemithorax, together with left‐sided pleural effusion. After re‐evaluation of the pathological specimens retrieved from the local hospital, he was finally diagnosed with pleural metastasis secondary to anaplastic meningioma (WHO classification, grade 3). Generally, brain meningiomas are believed to be benign and seldom metastasize to other organs. However, the present case clearly demonstrated the unique clinical presentation of anaplastic meningioma, also known as malignant meningioma, which mimicked the pathological and radiological findings of a malignant mesothelioma.


Introduction
Extracranial metastases from brain meningioma have a low incidence of 0.76% [1] and occur mostly in the lungs, bone, intraspinal, abdominal viscera, mediastinal, and cervical lymph nodes. However, pleural metastasis can occur as an extremely rare complication of brain meningioma regardless of the tumour grading [2]. Here, we report a case with metastatic meningioma to the pleura that radiologically and pathologically mimicked malignant mesothelioma.

Case Report
A 64-year-old man was referred to our hospital because of recent dyspnoea of a duration of 1 month. One month prior to his referral, he was diagnosed with biphasic malignant mesothelioma at his local hospital based on pathological findings of the pleura after videoassisted thoracic surgery. He was an ex-smoker with a history of 42 pack-years and had denied any exposure to dust. Two years before referral, a resection of an identified meningioma located at the right parasagittal sinus was performed, and he was diagnosed with anaplastic meningioma, (World Health Organization (WHO) classification, grade 3) [3]. However, the tumour in the frontal convexity recurred 1 year later; therefore, a second operation and brain irradiation were performed. At his last evaluation, 1 month before presenting to our hospital, enhanced magnetic resonance imaging of the brain showed a residual tumour measuring 1.6 cm in size (Figure not shown).
A chest X-ray performed 4 months earlier (Fig. 1A) appeared normal. However, his right pleural effusion progressed in the following 2 months, and thoracic drainage was performed at his local hospital. After removal of the right-sided pleural fluid 1 month prior to his referral to us, a chest X-ray demonstrated an infiltration of the right lower lung field with blunting of the costophrenic angle ( Fig. 1 B), enhanced parietal pleural thickening (arrow head, Fig. 1C), and pleural effusion on enhanced thoracic computed tomography (CT). Furthermore, a small number of tiny nodules were also noted in the parietal pleura ( Fig. 1C, arrow head).
Surprisingly, thoracic CT performed at his first visit to our hospital depicted the emergence of a large inhomogeneously enhancing pleural thickening measuring approximately 3 cm, which surrounded the right hemithorax (Fig. 1D), together with left-sided pleural effusion. Accordingly, he was tentatively diagnosed with a severe rapidly progressing biphasic malignant mesothelioma.
Therefore, he was finally diagnosed with pleural metastasis secondary to anaplastic meningioma (WHO class-ification, grade 3). One month following the diagnosis, he died of respiratory failure.

Discussion
The present case demonstrated that meningioma (also known as malignant meningioma) could metastasize to the pleura and that metastatic pleural meningioma seems to resemble malignant mesothelioma radiologically and pathologically.
With regard to all types of metastatic meningioma, Enam et al. [1] have reported that the incidence of metastasis to other organs was only 0.76%. The exact frequency of meningioma metastasizing to the pleura is unknown, but previous studies have reported the range to be from 5.5% [2] to 9.0% [4], suggesting that it is an extremely rare event. However, for WHO grade 3 meningioma [3], the rate of metastasis to other organs was as high as 42.8% [1], and only 4.3% to the pleura [2].
In terms of radiological findings, the present case seemed to resemble malignant mesothelioma; however, it was distinguished by the rapid expansion of the pleural thickening, which covered the entire right hemithorax in 1 month, and the absence of thoracic deformity, regardless of the severity of the pleural thickening. Furthermore, the negative results of the immunohistochemical staining for calretinin and cytokeratin in the atypical epithelioid cells excluded the possibility of identifying them as epithelial or biphasic malignant mesothelioma cells.
In conclusion, brain meningiomas, generally considered benign tumours, could rapidly and aggressively metastasize to the pleura as in the present case.

Disclosure Statement
Appropriate written informed consent was obtained for publication of this case report and accompanying images.