CASE REPORT Malignant Pleural Mesothelioma With Heterologous Osteoblastic Elements Computed Tomography, Magnetic Resonance, and Positron Emission Tomography Imaging Characteristics of a Rare Tumor Rodrigo A. Salgado, MD,* Robert Corthouts, MD,* Paul M. Parizel, MD, PhD,* Paul Germonpre ´, MD, PhD,† Laurens Carp, MD,‡ Paul Van Schil, MD, PhD,§ and Eric Van Marck, MD, PhD k Summary: The imaging findings of a rare mixed type of malignant mesothelioma of the pleura with heterologous bone stroma in a pa- tient without a previous history of asbestos exposure are reported. Imaging findings of this type of bone-forming pleural mesothelioma are scarcely reported in the literature, with only a few case reports describing findings on conventional radiography, computed tomog- raphy, and magnetic resonance imaging. To the best of our knowl- edge, no positron emission tomography imaging characteristics of this type of mesothelioma have been previously reported. Key Words: computed tomography, magnetic resonance, mesothe- lioma, neoplasm (J Comput Assist Tomogr 2005;29:653–656) T his case illustrates the imaging findings of a rare mixed type of malignant mesothelioma of the pleura with heter- ologous bone stroma in a patient without a previous history of asbestos exposure. CASE REPORT A 64-year-old woman complained of shortness of breath and vague right-sided thoracic pain. A conventional chest radiograph (not shown) demonstrated a moderate pleural effusion in the right lung base. A subsequent contrast-enhanced computed tomography (CT) scan additionally revealed minimal pleural calcification on the right side (not shown). The mediastinal and hilar structures were unremark- able, and the left lung was normal. There was no history of asbestos exposure. An open biopsy was performed, which revealed only mesothelial hyperplasia with mature osseous metaplasia. There were no signs of malignancy. Six months later, the patient was referred to our university hospital for further evaluation. Her symptoms had increased in se- verity. A new chest radiograph demonstrated extensive thickening and densification of the right pleura (not shown). A 16-slice spiral CT scan (Sensation 16; Siemens Medical Systems, Erlangen, Germany) of the thorax before and after intravenous contrast administration confirmed extensive uniform thickening and densification of the right pleura completely covering the right lung, with a complex loculated pleural effusion at the lung base (Fig. 1). A subsequent magnetic resonance (MR) examination (Symphony 1.5-T system; Siemens Medical Systems) further documented this diffuse right-sided uni- lateral pleural mass (Fig. 2). The pleural thickening was slightly heterogeneous and isointense on T1-weighted images. The areas of extensive pleural densification, as documented on CT, did not create a signal void on MR imaging. It was therefore suggested that the pleural densification could not be composed of typically calcified pleural plaques. Furthermore, after intravenous gadolinium injection, a diffuse and intense enhancement of this pleural mass on T1-weighted images was observed. Magnetic resonance images also showed a blurred border and obliteration of the normal fat plane between the mediastinal pleura and the adjacent vascular and hilar structures, indicating possible tumoral infiltration. There were signs of invasion of the pericardium and the lung base not clearly appreciated on the CT examination. An 18 F-fluorodeoxyglucose positron emission tomography (PET) scan (ECAT ART PET scanner; Siemens Medical Systems) showed diffuse moderately increased tracer uptake in the right pleura (Fig. 3). A new open biopsy showed a malignant pleural mesothelioma of the mixed epithelial-mesenchymal type with heterologous osteo- blastic stroma elements. DISCUSSION Malignant mesothelioma is a rare tumor that arises from the mesothelial surfaces of the pleura and peritoneum but, in exceptional cases, can also arise from the tunica vaginalis and pericardium. Most cases of mesothelioma (80%) are pleural in origin, 1 presenting as locally aggressive tumors. Histologically, malignant mesotheliomas are typically classified in 3 main subtypes. The most frequent type is the epithelial form, which is encountered in 50% to 60% of all cases. The sarcomatoid Received for publication May 3, 2005; accepted June 6, 2005. From the *Department of Radiology, University Hospital of Antwerp, Edegem, Belgium, †Department of Pneumology, University Hospital of Antwerp, Edegem, Belgium, ‡Department of Nuclear Imaging, University Hospital of Antwerp, Edegem, Belgium, §Department of Thoracic and Vascular Surgery, University Hospital of Antwerp, Edegem, Belgium, and k Department of Pathology, University Hospital of Antwerp, Edegem, Belgium. Reprints: Rodrigo Salgado, Department of Radiology, University Hospital Antwerp, Wilrijkstraat 10, Edegem, Belgium (e-mail: rodrigo.salgado@ uza.be). Copyright Ó 2005 by Lippincott Williams & Wilkins J Comput Assist Tomogr  Volume 29, Number 5, September/October 2005 653