Teaching File

Institute of Radiology "P. Cignolini" - University of Palermo, Italy

Benign fibrous mesothelioma: a case report


March 20, 1996


Introduction

It is a rare tumor of mesodermal origin which amounts to less than 5 % of all neoplasms concerning the pleura.
It occurs in patients of all ages. There is neither sex predilection nor evidence of a relationship to asbestos exposure. It has a good prognosis if compared to the diffuse malignant form (1-2).
It is usually discovered as an asymptomatic lesion on chest radiograph though some patients may complain a mild chest pain (3). It is reported it may recurs in up to 16 % of cases after surgical excision, but malignant transformation in unusual (4).

Findings

  • Axial CT unenhanced scan
  • Axial CT unenhanced scan - lung window
  • Enhanced axial CT scan
  • Axial MRI T1W (TR/TE: 380/20) and T2W (TR/TE: 2600/100) images
  • Sagittal Gd-DTPA-enhanced MRI T1W image
  • Gross specimen

    Case report

    70-year-old male.
    Surgical excision of three liver hydatid cysts 8 months before.
    A follow up chest radiograph showed a large, ovalar, smooth mass in the right middle-posterior hemithorax.
    The mass formed an acute inferior angle with the pleura whereas the superior angle were obtuse.
    Chest CT showed a smooth, ovalar, slightly unhomogeneous mass with a maximum diameter of about 6 cm, in the upper segment of the right lung inferior lobe.
    The mass abutted the pleura and formed acute angles and a tapering margin with adjacent pleura.
    The mass had a low-intermediate attenuation on unenhanced scans and a slight contrast uptake.
    Chest MRI confirmed the CT findings. The mass abutted the pleura and formed acute angles and a tapering margin with the adjacent pleura, in the transaxial plane. Whereas it formed an acute inferior angle and an obtuse superior angle with the pleura in the sagittal plane.
    It was with intermediate signal intensity on SE T1 weighted images and high signal intensity on SE T2 weighted images . Some hypointense foci were present within the mass in all the sequences.
    The mass showed a contrast uptake after endovenous injection of Gd-DTPA, but no contrast enhancement was seen in the hypointense foci .
    The gross specimen measured about 6 cm and had a short pedicle. The mass consisted of fibrous tissue with some microcysts filled with a viscous fluid.
    Bundles of reticular and collagen fibers and fibroblast-like mesothelial spindle-shaped cells were found on microscopic examination.
    The histological diagnosis was benign fibrous mesothelioma.

    Discussion

    The patient had undergone abdominal surgical intervention for three hydatid cysts of the liver 8 months before. The finding on chest radiograph of a large oval mass in the right hemithorax was strongly suggestive of hydatid cyst. The contrast enhancement shown by the mass after iodine contrast medium on CT and after paramagnetic contrast agent on MRI, ruled out the hypothesis of the thoracic hydatid cyst.
    The mass formed a smoothly tapering margin with the adjacent pleura. The angles at the junction of the mass with the pleura were acute instead of obtuse.
    The finding of a smoothly tapering margin showed to be a more reliable sign of the pleural origin of the mass than the observation of obtuse angles, as reported by other investigators (4).
    The mass showed smooth and sharply delineated contours, supporting the hypothesis of a benign lesion (4).
    In previous cases the signal intensity of benign fibrous mesothelioma on MRI was intermediate on SE T1 weighted images and hypointense on SE T2 weighted images (5-6). Our experience was in disagreement with this finding, because benign fibrous mesothelioma was hyperintense on SE T2 weighted images, despite the intermediate signal on T1 weighted images.
    In conclusion a smoothly tapering margin of a mass with the pleura is a reliable sign of its pleural origin. A smooth, sharply delineated contour of the mass may support the hypothesis of a benign lesion. The signal intensity of the mass on MRI is not always helpful in the diagnosis of benign fibrous mesothelioma, since the latter may be hyperintense, instead of hypointense, on SE T2 weighted images, as shown in our experience.

    REFERENCES

    1) Dedrick CG, Mc Loud TC, Shepard JO, Shipley RT. Computed Tomography of localized pleural mesothelioma. AJR 1985; 144: 275-280

    2) Mendelson DS, Meary E, Buy JN, Pigeau I, Kirschner PA. Localized fibrous mesothelioma: CT findings. Clin Imaging 1991; 15: 105-110

    3) Kyung SL, Jung GI, Kyu OC, Chang JK, Byoung HL. CT findings in benign fibrous mesothelioma of the pleura AJR 1992; 158: 983-986

    4) Briselli M, Mark EJ, Dickersin R. Solitary fibrous tumors;of the pleura: eight new cases and review of 360 cases in the literature. Cancer 1981; 47: 2678-2689

    5) Herold CJ, Zerhouni EA. The mediastinum and lungs. In Higgins CB, Hricak H, Helms CA(Eds). MRI of the body 1992; 461-523 Raven Press

    6) Harris GN, Rozenshtein A, Schiff MJ. Benign fibrous mesothelioma of the pleura: MR imaging findings AJR 1995; 165: 1143-1144


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